12 Things Everyone Should Know About Behavior Genetics
They're some of the most important findings in psychology - yet many psychologists know little about them
“The discovery of such big and often counterintuitive findings is a cause for celebration in psychology.”
Welcome to the latest installment of my “12 Things Everyone Should Know” series! If you’ve missed the earlier entries, you can find the full collection here.
Behavior genetics is one of psychology’s great success stories. While many other areas have struggled to replicate their core findings, behavior genetics has consistently produced the goods: a stash of highly replicable results bearing on one of the most important questions in intellectual history: the question of nature and nurture.
Despite its success and significance, however, behavior genetics is often given short shrift in psychology. It’s easy to get a psychology degree without learning much about the field - and even many professional psychologists know little about this cornerstone of their discipline. Worse than that, many still operate on an implicit “blank slate” model of the human mind, neglecting genetic influences altogether.
What explains psychologists’ failure to get to grips with behavior genetics? A big part of it, I suspect, is that many are still squeamish about genetic explanations for the mind and behavior. The aim of this post, therefore, is to cut through the discomfort, and spotlight some of the most important findings in the area.
So, without further ado, here are 12 things everyone should know about behavior genetics.
1. All traits are partially heritable
This surprising observation goes by the name the First Law of Behavior Genetics. All traits - not just some or many, but all - are partially heritable. What this means is that differences among individuals in every measured trait are correlated to a significant degree with differences among them in their genes. This is true of physical traits like height, weight, and BMI; it’s true of psychological traits like intelligence, personality, and self-control; and it’s even true of weird stuff like political attitudes, mobile phone use, and time spent on social media (see here for an overview). The most important evidence for the First Law is the fact that identical twins are more similar than non-identical twins on all traits, even when they grow up apart. The graph below, created by Spencer Greenberg, shows the heritability estimates for a selection of physical and psychological traits.
2. No trait is 100% heritable.
Although all traits are shaped in part by genes, none is shaped by genes alone. If any trait were shaped by genes alone, identical twins would be literally identical for that trait. They never are, though; they’re surprisingly similar, but not identical. Heritability estimates always fall short of 100% - and usually considerably so. We call the field behavior genetics, but we could equally well call it behavior environmentalism, because its findings testify to the importance of the environment just as much as the importance of genes. Indeed, the field provides some of the strongest evidence available that the environment is crucial in shaping individual differences.