Three Laws of Behaviour Genetics and What They Mean

Behavior genetics has demonstrated that genetic variance is an important component of variation for all behavioral out comes, 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 de velopment, while some behavior genetic partisans have concluded that family environ ment 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.