Heritability, heterogeneity, and group differences

Idea: As conventionally interpreted, heritability indicates the fraction of variation in a trait associated with “genetic differences.” A high value indicates a strong genetic contribution to the trait and “makes the trait a potentially worthwhile candidate for molecular research” that might identify the specific genetic factors involved. I contest the conventional interpretation and contend that there is nothing reliable that anyone can do on the basis of estimates of heritability for human traits. While some have moved their focus to cases in which measurable genetic and environmental factors are involved, others see the need to bring genetics into the explanation of differences among the averages for groups, especially racial groups.

a. Heritability & critique
Heritability is a quantity derived from analysis of variation in traits of humans, other animals, or plants in ways that take account of the genealogical relatedness of the individuals whose traits are observed. Such “quantitative genetic” analysis does not require any knowledge of the genes or “measurable genetic factors” involved.
Turkheimer is “on the left” of behavioral genetics, being much less gung ho about the implications of its findings. Here he gives a clear overview of what the field has shown.
Plomin articulates the confident consensus of behavior genetics, namely, that they’ve debunked the supposed environmentalist orthodoxy in social science that says that everything is social and have established a basis for connecting with molecular genetics to identify the actual genetic factors.
Rutter, a senior psychological researcher (who once worked with Brown on social determinants of mental illness), tries to moderate the “polarizing claims” and “unwarranted extrapolations.”
Taylor 2010 casts doubt on the findings that underlie both Turkheimer and Plomin’s articles by exposing problems with the concepts and methods used to arrive at those findings. Taylor ends with a nudge towards methods that use measured genetic factors as well as measured environmental factors (the latter being the staple of social epidemiology).

b. Interaction of measured genes and measured environments
Moffitt 2005 provides a review of what’s involved in trying to identify interactions between measured genetic and environmental factors. (Use Taylor 2010 to get clear about the difference between this kind of interaction and the classical genotype x environment interaction in quantitative genetics.) Caspi 2002 is one of two 2002 papers that caused a lot of splash. Davey-Smith picks up on the current consensus that the 2002 studies have been hard to replicate and invokes Mendelian randomization as a way to strengthen causal inference about interactions between measured genetic and environmental factors.

c. Data & models about heritability & change (or lack of it)
Dickens 2001 provides a resolution of the paradox that heritability of IQ test scores is reported to be high, but there has been a large increase in average IQ test scores from one generation to the next. We know that genes haven’t changed from one generation to the next, so Dickens’ account is also exposing a flaw in the logic that because heritability of IQ test scores is high within racially defined groups and because there is a large difference in average IQ test scores between whites and blacks, genetic factors are probably involved in that difference.
Rushton 2005 however thinks that 30 years of research has validated that idea.
Taylor 2010 refers to Dickens 2001, but gives a somewhat different spin on its implications.

(This post continues a series laying out a sequence of basic ideas in thinking like epidemiologists, especially epidemiologists who pay attention to possible social influences on the development and unequal distribution of diseases and behaviors in populations [see first post in series and contribute to open-source curriculum http://bit.ly/EpiContribute].)


Caspi, A., J. McClay, et al. (2002). “Role of Genotype in the Cycle of Violence in Maltreated Children.” Science 297(5582): 851-854.
Davey-Smith, G. (2009). “Mendelian randomization for strengthening causal inference in observational studies: Application to gene by environment interaction.” Perspectives on Psychological Science, in press.
Dickens, W. T. and J. R. Flynn (2001). “Heritability estimates versus large environmental effects: The IQ paradox resolved.” Psychological Review 108(2): 346-369.
Moffitt, T. E., A. Caspi, et al. (2005). “Strategy for investigating interactions between measured genes and measured environments.” Archives of General Psychiatry 62(5): 473-481.
Plomin, R. and K. Asbury (2006). “Nature and Nurture: Genetic and Environmental Influences on Behavior.” The Annals of the American Academy of Political and Social Science 600(1): 86-98.
Rushton, J. P. and A. R. Jensen (2005). “Thirty years of research on race differences in cognitive ability.” Psychology, Public Policy, and Law 11: 235-294.
Rutter, M. (2002). “Nature, nurture, and development: From evangelism through science toward policy and practice.” Child Development 73(1): 1-21.
Taylor, P. J. (2010). “Three puzzles and eight gaps: What heritability studies and critical commentaries have not paid enough attention to.” Biology & Philosophy, 25:1-31. (DOI 10.1007/s10539-009-9174-x).
Turkheimer, E. (2000). “Three laws of behavior genetics and what they mean.” Current Directions in Psychological Science 9(5): 160-164.


2 thoughts on “Heritability, heterogeneity, and group differences

  1. Chuck

    “Dickens 2001 provides a resolution of the paradox that heritability of IQ test scores is reported to be high, but there has been a large increase in average IQ test scores from one generation to the next. We know that genes haven’t changed from one generation to the next, so Dickens’ account is also exposing a flaw in the logic that because heritability of IQ .”

    The IQ paradox does not expose “the flaw in the logic.” The “flaw in the logic” has always been recognized. Let me quote from Jensen (1974): “Such (GE) interactions with respect to human intelligence, and particularly genetic racial differences in intelligence, cannot be ruled out on the basis of present evidence… (Educability and group differences, p. 174).” What the IQ paradox suggest to you is that there are significant, yet undetected, GE interactions with respect to IQ. What the IQ paradox suggests to me is that there is something yet unknown about IQ. To what degree there are such interactions is an empirical question, as always has been recognized. It is not a question “resolved” by showing that a non-conventional model of heritability is possible or, as Jensen (1974) put it, citing examples of GE interactions from “plant and animal breeding experiments.” More generally, to what degree heritability studies are useful for directing genetic research is also an empirical, or meta-empirical, question. In the next few years we will see if genomic studies turns up anything with respect to IQ — currently, all Jensenite eyes are on that large study going on at BGI. It may indeed turn out that heritability studies are the alchemy, as you put it, of the day — but we will know shortly.

    Now, what about empirical evidence for interactionism? As for significant GE interactions, one might cite Tucker Drob (2011), who didn’t show the type of GE interaction that you need (i.e different racial genotypes reacting to different norms), but something more akin to a threshold effect. On the other hand, one could cite Jinks and Fulker (1970), Jensen (1970), Finkel and Pedersen (2001), and van Leeuwen (2008) all which were unable to detect statistically significant GE interactions (1). Moreover, when it comes to the heritability of general intelligence, as opposed to just intelligence, one could cite Posthuma et al. (2003), De Moor et al. (2008), van Leeuwen et al (2009), and Betjeman (2009) as evidence against socially significant GE interactions.

    That last point is worth considering. While the IQ paradox may or may not suggest something about intelligence, to date it suggest nothing about (g) per se. In fact there really is a double part IQ paradox: How can we make sense of the generational rise in IQ? Given the generational rise, and apparent fluidity, in IQ differences, how can we make sense in the robustly biological, substantially genetic, and apparently fixed (at least by adulthood), nature of average individual differences in g (4), given that IQ and GQ are closely related. To the extent that the former can be said to show the flaw in conventional thinking about IQ, that latter can be said to show the flaw in contemporary interactionist thinking about GQ; together they suggest something unknown about the IQ-GQ relation. When it comes to group differences (in the US), this is important, since these differences are g-loaded. To put it another way, the IQ paradox, to date, leaves,as best I can see, the two part probabilistic GQ argument intact.

    Of course, it’s possible that the genetic component of g is fluid, something that can be readily be acquired (neoeugenics aside), but I have yet to find a shred of evidence for this, indirect or otherwise.

    1. Tucker-Drob, 2011. Emergence of a Gene× Socioeconomic Status Interaction on Infant Mental Ability Between 10 Months and 2 Years
    2. van Leeuwen, 2008. A twin-family study of general IQ
    3. Betjemann, et al., 2009. Genetic Covariation Between Brain Volumes and IQ,
    4. Shikishima, et al., 2009. Is g an entity? A Japanese twin study using syllogisms and intelligence tests

  2. Chuck

    I would note that the evidence for substantial subpopulation differences in GQ, or lack there of, is equivocal. The best way to resolve this would be to directly test the hereditarian hypothesis. Rowe (2005) and Hunt and Carlson (2007) outlined some simple, definitive ways of doing this. This whole, distasteful for some, issue could be resolved in a matter of months. To me, given that this question is still unresolved and that numerous people have been spilling quite a bit of ink on it lately, it’s suggestive to me that such studies have not been conducted.

    Rowe, 2005. Under the Skin: On the Impartial Treatment of Genetic and Environmental Hypotheses of Racial Differences* 1

    Hunt and Carlson, 2007. Considerations relating to the study of group differences in intelligence


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