The Fourth Law of Behavior Genetics
Complex traits are shaped by many genes of small effect
This is the fourth part of my five-part series on the Four Laws of Behavior Genetics and why they matter. You can access the full collection here.
In the first three parts of the series, we looked at the first Three Laws of Behavior Genetics. In this part, we’ll look at the Fourth Law, which - as you’ll see - is a very different kettle of fish. As with the earlier parts, we’ll start with an overview for everyone, followed by a deep dive for paid subscribers. Enjoy!
The Fourth Law Joins the Party
In the beginning, there were only three: the Three Laws of Behavior Genetics, as articulated by the behavior geneticist Erik Turkheimer. Here’s a quick recap.
The First Law of Behavior Genetics states that all psychological traits are partially heritable, including even traits as weird and wonderful as political attitudes and mobile phone use.
The Second Law of Behavior Genetics states that the effect of sharing a family home is smaller than the effect of genes - and that sometimes there’s no effect at all.
The Third Law of Behavior Genetics states that a lot of the variance among individuals in psychological traits isn’t due to either genes or the shared family home. It’s due to… something else. What that something else is isn’t clear, but much of it may be random developmental noise.
In this post, we’ll shift gears and look at the Fourth Law of Behavior Genetics, which was posited fifteen years after the first three by the psychologist Chris Chabris and colleagues. Whereas Turkheimer’s Laws are based on the findings of twin and adoption studies, the Fourth Law is based on recent advances in molecular genetics: the field that aims to map specific genetic variants onto phenotypic outcomes. This is the Fourth Law:
Most complex traits - including most psychological traits - are shaped by vast numbers of genetic variants, each of which has only a very small effect.
For example, there are no common variants that increase or decrease IQ by, say, 2 points. But there are hundreds or even thousands of variants that increase or decrease IQ by a tiny fraction of a point.
As we’ll soon see, the Fourth Law of Behavior Genetics has important implications for our understanding of the nature and nurture of the human mind. It solves one of the great mysteries of behavior genetics: the missing heritability problem. It implies that a major area of research within molecular genetics is essentially worthless, and its findings largely false. And it points the way to critical breakthroughs in future research - breakthroughs that could greatly enhance the quality of human life.