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.