Genetics and Human Agency: The Philosophy of Behavior Genetics Introduction to the Special Issue

Abstract

Behavior genetics has a fraught history with philosophy. In the early days of the field it seemed as though the only philosophical discussion that was possible was between the hereditarian descendants of Galton–Cyril Burt, Raymond Cattell, Hans Eysenck and Arthur Jensenand their critics from the intellectual left: Richard Lewontin, Leon Kamin and Steven J. Gould. Many of the participants on both sides, of course, were neither geneticists nor philosophers, but they nevertheless set the tone of the debate: the discussion about the genetics of behavior was a clash between a harsh and often racialized determinism and a politically motivated commitment to individual freedom and progressive social values. For at least a century, unsurprisingly, it was a standoff. But that familiar version of the nature-nurture debate overlooked a simple empirical fact and a deep theoretical gap. The empirical fact was that by any plausible empirical standard, individual differences in human behavior are heritable. Identical twins are more similar than fraternal twins, siblings more similar than half-siblings, adopted children more similar to their biological than their adoptive parents. When it became possible to confirm these observations in unrelated people using measured DNA, the results were confirmed: individuals with more similar DNA were more similar in their behavior, for every human difference that could be measured. The theoretical gap is harder to state in a few sentences but just as important. Heritability is about populations and their variances; behavioral science ultimately is about individual people, and why they behave and experience the world as they do. If the heritability of differences in extraversion is equal to 0.4, what are the implications for how individual people develop their personalities? The simple hereditarian answer is that extraversion is in some (unspecified) sense fixed at conception. By extension, our psychological intuition that we make developmental choices about our behavioral characteristics under the influence of our families and the local and cultural environments is false, a psychological illusion. But if our behavioral differences are indeed the result of distal genetic differences, how exactly does that happen? The big biogenetic explanation that has always been promised about the “gene to behavior pathway” linking DNA to complex psychological experience has remained forever on the theoretical and empirical horizon. Variance partitioning has never been enough to convince the experiencing human that personality is predetermined by genes, and it never will be. Now that the twin and adoption studies are done, and furthermore now that GWAS has been conducted on millions of individuals, what has behavior genetics elucidated about extraversion? It is, undeniably and replicably, heritable, but what else? We know about some variations on heritability: how it changes with age or gender, or across social classes, but those findings tend to be more about heritability itself, rather than being about extraversion per se, where it comes from and how it develops. The knowledge gap begins in developmental biology (and many of the most trenchant critics of behavior genetics have been biologists, from Richard Lewontin to Gilbert Gottlieb to Denis Noble to Kenneth Weiss), but it is also psychological, experiential, and ultimately philosophical. Questions of how biological givens about humans taken as a group (leading to evolutionary questions about human nature) or as individuals (leading to questions of self-determination and personal responsibility) underlie many if not most of the deep philosophical questions about human development. In Turkheimer (1998) I cited a remark by Toulmin to the effect that psychiatry was then the major contemporary arena of applied philosophy, and predicted that moving forward the major theoretical questions would involve * Eric Turkheimer [email protected]