The First Law of Behavior Genetics

All psychological traits are partially heritable

This is the first part of a five-part series about the Four Laws of Behavior Genetics and why they matter. I’ll be releasing one new part a month. In this first part, I’ll give an overview of the Four Laws, then do a deep dive into the First Law. The overview is free; the deep dive is for paid subscribers. Check out the other benefits of a paid subscription here.

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The Four Laws of Behavior Genetics

Some of the most exciting questions in science are questions of origin.

For cosmologists, the great origin questions are: How did the universe begin? And how did galaxies, stars, and planets form?

For biologists, the great origin questions are: How did life emerge from non-life (abiogenesis)? And how do adaptations and new species come into existence?

For psychologists, on the other hand, the great origin questions are: What makes us who we are? Where did the mind come from? And why is my mind different from your mind?

Behavior genetics is the field that deals with the last question: the origins of individual differences. More precisely, it’s the field that asks: To what extent are individual differences due to nature and to what extent are they due to nurture?

If you haven’t already heard of behavior genetics, you’ve almost certainly heard of its main tools: twin studies and adoption studies.

Using these tools and others like them, behavior geneticists have made considerable headway in solving the ancient nature-nurture riddle. Indeed, at the turn of the century, the behavior geneticist Eric Turkheimer announced that “The nature-nurture debate is over.”

The most important outcomes of the debate are captured in the Four Laws of Behavior Genetics: four well-supported generalizations that have emerged from more than half-a-century of research in this fascinating field.

In the beginning, there were just Three Laws. These were proposed by Turkheimer in the same paper in which he announced the end of the nature-nurture debate. (And it’s only fair to mention that Turkheimer doesn’t agree with the way that many have interpreted the laws, which he thinks overstates and misconstrues the role of nature.) The Fourth Law was added 15 years later in a paper by the psychologist Chris Chabris and colleagues.

Here are the Four Laws, along with some representative evidence for each of them.

1^st Law: All psychological traits are partially heritable

Representative evidence: Identical twins reared apart are more similar than fraternal twins reared apart, who in turn are more similar than non-relatives reared apart.

2^nd Law: The effect of being raised in the same family is smaller than the effect of genes

Representative evidence: Identical twins reared together little or no more similar than identical twins reared apart, at least by adulthood. Likewise, adopted siblings reared together aren’t much more similar than non-relatives reared apart.

3^rd Law: A lot of the differences between people in psychological traits aren’t attributable to either genes or the family environment

Representative evidence: Identical twins reared together aren’t identical, despite sharing all their genes and their family environment.

4^th Law: Most complex traits are shaped by many genes of small effect

Representative finding: There are no common gene variants that increase or decrease IQ by, say, 5 points. However, there are hundreds or even thousands of gene variants that increase or decrease IQ by a tiny fraction of a point.

Each of these generalizations is supported by a large mountain of studies. Indeed, the Four Laws are among the best-replicated findings in psychology.

For that reason, every psychologist, and anyone wishing to understand the human mind and behavior should have a good working knowledge of the laws. The aim of this series is to provide that. Let’s go through each law in turn, then, starting - naturally enough - with the first law.