Questions on _g_ and intelligence

[Mandrake]

From the excerpts there, it doesn't sound like the authors are saying environmental influences are lost at all. Instead, it seems to be suggesting a fairly reasonable argument that underlying genetic predisposition and environmental factors interact with one another to influence adolescent behavior.

I previously referenced this paper from Intelligence:
Genetics and Intelligence: What’s New?
ROBERT PLOMIN, STEPHEN A. PETRILL
Institute of Psychiatry, London

On page 12, it says:
Until recently, environmental factors that affect intelligence were thought to operate primarily in a shared manner. For example, our earlier review of genetic influences on intelligence concluded that shared environment accounted for about 25% of the variance of IQ scores. The strongest evidence for the importance of shared environment comes from the correlation for adoptive siblings, that is, pairs of genetically unrelated children adopted into the same adoptive families. As shown in Figure 2, adoptive siblings correlate about .30 for IQ, suggesting that about a third of the variance in IQ can be attributed to shared family environment. However, the studies reviewed in Figure 2 happened to assess adoptive siblings as children. In 1978, the first study of older adoptive siblings yielded a strikingly different result: The IQ correlation was -.03 for 84 pairs of adoptive siblings from 16 to 22 years of age (Scat-r & Weinberg, 1978). Other studies of older adoptive siblings have also found similarly low IQ correlations. The most com-pelling evidence comes from a lo-year follow-up study of 181 adoptive siblings. At the average age of 8 years, their IQ correlation was .26. However, 10 years later, their IQ correlation was - .Ol , suggesting that shared family environmental effects on IQ decline to negligible levels after adolescence (Loehlin, Horn, & Willerman, 1989) (see Figure 7).

We are confident that the question is when, not whether, genes will be found that are associated with intelligence. Indeed, many genes have already been found that are associated with low intelligence. More than 100 rare single-gene disor-ders include mental retardation among their symptoms (Walhsten, 1990).
The figure 7 is on page 13 and shows that the shared environment shows up in childhood, then vanishes completely.

 

[Mandrake]

Concordance rates of IQ scores

This may be of interest to some of you:

Concordance rates of IQ scores

Evidence from family studies provides the main supporting evidence from which arguments about the relative roles of genetics and environment are constructed.

A large number of the study of twins reared apart was undertaken by Thomas Bouchard of the University of Minnesota starting in 1979. He “collected” pairs of separated twins from all over the world and reunited them while testing their personalities and IQs. Other studies at the same time concentrated on comparing the IQs of adopted people with those of their adopted parents and their biological parents or their siblings. Put all these studies together, which include the IQ tests of tens of thousands of individuals, and the table looks like this:

• Same person tested twice 87%
  
  Identical twins reared together 86%
  
  Identical twins reared apart 76%
  
  Fraternal twins reared together 55%
  
  Biological siblings reared together 47% (studies show that reared apart about 24%)
  
  Parents and children living together 40%
  
  Parents and children living apart 31%
  
  Adopted children living together 0%
  
  Unrelated people living apart 0%

from
Ridley, M. (1999). Genome: The autobiography of a species in 23 chapters. London: Fourth Estate Ltd.

 

[hitssquad]

The Physics Forums 'Science Expert' medal and its nominators

Let me add that one contributor to these discussions (screen name "hitsquad") is well informed and has posted comments that are identical to what I would have written about the same issues. This person has addressed the questions pertaining to intelligence with facts that are scientifically valid and known to those who have studied the subject in depth.

Go for it. I won't complain.

 

[nuenke]

From the excerpts there, it doesn't sound like the authors are saying environmental influences are lost at all. Instead, it seems to be suggesting a fairly reasonable argument that underlying genetic predisposition and environmental factors interact with one another to influence adolescent behavior.

The book looks at three causes for differences in intelligence and behaviors. Using a very large group, and using longitudinal studies, as well as including all of the relationships (twins, adopted children, families with half brothers/sisters, etc.), they determined that when young, the shared environment may be important, but as children grow older, it is almost exclusively genes and the non-shared environment that determines these factors. That is, the family environment has very little impact on children once they enter their teens - they find, produce and gravitate towards their own individual environments depending on their genes and other unknown factors. That is, no one really knows what the interaction is, but it is not the family that makes the person.

This is pretty much accepted by behavior geneticists. Also see the Nurture Assumption by Judith Rich Harris, 1998. She discovered this, if I remember right, because she edited a lot of research papers and noticed irregularities. Her book is a much easier read, and explains how children find their own niches.

Of course, this new research really hurts educators, psychologists, social workers, etc. There is a huge industry built upon naïve environmentalism, and they are not about to lose their livelihoods because new research has made many of their programs quite meaningless.

 

[Moonbear]

That is, the family environment has very little impact on children once they enter their teens - they find, produce and gravitate towards their own individual environments depending on their genes and other unknown factors. That is, no one really knows what the interaction is, but it is not the family that makes the person.

Can you please expand on this? Are they saying environment is not at all a factor, or that the social influences shift from family to some other source? Part of the social shift in adolescence is becoming independent of your parents and instead identifying more with same-aged peers.

Of course, this new research really hurts educators, psychologists, social workers, etc. There is a huge industry built upon naïve environmentalism, and they are not about to lose their livelihoods because new research has made many of their programs quite meaningless.

You mean those who would say it's ONLY social factors, and NEVER biological? Sure, any view that rigid isn't going to last long in any field of science.

Something that still doesn't seem clear to me is which definitions of environmental are being employed here. I suspect we may not all have the same idea in mind. When I think of environmental influences, I think of anything coming from outside our own body, including physical/chemical/biological external influences (including intrauterine environment of the fetus) as well as social influences from both family and other people (teachers, friends, neighbors). In a number of the posts here, I get the impression environmental is being used synonymously with social influences only, i.e., nature vs nurture. Conversely, I also keep getting the impression that some are considering anything that is not due to social influence must be due to genetic influence rather than other physical/biological factors, and I'm not sure where everyone is categorizing those factors. It is quite possible this is leading to a good deal of our misunderstandings in this thread because we aren't all thinking the same definitions.

In Plomin's 2004 article, he discusses that one of the reasons why it may be difficult to pinpoint a genetic link to IQ is that a single gene may only account for something as small as 1% of the variation, such that you can never detect a difference by looking for one gene at a time. So, for those who are willing to embrace the genetic linkage, and would accept Plomin as an authority on the subject, why could the same not be true of environmental factors? Often, we try to study single environmental factors to avoid the difficulty in interpreting findings with numerous variables. What if we can't detect any differences for a single environmental factor, such as SES, for the same reasons we can't detect significant differences related to a single gene? And if we did find a gene, we'd need to show that it then results in a functional protein that also differs in expression levels. And is that protein expression altered by environment? Or does it alter the way we respond to our environment? Or both, in a feedback loop? There is prior evidence that both can occur. In behavioral endocrinology, hormones influence behavior, but then behavior also influences hormone secretion. It's incredibly difficult to tease these two directions of interaction apart.

 

[hitssquad]

Definitions of heritability and environment and introduction to concept of variance

That is, the family environment has very little impact on children once they enter their teens - they find, produce and gravitate towards their own individual environments depending on their genes and other unknown factors. That is, no one really knows what the interaction is, but it is not the family that makes the person.

Can you please expand on this? Are they saying environment is not at all a factor, or that the social influences shift from family to some other source?

In regards to heritability, we consider phenotypic variance accountable for on the one hand by environemental variance and on the other hand by genetic variance. If a teen "gravitates toward" specific environments, genetics would be accounting for the variance in environment and hence ultimately accounting for variance in phenotype. What is proposed is not that the teens are shifting from the family to random outside influence, but to ouside influence that caters to the teens' biological programming, of which genetics accounts for the bulk of the variance.

Part of the social shift in adolescence is becoming independent of your parents and instead identifying more with same-aged peers.

And what is proposed is that peer selection (among other selections by teens) is not random, but is largely a product of genetically-determined tendency. There is no accounting for taste, as they say, and it is proposed that variance in taste in environment may be more largely accountable for by variance in genetic code than by variance in environment.

Something that still doesn't seem clear to me is which definitions of environmental are being employed here.

Heritability refers to variance in phenotypic outcome accountable for by genetics as opposed to accountable for by environment. Environment is all factors left over when genetic code is controlled for.

When I think of environmental influences, I think of anything coming from outside our own body

Then you are mistaken, because in terms of heritability environment is any factor that is not genetic. That includes the body itself. For example, genetic code expresses throughout a person's life. But the state of the body has visible effects on genetic expression. When people reach physical maturity, their genes still express proteins that code for growth factors. Yet they do not grow any more. This is because of states in the body that prevent growth in the presence of growth factor. One of these states is the hardening of the ends of bones. If the ends of bones remained soft, as they are when we are children, growth factors might continue to stimulate bone growth indefinitely - and people would grow to towering heights throughout their lifetimes, as trees do.

In a number of the posts here, I get the impression environmental is being used synonymously with social influences only, i.e., nature vs nurture.

Yes. This is a classic confusion of the term "environment." What normally happens in nature/nurture discussions is that the equivocation fallacy is committed in regards to the definition fo the term environment. One moment it means all environment (all variance not genetic), and the next moment it means only intellectual stimulation. Arthur Jensen suggested that when we discuss environmental variance we be clear about whether we are speaking in terms of biological variance; variance in intellectual stimulation; or all non-genetic cariance (all environmental variance).

In Plomin's 2004 article, he discusses that one of the reasons why it may be difficult to pinpoint a genetic link to IQ is that a single gene may only account for something as small as 1% of the variation, such that you can never detect a difference by looking for one gene at a time. So, for those who are willing to embrace the genetic linkage, and would accept Plomin as an authority on the subject, why could the same not be true of environmental factors?

There is no environmental counterpart to the discrete gene. In regards to heritability, environment is taken as one big lump and genetics is taken as one big lump. It stands to reason that if it has been established that genetic variance accounts for any amount of variance in a given phenotypic trait that we should also be able to find variance in specific genes that accounts for the same phenotypic trait variance. But searching for genes is not directly a part of heritability studies. Heritability of g in various human populations has already been established by selectively controlling for variance in environment and by selectively controlling for variance in genetic code.

What if we can't detect any differences for a single environmental factor, such as SES, for the same reasons we can't detect significant differences related to a single gene?

SES is not comparable to a discrete gene. You can silence a discrete gene, but you cannot silence SES. SES is an axis. A gene is quantum piece of code.

And if we did find a gene, we'd need to show that it then results in a functional protein that also differs in expression levels.

No. The gene in question might be coding for the silencing or unsilencing of other parts of the genome. Discrete production of a protein by the IQ-related gene in question may not be necessary for that to occur.

And is that protein expression altered by environment?

Of course it is affected by environment. Studies of heritability have nothing to do with teasing out discrete affectors. They simply control for variance of environment and genetics and see how that results in changes in variance of expression of one or more phenotypic traits.

 

[Moonbear]

Then you are mistaken, because in terms of heritability environment is any factor that is not genetic. That includes the body itself. For example, genetic code expresses throughout a person's life. But the state of the body has visible effects on genetic expression. When people reach physical maturity, their genes still express proteins that code for growth factors. Yet they do not grow any more. This is because of states in the body that prevent growth in the presence of growth factor. One of these states is the hardening of the ends of bones. If the ends of bones remained soft, as they are when we are children, growth factors might continue to stimulate bone growth indefinitely - and people would grow to towering heights throughout their lifetimes, as trees do.

I wouldn't have chosen the word mistaken. It appears to be a difference in terminology from my field in which environmental does mean anything from outside the body. This is why I asked, because it is becoming apparent that we aren't all talking about the same thing.

Yes. This is a classic confusion of the term "environment." What normally happens in nature/nurture discussions is that the equivocation fallacy is committed in regards to the definition fo the term environment. One moment it means all environment (all variance not genetic), and the next moment it means only intellectual stimulation. Arthur Jensen suggested that when we discuss environmental variance we be clear about whether we are speaking in terms of biological variance; variance in intellectual stimulation; or all non-genetic cariance (all environmental variance).

Well, at least that much we can agree on. We need to be explicit in our definition of environment, or, more importantly, various authors' definitions of environment, when discussing it here. I think this is leading to a lot of additional confusion in this discussion.

 

[Mandrake]

Of course, this new research really hurts educators, psychologists, social workers, etc. There is a huge industry built upon naïve environmentalism, and they are not about to lose their livelihoods because new research has made many of their programs quite meaningless.

Yes indeed! The political and job related motivations for promulgating false concepts concerning environmental factors is huge. It has resulted in massive spending without returns. We have an inverse relationship between school spending and educational results precisely because the spending has been narrowly focused on population groups that will not show cognitive improvements as a result of the programs advertised to produce gains. The politics of this issue are so robust that I doubt that the subject can be discussed openly in any educational or political forum. The special interest side will simply shout down the scientific findings and use their usual "dirty racist" name calling as their only weapon. This struggle is similar to the Christian church as it sought to stop the advancement of science.

 

[Mandrake]

The Shared Environment

Can you please expand on this? Are they saying environment is not at all a factor, or that the social influences shift from family to some other source?

There are shared and not-shared environmental factors. Family factors are the shared ones. I previously quoted from Plomin (this thread) that the shared factors vanish in adolescence. In childhood, the shared factor is about 25%. From there, it goes to zero.

Part of the social shift in adolescence is becoming independent of your parents and instead identifying more with same-aged peers.

More likely is that genetic expression increases.

When I think of environmental influences, I think of anything coming from outside our own body, including physical/chemical/biological external influences (including intrauterine environment of the fetus) as well as social influences from both family and other people (teachers, friends, neighbors).

Jensen divides environmental factors into macro and micro, where the macro environment is that which is due to social interactions (family, institutional, etc.) and the micro environment is due to chemical and biological elements. The micro environment includes the intrauterine environment. When you subtract the variance due to genetics (considering intelligence), the factors that remain are environment and error. Together they add to about20-30% of the total variance.

Obviously, adoption studies are important to the observation of the evaporation of the shared environmental component.

By adolescence (by which time parental influence is weak in the modern West), unrelated children who have grown up as adoptees in the same family show quite simply no similarity at all in their levels of g . As Neisser et al. (1995) conclude in their review for the American Psychological Association, "Severely deprived, neglectful or abusive environments must have negative effects on a great many aspects of development, including intelligence. Beyond that minimum, however, the role of family experience is now in serious dispute." [Brand, C. (1996). The _g_ Factor: General Intelligence and Its Implications. Chichester, England: Wiley]

In Plomin's 2004 article, he discusses that one of the reasons why it may be difficult to pinpoint a genetic link to IQ is that a single gene may only account for something as small as 1% of the variation, such that you can never detect a difference by looking for one gene at a time. So, for those who are willing to embrace the genetic linkage, and would accept Plomin as an authority on the subject, why could the same not be true of environmental factors?

It may well be true that environmental factors account for only 1% of the variance each. I don't see the problem. We know the sum of the genetic influence and we know the remainder is due to the sum of all environmental and error components.

Often, we try to study single environmental factors to avoid the difficulty in interpreting findings with numerous variables. What if we can't detect any differences for a single environmental factor, such as SES, for the same reasons we can't detect significant differences related to a single gene?

It is relatively unimportant to identify causes of 1% in variance when we can identify the sum. There is no dispute that the sum of all environmental factors is equal to 100% minus the variance in the genetic component, minus the variance due to error. The end finding is completely consistent with the findings of scores of careful research programs.

Jensen: "One commentator likened the latest phase of the nature-nurture IQ debate to "a stomping match between Godzilla [that is, genes] and Bambi [that is, environment]."
See page 100 Miele (2002) - Intelligence, Race, and Genetics: Conversations with Arthur R. Jensen

 

[Waterdog]

This may be of interest to some of you:

Concordance rates of IQ scores

Evidence from family studies provides the main supporting evidence from which arguments about the relative roles of genetics and environment are constructed.

A large number of the study of twins reared apart was undertaken by Thomas Bouchard of the University of Minnesota starting in 1979. He “collected” pairs of separated twins from all over the world and reunited them while testing their personalities and IQs. Other studies at the same time concentrated on comparing the IQs of adopted people with those of their adopted parents and their biological parents or their siblings. Put all these studies together, which include the IQ tests of tens of thousands of individuals, and the table looks like this:

• Same person tested twice 87%
  
  Identical twins reared together 86%
  
  Identical twins reared apart 76%
  
  Fraternal twins reared together 55%
  
  Biological siblings reared together 47% (studies show that reared apart about 24%)
  
  Parents and children living together 40%
  
  Parents and children living apart 31%
  
  Adopted children living together 0%
  
  Unrelated people living apart 0%

from
Ridley, M. (1999). Genome: The autobiography of a species in 23 chapters. London: Fourth Estate Ltd.

The Minnesota Twins study really doesn't account for environment very well. The employ an inventory of items such as whether the households in which the separated twins are raised possesses power tools or encyclopedias. Based on this they have argued that the environments of the twins in the study were heterogeneous. However, the twins are not from "all over the world," as you say. They come from a few culturally homogeneous countries such as the US, UK, Australia, West Germany, and a few others that are quite similar as well--both internally homogeneous and similar to each other. All of these countries provide basically the same opportunities for developing IQ and the same cultural emphasis on IQ-related tasks. In addition, the MN twin study's inventory of items that supposedly shows how heterogeneous the households are could be described as rather arbitrary or even silly. There is a reason that so-called "social science" is distinguished from "hard science" such as physics or chemistry--it's simply impossible to really control any variables in a social science.

No one would deny that MZ twins are more similar then unrelated strangers, but this would only come as news to the most radical environmentalists. For most reasonable people, who believe that variation of traits in a population is partly due to environment and partly to genetics, the MN twins study is really not that enlightening, since it only tells us what we already knew. The beanfield analogy is most apt here. They are studying beans in one field where soil, light, and water are all constant. Not surprising at all that in that case, beans with the same genes don't show much variation. They have not looked at how beans with identical genes would develop in fields with radically differing soil quality, sunlight, water, etc. In fact, the authors of the MN twins study explicitly state that their study of within-group variance cannot be used to draw any conclusions about between-group variance. This last point is usually ignored by the radical genetic determinist crowd who misuse their work.

 

[Mandrake]

The Minnesota Twins study really doesn't account for environment very well. The employ an inventory of items such as whether the households in which the separated twins are raised possesses power tools or encyclopedias. Based on this they have argued that the environments of the twins in the study were heterogeneous. However, the twins are not from "all over the world," as you say.

Did I mention the Minnesota Twins study? Do you think this is the only twins study reported in the literature? Besides the Burt studies, which are sometimes not cited, there are four other studies that produced essentially identical results. All of these are for MZA's:

Newman et al.
Shields (1%2)
Juel-Nielsen
Minnesota Study

I just now did a search on "twin studies" within the Journal Intelligence. There were 21 hits. I looked at the first few. Each was from a different researsh team and each from a different country. The last one I checked studied just under 7,000 twin sets. Your attempt to refute my comment is incomprehensible. Do you really think that there has only been one twin study and that other nations have not studied twins? Perhaps that is not what you intended to imply? I hope not, but that is the implication I read into the comment.

They come from a few culturally homogeneous countries such as the US, UK, Australia, West Germany, and a few others that are quite similar as well--both internally homogeneous and similar to each other. All of these countries provide basically the same opportunities for developing IQ and the same cultural emphasis on IQ-related tasks.

We know from adoption studies, intensive intervention programs, and longitudinal studies that the shared environment vanishes by age 17. Have you seen real (as in peer reviewed) data showing that any macro environmental effect has ever caused a permanent boost in _g_? How about even a one year boost in _g_?

No one would deny that MZ twins are more similar then unrelated strangers, but this would only come as news to the most radical environmentalists.

MZ twins are more similar than any other category of humans.

 

[Waterdog]

Did I mention the Minnesota Twins study?

Yes, you did.

Do you think this is the only twins study reported in the literature?

No. However, it is the most commonly discussed by the semi-literati on internet discussion boards, and hence the most relevant to analysis.

Besides the Burt studies, which are sometimes not cited, there are four other studies that produced essentially identical results. All of these are for MZA's:

Newman et al.
Shields (1%2)
Juel-Nielsen
Minnesota Study

You seem to have three other studies there, not four. Perhaps you meant besides Burt?

I just now did a search on "twin studies" within the Journal Intelligence. There were 21 hits. I looked at the first few. Each was from a different researsh team and each from a different country. The last one I checked studied just under 7,000 twin sets.

No one suggested N was insufficient, did they? The question is where the subjects come from and where they are placed, not how many there are. So, what are the countries? How did the researchers try to control for environment? OK, I did the same search and noted Australia, Germany, the United Kingdom, etc. What does that have to do with the argument I made? Can you cite a study of pairs of twins who were raised in truly different environments, e.g., one set raised in Norway and their MZ twins raised in Ghana? That would tell us something. Else, you are simply stating the obvious: genetically identical organisms that develop in similar environments will be similar. Why is this finding noteworthy? Why do we need dozens of studies to demonstrate what is obvious?

Your attempt to refute my comment is incomprehensible.

Heh.

Do you really think that there has only been one twin study and that other nations have not studied twins? Perhaps that is not what you intended to imply? I hope not, but that is the implication I read into the comment.

Your statement here makes it obvious that you didn't understand what I wrote. The problem is that each pair of twins develops within a similar environment. Doesn't matter whether you have twins from different countries if the members of each pair have similar environments. Then you haven't controlled for environment, even if the environment does differ between different pairs.

We know from adoption studies, intensive intervention programs, and longitudinal studies that the shared environment vanishes by age 17. Have you seen real (as in peer reviewed) data showing that any macro environmental effect has ever caused a permanent boost in _g_? How about even a one year boost in _g_?

Ah, the fetish of the peer review. Do you know that many of the early behavioral geneticists, people who you now probably think of as great minds, couldn't get their stuff published because the peer reviewers for the major journals didn't like it? Peer review is a process whose goal is to produce high quality in publications, but it is far from perfect. Good articles get rejected and bad articles get accepted. Just because something is "peer reviewed" doesn't mean it's true.

As for boosting, "g," I doubt it's possible to raise someone's IQ test scores significantly once they reach school age. I would agree with you there. The most important environmental effects have to do with pre-natal care, followed by the development of the neural network in early childhood. Neuroscientists have documented (yes, in *peer reviewed* publications) that the brain develops its structure of neurons iin response to environmental stimuli. Since MZ twins tend to be placed in similar homes in the same countries, it is not surprising that they have similar IQ's as adults. They get the same nutrition and other environmental effects in the womb, that's for sure, and quite similar during the rest of early childhood. These are the crucial periods in neural development.

Anyway, no one would deny that there is a large genetic component. How could a biological organism develop with no reference to its own DNA? What I am arguing against is the idea that it has been "proven" that this is as high as .80. Environment has never been controlled for in any twin study, so this has not been proven at all. My guess is that, like bean sprouts, human brains with the same genes can develop in all sorts of different ways in response to different environmental stimuli.

MZ twins are more similar than any other category of humans.

Who ever said they weren't? Again, you clearly don't understand what I wrote.

More generally, here is a *peer reviewed* article critical of the use of twin studies in behavioral genetics. Now you are going to tell me that this article is wrong even though it's *peer reviewed*! lol.



Accession Number
Peer Reviewed Journal: 2003-01223-002.

Author
Ehrlich, Paul; Feldman, Marcus.

E-Mail Address
Feldman, Marcus: marc@charles.stanford.edu

Title
Genes and Cultures: What Creates Our Behavioral Phenome? [References].

Source
Current Anthropology. Vol 44(1) Feb 2003, 87-107.
Univ of Chicago Press, US

Abstract
(from the journal abstract) A central theme of the flood of literature in recent years in "evolutionary psychology" and "behavioral genetics" is that much or even most human behavior has been programmed into the human genome by natural selection. We show that this conclusion is without basis. Evolutionary psychology is a series of "just-so" stories rooted in part in the erroneous notion that human beings during the Pleistocene all lived in the same environment of evolutionary adaptation. Behavioral genetics is based on a confusion of the information contained in a technical statistic called "heritability" with the colloquial meaning of the term, exacerbated by oversimplification of statistical models for the behavioral similarity of twins. In fact, information from twin studies, cross-fostering, sexual behavior, and the Human Genome Project makes it abundantly clear that most interesting aspects of the human behavioral phenome are programmed into the brain by the environment. The general confusion created by the genetic determinists has had and will continue to have unfortunate effects on public policy. Commentaries on this article and a reply are appended. (PsycINFO Database Record (c) 2004 APA, all rights reserved)

 

[Mandrake]

Mandrake:
Do you think this is the only twins study reported in the literature?

No. However, it is the most commonly discussed by the semi-literati on internet discussion boards, and hence the most relevant to analysis.

Really? "Most relevant" is determined by "the semi-literati?" I didn't know that. Thank you.

You seem to have three other studies there, not four. Perhaps you meant besides Burt?

I wrote "Besides the Burt studies, ..."

Mandrake: I just now did a search on "twin studies" within the Journal Intelligence. There were 21 hits. I looked at the first few. Each was from a different research team and each from a different country. The last one I checked studied just under 7,000 twin sets.

No one suggested N was insufficient, did they?

Gee, I guess not. Thank you for adding that profound and helpful insight. I am sure that the other participants here appreciate it as well.

The question is where the subjects come from and where they are placed, not how many there are.

The question is the value of h^2. The MZA studies confirm other computations that show the value of h^2 to be between 0.70 and 0.80. The Texas Adoption Project data have been used to calculate h^2 via path analysis. The result was 0.78. MZA studies typically show about 0.75. Burt's studies showed 0.77.
[Miele - Intelligence, Race, and Genetics, P. 103]

So, what are the countries? How did the researchers try to control for environment?

The effects of family environment have been significantly different in some specific MZA pairs. Have you read the case studies? If so, you know that there are pairs from grossly different SES environments. But more importantly, the adoption studies include data sets in which children were reared in much different family environments than their biological peers. There are data sets for children adopted into different countries; to families of higher mean IQs and to families with lower mean IQs. There are also inter-racial adoption data. These studies are extensive, thoroughly reported in the literature, and all show that the family contribution to IQ in adulthood is essentially zero.

Can you cite a study of pairs of twins who were raised in truly different environments, e.g., one set raised in Norway and their MZ twins raised in Ghana? That would tell us something.

Please see my later comments on extreme environments. Looking at such large differences certainly allows for the introduction of micro environmental components, which are well known to account for all of the environmental component of the variance in adult intelligence.

There already exist good studies to account for large differences in the shared environment. The family environments in the Minnesota study were actually measured:
Sources of human psychological differences: the Minnesota study of twins reared apart
Thomas J. Bouchard Jr.; David T. Lykken; Matthew McGue; Nancy L. Segal; Auke Tellegen
Science, Oct 12, 1990 v250 n4978 p223(6)
A checklist of available household facilities (for example, power tools, sailboat, telescope, unabridged dictionary, and original artwork) provides an index of the cultural and intellectual resources in the adoptive home [17]. Each twin completes the Moos Family Environment Scale (FES), a widely used instrument with scales describing the individual's retrospective impression of treatment and rearing provided by the adoptive parents during childhood and adolescence [18]. The age- and sex-corrected placement coefficients for these and other measures are shown in Table 3, together with the correlations between twins' IQ and the environmental measure ([r.sub.ft]) and the total estimated contribution to MZA twin similarity. The maximum contribution to MZA trait correlations that could be explained by measured similarity of the adoptive rearing environments on a single variable is about 0.03(19). The absence of any significant effect due to SES or other environmental measures on the IQ scores of these adult adopted twins is consistent with the findings of other investigators [20]. Rearing SES effects on IQ in adoption studies have been found for young children but not in adult samples [21], suggesting that although parents may be able to affect their children's rate of cognitive skill acquisition, they may have relatively little influence on the ultimate level attained. [17.] M. McGue and T. J. Bouchard, Jr., in Advances in the Psychology of Human Intelligence, R. J. Sternberg, Ed. (Erlbaum, New York, 1989), vol. 5, p. 7. This checklist yields four relatively independent scales: scientific or technical, cultural, mechanical, and material possessions.

[18.] R. H. Moos and B. S. Moos, Manual: Family Environment Scale (Consulting Psychologists Press, Palo Alto, CA, 1986).
[19.] Formally, this is the maximum linear contribution; nonlinear effects are, of course, possible. For these data, however, investigation of higher-ordered relationships (quadratic and cubic) showed no associations that did not exist at the linear level, and there was no discernible nonlinearity detected in visual inspection of the scatterplots.
[20.] T. J. Bouchard, Jr., Intelligence 7, 175 (1983).
[21.] C. Capron and M. Duyme [Nature 340, 552 (1989)] have shown an SES effect in an adoption study of young children; S. Scarr and R. Weinberg [Amer. Sociol. Rev. 43, 674 (1978)] did not find an SES effect in a study of young adult adoptees.

Else, you are simply stating the obvious: genetically identical organisms that develop in similar environments will be similar. Why is this finding noteworthy? Why do we need dozens of studies to demonstrate what is obvious?

Some people have been of the opinion that family environments, institutional environments, and other macro environmental factors could be structured to boost intelligence. After the spending of many billions of dollars on such programs as Head Start, we now know that macro environmental factors are not present in adults.

I am curious as to your take on the conclusions drawn by such respected scientists and Bouchard and Lykken. One must assume that you believe that you have a superior vantage point and better understanding of the subject than the researchers who conducted the studies. Is that correct? How is it that you gained this superior insight? The findings of the Minnesota Twins study were not shocking, since they were in agreement with similar conclusions drawn a bit earlier and from different observations. For example, see: R. Plomin and D. Daniels, Behav. Brain Sci. 10, 1 (1987); L. J. Ea ves, H. J. Eysenck, N. G. Martin, Genes Culture and Personality: An Empirical Approach (Academic Press, New York, 1989).

Your statement here makes it obvious that you didn't understand what I wrote. The problem is that each pair of twins develops within a similar environment.

Your continued assertion to this effect is at odds with the research report I cited. I have not seen a claim that IQ cannot be lowered, even significantly so, by extremes in environment. The issue is largely unknown and is usually excluded by wording such as that used by Bouchard: "... this heritability estimate should not be extrapolated to the extremes of environmental disadvantage still encountered in society." He made that statement in recognition that, for example, his data set did not contain retarded subjects.

 

[Mandrake]

continued

Waterdog: Doesn't matter whether you have twins from different countries if the members of each pair have similar environments. Then you haven't controlled for environment, even if the environment does differ between different pairs.

Your contention that the environments are "similar" is not universally true. There are significant differences in some of the pairs. One set of twins, Oskar Stohr of Germany and Jack Yufe of California were separated after their birth in Trinidad and grew up in very different cultural surroundings. Yufe was brought up a Jew by his Jewish father in Trinidad and Stohr was raised in occupied Czechoslovakia and went to a Nazi-run school.

Mandrake: We know from adoption studies, intensive intervention programs, and longitudinal studies that the shared environment vanishes by age 17. Have you seen real (as in peer reviewed) data showing that any macro environmental effect has ever caused a permanent boost in _g_? How about even a one year boost in _g_?

Waterdog: Ah, the fetish of the peer review. Do you know that many of the early behavioral geneticists, people who you now probably think of as great minds, couldn't get their stuff published because the peer reviewers for the major journals didn't like it? Peer review is a process whose goal is to produce high quality in publications, but it is far from perfect. Good articles get rejected and bad articles get accepted. Just because something is "peer reviewed" doesn't mean it's true.

Peer review is not a fetish, it is an important precaution that helps to weed out lunacy. When people discuss a topic in a forum, such as this one, there are a lot of assertions made and many claims are shown to be based on the opinions of journalists and people who have not established that they have any understanding of the subject at hand. Do you wish to argue that we are better served by material that is not peer reviewed? Peer review is not limited to the review by individual journals. In the case of psychometrics, there is peer review conducted on the Internet in a formal academic format. I have not argued that peer review inherently produces accuracy, but I do contend that it is a viable means of filtering noise.

As for boosting, "g," I doubt it's possible to raise someone's IQ test scores significantly once they reach school age. I would agree with you there.

Do you have any evidence that any macro environmental experience has been shown to boost intelligence in such a manner that the boost remains after the age of 17?

The most important environmental effects have to do with pre-natal care, followed by the development of the neural network in early childhood.

Agreed. This is well known and well documented. The intrauterine environment is totally a micro environmental factor. Essentially all of the non-genetic, non-error, parts of the variance in intelligence among adults is due to the micro environment.

They get the same nutrition and other environmental effects in the womb, that's for sure, and quite similar during the rest of early childhood. These are the crucial periods in neural development.

Jensen has commented that "most of what happens during pregnancy makes twins more different, rather than more alike, and thus causes us to underestimate the true heritability." [Miele (2002) - Intelligence, Race, and Genetics: Conversations with Arthur R. Jensen, P. 102]

Anyway, no one would deny that there is a large genetic component. How could a biological organism develop with no reference to its own DNA? What I am arguing against is the idea that it has been "proven" that this is as high as .80.

The various computations are in agreement. Path analysis and MZA data show the same results from many data sets. Heritability can be determined by the analysis of inbreeding depression and by vairous othe methods. The numbers are consistent enough that the burden of proof must lie on the person who says they are wrong.

 

[Nereid]

There are no elite competitive athletes that I am aware of who do not use performance-enhancing drugs. Most performance-enhancing drugs are not barred by sports bodies.

so what is all the fuss about re the Olympics and drugs? IIRC, there is a whole new bureaucracy devoted to testing athletes for 'banned substances' (aka 'performance enhancing drugs'), and fierce denials from those who win (e.g. that US Tour de France winner)

Caffeine is such a drug (motor time enhancing and also peak explosive strength enhancing and athletic endurance enhancing), and it is interesting enough that virtually all athletes use it to enhance performance.

But didn't I read that an Olympic athlete was stripped of his medal precisely because he tested too high on caffeine? (His excuse was that he was a caffeine addict, typically drinking 10 cups a day)

No. g tends to decline commensurately with decline of physical parameters. Some people age more slowly than others. This seems to be largely mediated by their body's relative production levels of antioxidants such as SOD and uric acid.

People can radically increase their rates of mental decline by abusing alcohol and other drugs and exposing themselves to biologically damaging levels of various chemicals. Lack of micronutrient intake, relative to that of same-age peers, will also increase rate of mental decline, as will lack of food-based antioxident (flavonoids, polyphenolics) intake, again relative to that of same-age peers.

Some adults have managed to virtually halt, relative to that of same-age peers, both their mental and physical age-related decline. These persons have adopted comprehensive anti-senescence regimens. Many of these persons refer to themselves as life extensionists.

very interesting! Thanks hitssquad.

Persons with lower IQs to begin with tend to be relatively incompetent at preserving their own health. Thus, the present author would expect that persons with lower IQs to begin with would also suffer greater age-related declines in IQ relative to those of same-age peers. See the recent paper by Gottfredson on IQ and relative hygiene competency.

so presumably a goal of an enlightened public health policy might be to retard this decline, much as (for example) public health policy might be directed to encouraging exercise, healthy diets, quitting smoking, etc?

Females are known to age more slowly than men, physically. Based on that, I would expect that female cognitive decline is generally also retarded.

What data is there to show, for example, that females or a certain (relatively advanced) age are more intelligent than males (who are otherwise equal)?

Age-related hormone decline is known to dramatically affect cognitive parameters. Hormone replacement therapy is known to have an opposite dramatic effect on cognitive recovery in older persons. This effect is mainly attributed to myelination, which is known to be largely mediated by the so-called "sex hormones".

to what extent has this been nailed down to _g_? How does the 'sex hormone'/'myelination' relationship work? To what extent has it been validated by neuroscientists? How does it affect males vs females?

There has never been a single documented case of any human reaching the age of 70 in good cognitive health. Before the recent life extension revolution, cognitive decimation on the order of several standard deviations was the norm for 70-year-olds. Persons who are relatively, compared with same-age peers, physically and mentally healthy in their senior years are relatively, compared with same-age peers, physically and mentally healthy in their senior years. Maybe you wanted to know if they were also relatively healthier than average when they were younger. Answer: generally, they were.

This seems to suggest there is some kind of relationship between general health and _g_, specifically between physical health and _g_, at least in adults. How does this relate to Jensen's findings that _g_ in adults is largely hereditary?

 

[Nereid]

That seems to be the implication of the latest information. We are at the beginning, not the end, of the resolution of how and where the brain processes thoughts.

Then equally claims which go beyond the very tentative first steps must be taken with a great many grains of salt (Rushton's spring to mind here).

Lynn has convincingly demonstrated that the mean IQ for women is 4 points below the mean for men. This difference is entirely due to group factors and as such does not conflict with Jensen's frequently reported finding that there is no difference in the mean _g_ for men and women. The primary group factors at work are presumably spatial and quantitative.

Can you say more please? What are 'group factors'? Which groups did Lynn test? When? How did he control for all the variables that we have so far uncovered?

On the face of it, Lynn's finding is in direct contradiction to hitssquad's earlier post.

Evolutionary adaptations are going to be driven by advantages in the existing environment that contribute to increased probability that the holders of the genetic allele will survive to reproduce and that their children will do the same. If the existing climate does not contribute to that result, why would you expect an adaptation? Lynn has argued that it was extreme climate that caused increased spatial performance in Mongoloids (contributing to a slight IQ advantage relative to Caucasoids). He also speculated that this spatial advantage may have come at the price of decreased verbal abilities (both differences are measurable).

Interesting. What time periods are we talking of here? what (historical) geographical regions? Why would one not expect (say) the Saami to have the same adaptations? or the native Americans and Indians who lived at extreme altitudes?

At present, the evidence points to IQ contributions in various parts of the brain, not just the prefrontal cortex. There was a strong hint of this in earlier research. That research involved the destruction of 48 locations of rat brains (there were meticulous control groups, pairs, etc.), followed by measurements of _G_ (upper case is used to designate the general factor in animals). The total findings are quite revealing and are reported on page 165 of The _g_ Factor. I am uninclined to type the whole result. Part: "Probably the most important finding is the very high correlation between the various tasks' _G_ loading and the number of brain structures that are significantly involved in the task performance -- a rank-order correlation of _.91."

So psychometricians have made significant and original advances in mammalian biology too? Do you have papers from mammalian biologists (or neuroscientists) commenting on these contributions?

Heavily loaded task = 17 brain structures
Simple task + 4 brain structures

"The _G_ factor correlated -.45 with the presence of _any_ brain lesion."

Britt Anderson determined that the _G_ factor scores for rats correlates with brain weight (they killed the unfortunate rodents) at r= +.48.

The subject of brain size has drawn a great deal of research attention for a very long time. I searched the INTELLIGENCE database and found 21 hits on "brain size." Some of the papers are very interesting, but way to long to discuss as part of this post. Since anyone seriously interested in psychometrics will have (and will have read) a copy of The _g_ Factor, they can review Jensen's comments through the entire chapter titled "Causal Hypothesis." Jensen goes through the math on page 442 to show that measurement data suggest that about 6 points of the W-B IQ gap are due to differences in brain volume. In comparing Negroid, Caucasoid, and Mongoloid means, he says "The regression of median IQ on mean cranial capacity is almost perfectly linear, with a Pearson r= +.998." After giving reasons, he goes on to say "Thus it appears that the central tendency of IQ for different populations is quite accurately predicted by the central tendency of each population's cranial capacity."

Can you confirm please that Jensen's research was done entirely with US subjects, within a narrow time period? Also, how do the regresssions look when broken into gender and age cohorts?

The same chapter includes a through discussion of the male-female difference.

And Jensen's conclusions are at odds with Lynn's?

 

[Nereid]

I assume you understand that Cattell's example was intended to be understood as a statistical and not an anecdotal observation. When Cattell stated "The strong genetic component in criminality has already been proven up to the hilt," he was not implying that he had based that conclusion on a single observation, nor even on a set of personal observations. He was telling us that the subject has been reported in many sources and that the results are in agreement. Likewise, your family example, while interesting, is anecdotal.

In which countries did Cattell do his research?

The relationship of crime to IQ is also interesting. Cattell was not commenting on this, but it is discussed in The Bell Curve, in the chapter titled "Crime." [chapter 11] The discussion points out that criminal behavior correlates negatively with IQ. It should be noted that the content of chapters 1-12 in The Bell Curve are based entirely on a single population group (non-Latino whites).

Presumably this is based entirely upon US data.

The heritability of intelligence can be firmly established without examining ANY gifted individuals. Intelligence is heritable at all IQ levels. The mechanism for describing the heritability of intelligence is to find the mean for the parents and to locate the regression point between that mean and the population group mean. That point then becomes the mean value for the normal distribution that applies to the children of the parents in question.

Unless you have oversimplified, or I have misunderstood, all this establishes is a relationship between parents and children; it doesn't begin to disentangle the many, many factors involved - genes, fetal environment, infant's and childhood experiences, ...

This field of study is particularly associated with researcher Robert Plomin, who discovered IGF2R on chromosome 6. Whether or not the specific genes have been identified, heritability of a trait can be established. The concept of heredity was known long before genetic research identified any trait specific genes.

One would have thought that something as overwhelmingly clear as the claimed _g_/hereditability relationship would have been very easily identified, at least in terms of *some* genes.

As for the heritability of intelligence, it is quantified by path analysis and by MZA studies. The data typically fall around h^2 = 72% for young adults, increasing to 80% for old adults. [These are variances, not r values.] Inbreeding depression studies show that intelligence behaves similarly to other traits that are depressed by inbreeding. The only explanation for this is a genetic cause. This one observation is so strong that it cannot be refuted by any amount of hand waving.

In which countries has this research been done? With what groups of subjects? What were the age and gender relationships (we have already seen that 'mental faculties', and IQ, decline at quite different rates for different people, with clear gender effects)? How did the researchers define 'young adult' and 'old adult'?

The other part of h^2 is the environmental component. This component has been studied in great detail for decades, including the conduct of costly and lengthy experiments. Adoption studies (including interracial ones) have shown that adopted children initially show some correlation to the mean IQs of their adoptive parents, but that this vanishes by late adolescence. As adults they resemble the IQs of their biological parents to approximately the same extent as do children who were reared by their biological parents. Adopted children have a tiny negative correlation to their adoptive siblings.

In which countries were these studies done? How did the researchers establish the 'race' of the subjects? How did they determine 'late adolescence'? How did the results vary with the age at which the children left the familial home? How did the adopted children differ from biological one in terms of the birth order _g_ effects?

Intervention programs have attempted to alter the environmental component, but they have demonstrated that such efforts are doomed to failure. The final conclusion is obvious. Intelligence is determined genetically.

In which countries were such intervention programs run? What interventions were attempted? How were the effectiveness of the interventions determined (not the outcome, but whether the intervention was, in fact, what the researchers thought)?

I don't think your comment is accurate. You have made an assertion, but it is in disagreement with reality. Although some genes do not express themselves immediately, there is a strong early indication of the correlation between IQ and performance.
The _g_ Factor: General Intelligence and Its Implications:
"IQ rises in predictive value relative to other measures as years go on" (p.77). In a long-term follow-up of a random sample of state-school five-year-olds on the Isle of Wight, IQ correlated strongly (at .50) with children's later educational attainments, when they were fifteen. Such prediction for individuals across ten supposedly formative years is unparalleled in social science."

How do these 'unparalleled' predictions compare with the decades long work of non-intelligence psychometricians? Is the *only* result of this clarity, from the whole of psychology, that of _g_ and hereditability?

 

[Nereid]

If 'matrix relations' tests have a _g_ loading of 0.94, what else is there is in these tests? In well constructed matrix relations tests, what is the typical individual variance (i.e. an individual takes the same (type of) test many times)?

This questions seems so far to have gone unanswered, so I'm posting it again.

 

[Nereid]

Related to the discussion of children, as having a different envrionmental component to the phenotype:

Genetics and intelligence: What's new?
Intelligence, Volume 24, Issue 1, January-February 1997, Pages 53-77
Robert Plomin and Stephen A. Petrill

In this paper, the shared environment variance is given as 25% in childhood and zero after adloscence.

To what extent do the researchers control for 'adolescence' (presumably related to hormones and physical growth) vs 'leaving the family' (going to live in a college dorm, getting married and moving out, gettin a job far from home, joining the army, whatever)? In which countries were these studies done?

 

[Nereid]

Intelligence appears to have at least some negative correlation with parity.

How much (range)? How extensively has this been tested?

I don't know. It is something that I have seen mentioned in various books and papers as part of the discussion of within family variance. Unfortunately, Jensen's books are rather poorly indexed (unlike The Bell Curve) so it is always difficult to find material in them.

Miller says that the levels of testosterone that children are exposed to may be related to parity. Presumably he meant the intrauterine environment.

Searching my files didn't work. The problems involve multiple meanings of the term "parity" and the difficulty in removing "disparity" from the search.

This is another of my questions that hasn't yet been answered (so I'm repeating it).

 

[Nereid]

I am a physicist with a strong interest in psychometrics. I know the people who deal with intelligence, but that's it. I am familiar with the Myers-Briggs test you listed, but it only relates to intelligence in that intelligent people tend to have a profile that is not common in the general population (INTJ). Adding INTJ and INTP, accounts for 75% of the Mensa level population (per Mensa's report).

OK. So, my understand is that:
- there is a field of scientific research called psychology
- within this, there is a discipline which uses quantitative methods to measure aspects of humans' brain capabilities (in the sense of using them, not in the sense of numbers of nerves, for example); it's called psychometrics
- within psychometrics, there is a sub-discipline called 'intelligence studies', and at least one other, to do with all other aspects of measurable brain capabilities.

Presumably, the two sub-disciplines have a strong common approach (after all, they study essentially the same thing).

So, what are the results of studies done on other (non-intelligence) psychometric variables (e.g. DISC?) and their relationship to intelligence?

What non-intelligence psychometric variables have been demonstrated to have as strong a hereditability as _g_? What 'environmental' variables have been shown to have important relationships with non-intelligence psychometric variables?

 

[Nereid]

In regards to heritability, we consider phenotypic variance accountable for on the one hand by environemental variance and on the other hand by genetic variance. If a teen "gravitates toward" specific environments, genetics would be accounting for the variance in environment and hence ultimately accounting for variance in phenotype. What is proposed is not that the teens are shifting from the family to random outside influence, but to ouside influence that caters to the teens' biological programming, of which genetics accounts for the bulk of the variance.



And what is proposed is that peer selection (among other selections by teens) is not random, but is largely a product of genetically-determined tendency. There is no accounting for taste, as they say, and it is proposed that variance in taste in environment may be more largely accountable for by variance in genetic code than by variance in environment.



Heritability refers to variance in phenotypic outcome accountable for by genetics as opposed to accountable for by environment. Environment is all factors left over when genetic code is controlled for.



Then you are mistaken, because in terms of heritability environment is any factor that is not genetic. That includes the body itself. For example, genetic code expresses throughout a person's life. But the state of the body has visible effects on genetic expression. When people reach physical maturity, their genes still express proteins that code for growth factors. Yet they do not grow any more. This is because of states in the body that prevent growth in the presence of growth factor. One of these states is the hardening of the ends of bones. If the ends of bones remained soft, as they are when we are children, growth factors might continue to stimulate bone growth indefinitely - and people would grow to towering heights throughout their lifetimes, as trees do.



Yes. This is a classic confusion of the term "environment." What normally happens in nature/nurture discussions is that the equivocation fallacy is committed in regards to the definition fo the term environment. One moment it means all environment (all variance not genetic), and the next moment it means only intellectual stimulation. Arthur Jensen suggested that when we discuss environmental variance we be clear about whether we are speaking in terms of biological variance; variance in intellectual stimulation; or all non-genetic cariance (all environmental variance).



There is no environmental counterpart to the discrete gene. In regards to heritability, environment is taken as one big lump and genetics is taken as one big lump. It stands to reason that if it has been established that genetic variance accounts for any amount of variance in a given phenotypic trait that we should also be able to find variance in specific genes that accounts for the same phenotypic trait variance. But searching for genes is not directly a part of heritability studies. Heritability of g in various human populations has already been established by selectively controlling for variance in environment and by selectively controlling for variance in genetic code.



SES is not comparable to a discrete gene. You can silence a discrete gene, but you cannot silence SES. SES is an axis. A gene is quantum piece of code.



No. The gene in question might be coding for the silencing or unsilencing of other parts of the genome. Discrete production of a protein by the IQ-related gene in question may not be necessary for that to occur.



Of course it is affected by environment. Studies of heritability have nothing to do with teasing out discrete affectors. They simply control for variance of environment and genetics and see how that results in changes in variance of expression of one or more phenotypic traits.

So here comes a biological question from a non-biologist, perhaps somewhat on a par with a physics question from a non-physicist ... a gene is what the gene is; however, the extent to which it does its thing ('is expressed'?) is partly determined by its environment - it may be 'switched on', or not; its expression may be modified by other genes being switched on or not. The triggers may be all but inevitable (e.g. pregnancy, ubiquitous chemicals), or rare or entirely absent (e.g. certain autoimmune conditions?). Tracing something to a gene (or genes) in the former case should be, in principle, fairly straight-forward; but how could you tell the latter from a non-genetic effect?

 

[Tigers2B1]

In relation to personality types (discussed above) it's my understanding that in similar situations, introverts take in more information about their environment than extroverts. So – when both pertsonality types visit a wild party, the introvert tends to become overwhelmed /exhausted with this over-stimulation before extroverts – who may even tend to be energized.

On the other extreme, extroverts will have a much more difficult time enjoying what an introvert would consider a pleasant level of stimulation. A night of star gazing, for example, might drive an extrovert into fits of head banging just to create the extra stimulation needed to make him or her feel that "something is going on." As such, each personality type selects different areas of interest – especially when they are old enough to choose those interests for themselves (i.e., when they leave the family environment and enter that unique environment they create for themselves based, in part, on their differing brands of personality.)

 

[Mandrake]

Then equally claims which go beyond the very tentative first steps must be taken with a great many grains of salt (Rushton's spring to mind here).

There is obviously a lot of study ahead. The following paper specifically addresses the sex differences:
Sex Differences in Brain Gray and White Matter in Healthy Young Adults: Correlations with Cognitive Performance
Ruben C. Gur, Bruce I. Turetsky, Mie Matsui, Michelle Yan, Warren Bilker, Paul Hughett, and Raquel E. Gur
The Journal of Neuroscience, May 15, 1999, 19(10):4065–4072

The paper shows significant brain volumetric differences between the sexes:
Females have greater GM volume; Males have greater WM and CSF volumes (see figures 2 and 3)

M: Lynn has convincingly demonstrated that the mean IQ for women is 4 points below the mean for men. This difference is entirely due to group factors and as such does not conflict with Jensen's frequently reported finding that there is no difference in the mean _g_ for men and women. The primary group factors at work are presumably spatial and quantitative.

Can you say more please? What are 'group factors'?

The hierarchial factor analysis (as developed by Charles Spearman) of test items determines correlations between test items. Subsequent extractions determine those groups that are correlated and from those the _g_ factor can be extracted. In most tests, _g_ is the third order factor. The second order factors are often called "group factors." The precise group factors that emerge (there are only 7 or 8) are dependent on the design of the test. If a test is designed so that it does not contain test items that call upon a specific group factor, it will not be seen. Most standard IQ tests cover the full range of group factors.

It seems to me that a for someone to have a realistic understanding of _g_, he must invest enough study in factor analysis that he can understand that process. Word descriptions of _g_ inevitably mislead.

Which groups did Lynn test? When? How did he control for all the variables that we have so far uncovered?

The following paper addresses his ideas thoroughly:
Sex Differences in Intelligence and Brain Size:A Developmental Theory
Richard Lynn INTELLIGENCE 27(1): 1-12

I will quote some material from it and take up the remainder of your comments in a separate message.

From the age of 16 years onwards, the growth rate of girls decelerates relative to that of boys. The effect of this is that a discernible male advantage of about 4 IQ points develops from the age of 16 into adulthood, consistent with the larger average male brain size.

Fitting American data for adults to the model, it was proposed that males have higher mean IQs than females of 1.7 IQ points for verbal comprehension, 2.1 IQ points for reasoning (the average of 1.8 for verbal reasoning and 2.4 for non-verbal reasoning) and 7.5 IQ points for spatial ability. These scores can be averaged to a 3.8 IQ point male advantage for general intelligence, which can be rounded to 4 IQ points. Similar data were presented for Britain, Norway, Sweden, Indonesia and Northern Ireland, all of which showed a male advantage of similar magnitude. In addition, six data sets of the Wechsler adult intelligence test were assembled, four of which were from the US and one each from The Netherlands and China, in all of which males obtained higher mean full scale IQs than females ranging between 1 and 5 IQ points and averaging 3.1 IQ points. The Wechsler full scale IQ is proposed as an approximate alternative measure of general intelligence defined as the sum of verbal comprehension, reasoning and spatial abilities. Thus, we have in all 12 data sets all showing higher mean IQs in males among adults.

Packenberg and Gundersen (1997) compared 62 male and 32 female brains and estimated the numbers of neurons in the entire cerebral cortex. They report that the average male brain contained 22.8 billion neurons and the average female brain 19.3 billion, a statistically significant difference, but there was no sex difference in neuronal density. The average number of female neurons in this study is 85% that of males. This is almost identical to the sex difference in cranial capacity estimated by Rushton (1992), in which average female cranial capacity is 86% of that of males. From this, we can conclude that estimates of male and female cranial capacity provide an accurate estimate of the numbers of neurons in the average male and female brain.

... males do have higher mean IQs than females by approximately 4 IQ points, commensurate with their larger average brain size. This conclusion holds whether general intelligence is defined as the sum of the verbal comprehension, reasoning and spatial group factors (Lynn), as fluid intelligence or reasoning ability (Mackintosh, Jensen), or as Spearman's g measured from the first principal component or as the global IQ obtained from standard intelligence and aptitude tests (Jensen), so long as these fulfil the conditions stipulated by Jensen.

 

[Mandrake]

part 2 for Nereid

M: Evolutionary adaptations are going to be driven by advantages in the existing environment that contribute to increased probability that the holders of the genetic allele will survive to reproduce and that their children will do the same. If the existing climate does not contribute to that result, why would you expect an adaptation? Lynn has argued that it was extreme climate that caused increased spatial performance in Mongoloids (contributing to a slight IQ advantage relative to Caucasoids). He also speculated that this spatial advantage may have come at the price of decreased verbal abilities (both differences are measurable).

N: Interesting. What time periods are we talking of here?

The Ice Age. He argues that Mongoloids were geographically isolated by terrain and effectively held captive in an extreme environment that allowed only the most able to survive. The primary survival skill was hunting, which he argues calls upon spatial abilities. He speculated that the Mongoloid brain (at least for males) may have extended its function in the spatial area to such an extent that it traded off verbal abilities.

what (historical) geographical regions?

North of the Himalayas and east of the Urals. See his paper: The Intelligence of the Mongoloids: A Psychometric, Evolutionary and Neurological Theory. Personality and Individual Differences, 1987, 8, 813-844.

Why would one not expect (say) the Saami to have the same adaptations? or the native Americans and Indians who lived at extreme altitudes?

I am unfamiliar with the isolation and climatic stresses on these groups.

Quote:
At present, the evidence points to IQ contributions in various parts of the brain, not just the prefrontal cortex. There was a strong hint of this in earlier research. That research involved the destructin of 48 locations of rat brains (there were meticulous control groups, pairs, etc.), followed by measurements of _G_ (upper case is used to designate the general factor in animals). The total findings are quite revealing and are reported on page 165 of The _g_ Factor. I am not inclined to type the whole result. Part: "Probably the most important finding is the very high correlation between the various tasks' _G_ loading and the number of brain structures that are significantly involved in the task performance -- a rank-order correlation of _.91."

So psychometricians have made significant and original advances in mammalian biology too? Do you have papers from mammalian biologists (or neuroscientists) commenting on these contributions?

I have limited interest in this area. You can find appropriate discussions here:
Anderson, B. 1993. Evidence from the rat for a general factor that underlies cognitive performance and that relates to brain size: Intelligence? Neuroscience Letters, 153, 98-102.
Crinella, F. and Yu, J. 1995. Brain mechanisms in problem solving and intelligence: A replication and extension. Intelligence, 21, 225-246.

M: In comparing Negroid, Caucasoid, and Mongoloid means, he says "The regression of median IQ on mean cranial capacity is almost perfectly linear, with a Pearson r= +.998." After giving reasons, he goes on to say "Thus it appears that the central tendency of IQ for different populations is quite accurately predicted by the central tendency of each population's cranial capacity."

N: Can you confirm please that Jensen's research was done entirely with US subjects, within a narrow time period?

The _g_ Factor is a textbook, not a research paper. Jensen presents data from many sources. The name index, for example, is 8 pages long. The material in question was drawn from multiple studies, which are all referenced as you would expect in a scientific book. It is a bit tedious for me to compile all of the references. If this subject is of interest to you, I strongly recommend obtaining a copy of this book. It is the present standard reference.

Also, how do the regresssions look when broken into gender and age cohorts?

The references I gave you for Lynn contain sex differences in brain volumes and in the contents of those volumes. As I recall, he also addresses age. There is a developmental rate difference that appears between the sexes. Salthouse has written a lot on the subject. One paper that I recently read: Structural models of the relations between age and measures of cognitive functioning Timothy A. Salthouse. Intelligence 29 (2001) 93-115

The same chapter includes a through discussion of the male-female difference.

And Jensen's conclusions are at odds with Lynn's?

Lynn disagrees with some of the explanations offered by Jensen. Those are discussed in great detail in the paper I previously referenced. Lynn has also documented the difference in knowledge between the sexes:

Sex differences in general knowledge
Richard Lynn, Paul Irwing, Thomas Cammock
Intelligence 30 (2001) 27–39

The study has four principal points of interest. First, it confirms the results obtained in the Wechsler standardisation samples shown in Table 1 that males tend to have more general knowledge than females. The magnitude of the male advantage as expressed in the sex difference on the general factor amounts to 0.51d or approximately half a standard deviation. This is a rather considerable advantage.

... the results of the sex differences on the six domains of general knowledge confirm the view that the differences between males and females in information are substantially determined by their different interests. Males have greater knowledge than females in Current Affairs (0.82d), Physical Health and Recreation (0.75d), Science (0.58d), and Arts (0.31d), but females have substantially greater knowledge than males in Family (
0.46d), while there is no sex difference in Fashion (
0.01d). These variations should probably be regarded as functions of different interests typically possessed by males and females. Considered in terms of the evolutionary psychology theory of sex differences presented by Geary (1998), these sex differences in interests are likely to be biologically programmed.

 

[hitssquad]

Content of specificity loadings and stability of Raven's Matrices

If 'matrix relations' tests have a _g_ loading of 0.94, what else is there is in these tests?

• A very few nonverbal, nonspatial visualization, and nonnumerical tests (such as Raven's Progressive Matrices and Cattell's Culture-Fair Test of g), which are based entirely on figural materials, have been expressly devised to maximize relation eduction and to minimize group factors. When factor analyzed among a wide variety of other tests, they do, in fact, have among the highest g loadings, and they usually have nonsignificant loadings on any less general factors. The Raven Matrices is sometimes moderately loaded on a broad spatial group factor, but always far less than on the g factor. Also, the Raven has some specificity (5 to 10 percent) arising most probably from the matrix format of all its items. Cattell's test largely "averages out" such specifics by including a variety of figural item types...
  
  
• It is important to note that not every test of figural material involves the spatial ability factor. Raven's Progressive Matrices, for example, does not qualify as a spatial test. This is shown by the fact that when the Raven is factor-analyzed among a variety of tests including several tests that define a spatial factor, it does not significantly load on the spatial factor. The defining characteristic for spatial problems is that, in order to obtain the correct solution, the subject must visualize and manipulate the figural material mentally as if it were an object in three-dimensional space.

(Arthur Jensen. The g Factor. pp90, 534.)

In well constructed matrix relations tests, what is the typical individual variance (i.e. an individual takes the same (type of) test many times)?

• In psychometrics this is termed the test-retest reliability...
  
  
  
• TEST- RETEST CHANGE IN SCORES
  
  When the same test, or an equivalent or parallel form of the test, is administered to persons on two separate occasions, days, weeks, or even months apart, there is usually an increase in scores, called a "practice effect." It affects performance on the particular test and on highly similar tests. [4] Subsequent retesting on the same (or similar) tests, however, shows ever smaller gains. Typically the initial gain amounts to almost one-third of a standard deviation (about three to six points on the IQ scale).
  
  The increment in scores due to the practice effect, however, has little effect on the rank order of individuals' scores. For intervals of less than one year, the test-retest correlations are generally above .90. This indicates that the test measures essentially the same factors on both occasions, despite the average increment in scores...
  
  The higher a test's g loading, the less susceptible it is to a practice effect...
  
  The inverse relationship between g loadings and practice effects, and the relative absence of g in the increments themselves, may explain the low external validity of the IQ gains that result from specific training in the various cognitive skills assumed to be measured by IQ tests. The training-induced gains in IQ scores fail to predict external criteria (e.g., scholastic achievement) to the degree that would be expected if the induced gain in IQ represented a true gain in g, rather than merely a gain in the test's specificity. This "hollow IQ" phenomenon is discussed more fully in a later section on the attempts to raise IQ by special educational interventions.

(Arthur Jensen. The g Factor. pp51, 314-316.)



Regarding in particular the test-retest reliability of the various iterations of the Raven's Matrices, these Google hits seem to provide some answers:

• ...In terms of its psychometrics, Raven's Progressive Matrices: has good test-retest
  reliability between .70 and .90 ...
  
• ...http://www.le.ac.uk/pc/jm148/IntellLecture3.html reliability –children: one week: .85, eleven years: .46 ...
  
• ...http://www.cps.nova.edu/~cpphelp/RSPM.html correlations range from a low of .46 for an eleven-year interval to a high of .97 for a two-day interval. The median test-retest value is approximately .82. Coefficients close to this median value have been obtained with time intervals of a week to several weeks, with longer intervals associated with smaller values. Raven provided test-retest coefficients for several age groups: .88 (13 yrs. plus), .93 (under 30 yrs.), .88 (30-39 yrs.), .87 (40-49 yrs.), .83 (50 yrs. and over).

 

[Mandrake]

Unless you have oversimplified, or I have misunderstood, all this establishes is a relationship between parents and children; it doesn't begin to disentangle the many, many factors involved - genes, fetal environment, infant's and childhood experiences, ...

The heritability of intelligence can be determined without knowing every detail of the mechanism. We do not, for example, know how many genes are involved. The fetal environment is not listed as part of the genotype in the material I have read. Childhood experiences consist of micro and macro types. It is the micro environment that appears to contribute somewhat to the variance in adult intelligence.

In which countries has this research been done? With what groups of subjects? What were the age and gender relationships (we have already seen that 'mental faculties', and IQ, decline at quite different rates for different people, with clear gender effects)?

These are all good questions and are obviously of great concern to you, as I can tell by the fact that you have repeated the same questions dozens of times. I am afraid that my personal time limitations are such that it is not practical for me to continue to quote specific answers to such questions. In virtually every case, you can find the answer yourself by using a good Internet search engine. If I attempt to answer from memory, I will ultimately make errors, so I suggest that you simply make an effort to locate the appropriate material and read it.

How did the researchers define 'young adult' and 'old adult'?

The effect of shared environment is usually believed to be gone by age 17. The term young adult may vary by a few years, depending on the author, but should be taken to start around 17-25. The progression of increased heritability goes on throughout life. "Old," obviously refers to people who are several decades beyond "young." Such specifics are virtually always included in the research papers. I am simply not going to continue to verify them because I don't have the time.

In which countries were these studies done? How did the researchers establish the 'race' of the subjects? How did they determine 'late adolescence'? How did the results vary with the age at which the children left the familial home? How did the adopted children differ from biological one in terms of the birth order _g_ effects?

There are numerous studies in the literature. The best way to answer these questions is to locate and read some of the material, or to read The _g_ Factor. As you know, biological studies involve multiple variables. Researchers make an effort to control for the effects that are likely to cause distortions of the effect being studied. It is common practice to discuss such controls at length. When prior investigations have shown that particular variables are second or third order effects, they may be considered to be inconsequential enough that the variable under study can still be investigated adequately.

In which countries were such intervention programs run? What interventions were attempted? How were the effectiveness of the interventions determined (not the outcome, but whether the intervention was, in fact, what the researchers thought)?

There are many intervention programs discussed in the literature. Some were conducted with some scientific care and some were done entirely for political reasons. They are discussed in some detail in The Bell Curve, Brand's The _g_ Factor: General Intelligence and Its Implications, and Jensen's The _g_ Factor. Otherwise there are many papers available. Intervention programs have not been able to produce lasting increases in measured IQ, nor have they ever been shown to have boosted _g_. Short term gains in IQ have been reported (childhood only) and are attributable to the known shared environmental factor (which vanishes) and to measurement error caused by teaching to the test.

How do these 'unparalleled' predictions compare with the decades long work of non-intelligence psychometricians?

That would be a good research project for you to undertake and to report to the rest of us. I do not know anything about non-intelligence psychometricians. In the call for papers at the upcoming ISIR conference, papers were invited on subjects pertaining to intelligence.

 

[Mandrake]

genes

One would have thought that something as overwhelmingly clear as the claimed _g_/hereditability relationship would have been very easily identified, at least in terms of *some* genes.

I think I have posted information indicating that some genes have been found that are known to influence intelligence. The majority of them are known to cause decreased intelligence.

The material I have read points to the general belief that the number of genes that affect intelligence is relatively large and that the variance attributable to most of them is on the order of 1% or less. This creates a huge barrier to locating specific genes. I am not a geneticist, but I would be surprised to know that we have identified the genes that determine the shape of the human nose. It is obvious that this shape is highly heritable, yet it consists of many dimensional parameters. Do we know how many genes are involved? Have they been identified? If we wanted this information, would it be easy to find?

 

[Mandrake]

This questions seems so far to have gone unanswered, so I'm posting it again.

I thought I addressed it. The answer is "random error."

When _g_ is measured by one test or another test, it is somewhat colored by the structure of the test. That is the nature of testing. We have always known that the correlation between tests is not 1.0. Although the Raven's set is considered to be nearly a pure measure of _g_, it may or may not yield a _g_ that is identical to the WAIS.

 

[Mandrake]

To what extent do the researchers control for 'adolescence' (presumably related to hormones and physical growth) vs 'leaving the family' (going to live in a college dorm, getting married and moving out, gettin a job far from home, joining the army, whatever)? In which countries were these studies done?

The issue is not as complicated as you wish to make it. The shared environment simply does not appear in data sets where the ages are at or above 17. In any biological data, the data produce a scatter, which means that there may be cases above and below the regression line. In this case, the regression line has a nearly zero slope.

 

[Mandrake]

This is another of my questions that hasn't yet been answered (so I'm repeating it).

Have you attempted to answer the question with the help of a search engine? It may be difficult, for the reasons I gave in my prior message. The topic is discussed in the literature, but is tedius to locate by searching. It was addressed in Storfer's book Intelligence and Giftedness, but does not appear in the index. The same is true with the other books I have. Most are not throughly indexed.

 

[Tigers2B1]

Mandrake, you ever watch TV Land? On the TV Land channel there’s a program call Columbo, which features a private detective who catches his bad guy by simply using long lists of seemingly innocent questions – Columbo always seems confused --- in need of help. Then, near the end of the show, Columbo has this Got-Ya Moment when the ‘bad guy’ slips up on an answer to one of his voluminous questions.

Well, our Nereid is forever like Columbo – except he never has that Got Ya Moment.

For those unfamiliar with Columbo (but know Nereid) – I’ve linked this site ---

http://www.tvtome.com/tvtome/servlet/ShowMainServlet/showid-1011/Columbo/

 

[Nereid]

Mandrake, you ever watch TV Land? On the TV Land channel there’s a program call Columbo, which features a private detective who catches his bad guy by simply using long lists of seemingly innocent questions – Columbo always seems confused --- in need of help. Then, near the end of the show, Columbo has this Got-Ya Moment when the ‘bad guy’ slips up on an answer to one of his voluminous questions.

Well, our Nereid is forever like Columbo – except he never has that Got Ya Moment.

For those unfamiliar with Columbo (but know Nereid) – I’ve linked this site ---

http://www.tvtome.com/tvtome/servlet/ShowMainServlet/showid-1011/Columbo/

Care to predict what the Got-Ya will be Tiger2B1?

Oh, and FYI: Nereids

 

[Nereid]

The issue is not as complicated as you wish to make it. The shared environment simply does not appear in data sets where the ages are at or above 17. In any biological data, the data produce a scatter, which means that there may be cases above and below the regression line. In this case, the regression line has a nearly zero slope.

I'm trying to understand the extent to which the terms used are clearly defined; since we're dealing with Homo sap., there are an awful lot of factors that may be relevant.

For example, if the hypothesis is something like "_g_ reaches its genetically predetermined value at the end of adolescence", then we can only satisfactorily test that hypothesis if we know what 'the end of adolescence' is. Studying how a group of individuals vary in their _g_s over time, as they transition adolescence, may be less interesting if they all reach the end of adolescence at much the same age than if they have a wide range of 'end of adolescence' ages.

BTW, don't you mean "The shared environment simply does not appear in a list of significant variables (wrt _g_) in data sets where the ages are at or above 17" (or something similar)?

 

[Tigers2B1]

Care to predict what the Got-Ya will be Tiger2B1?

I don’t know --- 12 more pages of questions??