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A Very Little Bit of Knowledge: Re-Evaluating the Meaning of the Heritability of IQ

Richardson and Norgate have argued that two unwarranted assumptions underlie many of the conclusions drawn by adoption study researchers, namely that genetic and environmental influences on phenotypic variation are additive, and that adoption studies approximate a simple randomized-effects design in which genetic and environmental influences on variance do not contaminate one another. As they note, this issue is important because the primary conclusions of such studies – that genetic variation accounts for a significant proportion of variation in psychological characteristics such as IQ – have been widely accepted [Maccoby, 2001; Nuffield Council on Bioethics, 2002; Turkheimer, 2005]. To address the inappropriateness of the assumption of additivity, Richardson and Norgate refer to several works that support the ascendant view that biological and psychological characteristics are constructed during development, when genes interact with local environmental factors that can be influenced by the broader environment. However, it is worth noting that the literature in developmental biology is replete with findings that support this view, so much so that the earlier view that some genes are able to independently cause or contribute to developmental outcomes is effectively indefensible [Coen, 1999; Edelman, 1988; Keller, 2000; Gottlieb, 1998; Jablonka & Lamb, 2005; Lewontin, 2000; Lickliter & Honeycutt, 2003; Michel & Moore, 1995; Moore, 2002; Robert, 2004; Schaffner, 1998]. Because gene-environment interactions are now understood to drive the development of all of our characteristics, the assumption of additivity is never warranted [Nijhout, 2002; Robert, 2000]. Likewise, after reviewing the relevant data, Richardson and Norgate conclude that the assumption that adopted children’s genes are uncorrelated with their adoptive developmental environments is probably unwarranted in many cases as well.