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Three Laws of Behavior Genetics and What They Mean

Eric Turkheimer
Three Laws of Behavior Genetics and What They Mean
Current Directions in Psychological Science October 2000 vol. 9 no. 5 160-164
(read this paper)

Behavior genetics has demonstrated that genetic variance is an important component of variation for all behavioral outcomes, but variation among families is not. These results have led some critics of behavior genetics to conclude that heritability is so ubiquitous as to have few consequences for scientific understanding of development, while some behavior genetic partisans have concluded that family environment is not an important cause of developmental outcomes. Both views are incorrect. Genotype is in fact a more systematic source of variability than environment, but for reasons that are methodological rather than substantive. Development is fundamentally nonlinear, interactive, and difficult to control experimentally. Twin studies offer a useful methodological shortcut, but do not show that genes are more fundamental than environments.

The author proposes three laws of behavioral genetics which summarize results across a wide range of behaviors:

  1. All human behavioral traits are heritable.
  2. The effect of being raised in the same family is smaller than the effect of genes.
  3. A substantial portion of the variation in complex human behavioral traits is not accounted for by the effects of genes or families.

These results are widely accepted in the specialty of behavioral genetics, but the implications are unclear and perhaps troubling within the broader context of human self-understanding. See Human Nature, Human Diversity.

It's my perception that this paper is part of Eric Turkheimer's effort to get behavior genetics unstuck from the politicized nature/nurture morass. He proposes that these laws are well established, and that we should move on. Unfortunately, this has proved difficult. Outside behavior genetics, these laws are not only questioned, they are frequently dismissed as either:

  • having been refuted long ago, or as
  • being so morally appalling in their implications that they must not be considered.

And when we go beyond the three laws, it only gets worse for the theory that differences in ability are primarily caused by potentially controllable factors such as poverty and education. In particular, there is the reliable failure of attempts to pin down the important causes underlying the unexplained part of individual difference. This is the other_stuff term in the in the equation:

behavior = genes + family + other_stuff

This cause of individual difference is usually called non-shared environment. In popular writeups, non-shared environment is most often presented as “good teacher/bad teacher”, but the many quantitative studies of teacher performance tend to show only small effects. See Education and Social Structure. There is reason to suspect that a lot of the residue may be non-deterministic (random) developmental variation, see Nature, Nurture and Noise. Are random differences “innate”, whatever that means? At least the dreaded determinism is avoided, (both genetic determinism and social determinism).

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