Turkheimer's Projects: Genetics and Human Agency | Jayman on the Three Laws of Behavior Genetics

Posted at 16:29h in Uncategorized by Eric Turkheimer 1 Comment

Charles Murray links to an article about the three laws of behavior genetics, recently updated by others to five.  

As is often the case when people of a hereditarian persuasion write about the three laws, Jayman gets it almost entirely wrong, reacting only to the headline, “Everything is Heritable” and ignoring the great majority of the paper that completely undercuts his conclusions. The paper is actually titled, “Three Laws of Behavior Genetics And What They Mean”, and it is in fact much more about the what-they-mean part than about the laws themselves. And what they mean is: Behavioral differences are in a weak sense heritable, but that doesn’t mean that behavior in general has a genetic explanation. Quite the opposite.

At the beginning of the post, Jayman has the chutzpah to suggest that I have “since spent [my] time trying to undermine [my] own discovery.” Actually, it’s worse than Jayman suggests: I spent most of the paper itself undermining the interpretations he wants to assign to it. That’s pretty bad: I misunderstood the conclusions of my own paper!  You can read the paper yourself, it is linked here.  But since Jayman doesn’t show any indication that he actually took the time to read it, let me summarize for you.

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

The first law states that all behavioral differences are heritable, in what I had earlier called the “weak” sense: all this means is that on average, people who start out more similar genetically wind up more similar phenotypically, for anything and everything. There is that paper going around about how dog ownership is heritable. Of course it is. This doesn’t mean that there are genes for dog ownership, it doesn’t mean there is anything resembling a genetic mechanism determining dog ownership, it doesn’t mean that we have reduced free will regarding dog ownership. If you happen to be conducting studies of parent-child similarity for dog ownership, you need to take it into account; otherwise, it is as close as you can come to being consequence-free.

The second law refers mostly to the results of twin studies, in which the effect of the shared family environment often appear to be small. There are many reasons why this is so, but none of them add up to, “families don’t matter.” In fact the paper concluded that most of the difference between genes and family environment in twin studies is methodological. DNA is conveniently discrete and relatively static; environments are continuous and changeable. Behavior genetics was greatly forwarded by the existence of twins, with their simple accounting of rMZ=1.0, rDZ=.5. There is no equivalent in the environment. I proposed a little thought experiment:

How similar was my rearing environment to that of my siblings? And how similar was it to the environment of my adopted sibling, if I have one, or to the environment of my biological sibling who was raised by someone else? The apparent victory of nature over nurture suggested by the first two laws is thus seen to be more methodological than substantive. In a world in which there were occasional occurrences of “identical environmental twins,” whose experiences were exactly the same, moment by moment, and another variety who shared exactly (but randomly) 50% of their experiences, environmentalists could reproduce the precision of their rivals,and like the behavior geneticists could measure with great precision the total contribution of the environment while knowing almost nothing about the developmental processes that underlie it.

The third law says that most human behavior winds up in what is called the nonshared environment: unpredictable, even in twins. As it has turned out, the third law has continued to be true, even in the era of measured DNA and GWAS. The best polygenic scores for behavior account for no more than 10-15% of the variance. Elsewhere, I argue that the nonshared environment also turns out to be the location of free will, which Jayman contends (ridiculously) is undermined by the existence of heritability.

As for the fourth and fifth laws. The fourth law, which says that genetic effects are broken up into the minute effects of many genes, is certainly true. I think I probably disagree with the authors of that paper about the implications of that idea, but they basically get it right.

I had never heard about the fifth law until I read this post. It says that associations between phenotypes are confounded by genetic associations.  The author of the blogpost making this point, by the way, manages not to credit me for the first three laws. And although I am out of the law-numbering racket,  the presumably new one is a dumbing-down of what I have called “the phenotypic null hypothesis”: the multivariate structure of genetic variance is usually the same as the structure of the phenotypic variance. This is another one that hereditarians interpret as an indication of genetic primacy, but as usual they get it exactly backwards. If people who own dogs are more likely to spend time in parks, and it turns out there is a genetic correlation between dog-ownership and park-time, it is not because “the same genes” cause dog-ownership and park-time: it is because phenotypic dog-ownership causes park-time, and whatever genes are associated with either go along for the ride. That’s a complicated story which I’ll have to explain another time.

Scroll to Top