[EDIT: Researching immediately after I wrote this article, I skimmed—I’ll have to find time to read it later—Turkheimer’s paper Still Missing (PDF, 2011), where he walks back his original assertion.]
The question, “why are children in the same family so different?” is answered, “Because measurable differences in their environment make them that way.”
I finished Pinker’s The Blank Slate the other day, but I didn’t have much time to capture my reflections. I’m already onto my next book, Mill’s Utilitarianism, so I figured I record some thoughts before they become too distant.
In chapter 19, Pinker summarises Eric Turkheimer’s paper Three Laws of Genetic Behaviour Genetics and What They Mean (PDF, 2000, dead link):
- The First Law: All human behavioural traits are heritable.
- The Second Law: The effect of being raised in the same family is smaller than the effect of the genes.
- The Third Law: A substantial portion of the variation in complex human behavioural traits is not accounted for by the effects of genes or families.
Elaborating on this, he summarises Turkheimer’s assessment on the variance in personality contributable to three factors: genetics, society, and family. According to this theory, variations in personality due accounted for by environment, are composed as
50 percent accounted for by society (non-shared environment); 40 – 50 percent accounted for by genetics (biology) and 0 to 10 percent attributable to family (shared environment). In his assessment, he was able to eliminate usual-suspect factors like birth order and siblings.
“Genotype is in fact a more systematic
source of variability than environment.”
According to Pinker, the 0 to 10 percent is generous, and it could just as well be 50% society and the rest is genetics. That doesn’t leave a lot of room for psychoanalysis, a discipline who strongly pushes back on this concept—just with emotional appeal in lieu of science.
Philosophics 
One thought on “3 Laws of Behaviour Genetics”