The Second Law of Behavior Genetics

Genes have more impact than the shared family environment

This is the second part of my five-part series on the Four Laws of Behavior Genetics. You can access the full collection here.

In part one, we looked at the First Law: the finding that all psychological traits are (partially) heritable. In this part, we’ll look at the Second Law: the discovery that, when it comes to individual differences, genes have more impact than the family home - and in fact, that the family home sometimes has close to zero impact.

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The Boys From Brazil

In Ira Levin’s book The Boys from Brazil, and the later movie adaptation, the fugitive Nazi doctor Josef Mengele and his evil henchmen decided to recreate Hitler after his death at the end of World War II.

The conspirators had a two-step plan to accomplish their nefarious goal.

Step One: They took a strand of Hitler’s DNA, and used it to create 94 Hitler clones.

Step Two: They placed the Hitler clones with families that resembled Hitler’s family – families with an abusive father, for instance, and a doting mother. Then, when the clones turned 13, the conspirators started murdering the fathers, to replicate the death of Hitler’s father when he was 13.

In concocting their evil plan, Mengele and his henchmen were making three main assumptions about the origins of individual differences:

1. Genes matter a lot.

2. Genes aren’t all that matters.

3. The family home matters a lot as well.

In the first post in this series, we reviewed evidence suggesting that the first two of these claims are correct. Specifically, we saw that around half the variation in most psychological traits is due to variation in genes. That tells us that genes matter a lot - but also that non-genetic factors matter too.

At this point, almost everyone jumps to the same conclusion: that the primary non-genetic contributors are parents and the early home environment. In other words, the family home matters a lot, and thus all three of Mengele’s assumptions are true.

This view is widely held and entirely plausible… but it’s also entirely false. That, at any rate, is what I’m going to argue in this post. When it comes to shaping who we are, the family home matters considerably less than do genes - an observation that the behavior geneticist Eric Turkheimer dubbed the Second Law of Behavior Genetics. And not only does the family home matter less, but for some traits, it doesn’t seem to matter at all.

This is a deeply counterintuitive conclusion - and one that poses a serious challenge to some of the most famous theories in psychology.