The Three (Now Four) Laws of Behavioral Genetics

Erik Turkheimer put forward the first three in 2000, based on work he had been doing for a over a decade before that.


1. All human behavioral traits are heritable. [That is, they are affected to some degree by genetic variation.]
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.

A fourth has been added more recently, which I hear Turkheimer has agreed with.
4.    A typical human behavioral trait is associated with very many genetic variants, each of which accounts for a very small percentage of the behavioral variability. That is, in the same way that height is polygenic, founded on thousands of SNPs, each of which only contributes a very small increase in height, so too are behavioral traits, like aggressiveness, IQ, or Educational Attainment. While their are individual Mendelian traits that cause cause things to go all broken and make your impulsiveness extreme or cut your intelligence severely, there aren't any Smart Genes or Athletic Genes that make you much better all by themselves.

Regarding #3, there is a good deal of talk about what that substantial portion consists of. If it is not shared in families, and doesn't seem to be measurable in a neighborhood, or a town, or a school system, then what is it? Is it hidden genetics?  Epigenetics?  Current thinking is that it is about half prenatal, and half a large collection of experiences that differ child to child that call out different responses from the genes. Both of those could be called epigenetic depending on definition. Pregnancies are different in the same woman, and the fetus itself may be calling out for different responses from Mom. Brothers will have different friends, different teachers and coaches, read different books, see different movies and a thousand other things that have unpredictable effects.

I think a good deal of it is going to be more formally genetic myself.  But we shall see.