The Nature and Nurture of Self-Control
Self-control is around 60% heritable
The graph below compares levels of self-control for identical twins (monozygotic or MZ) and non-identical twins (dizygotic or DZ). Each point represents either the MZ or DZ twins from one study, with the studies arranged by sample size (x-axis). The higher the correlation (y-axis), the greater the similarity in self-control.
As you can see, on average, the identical twins are more similar in self-control than the non-identical twins, indicating that genes play a considerable role in shaping this important trait. More precisely, self-control is around 60% heritable - that is, genes explain roughly 60% of the differences between individuals in their levels of self-control.
Notice that the larger the sample size, the clearer the trend becomes. This is a nice illustration of the fact that larger samples give more accurate, reliable results.
The graph comes from the preprint of a now-published paper by Yayouk Willems and colleagues titled “The heritability of self-control: A meta-analysis.” The published version features a different graph that obscures the identical vs. non-identical twin comparison, which is why I went with the graph from the preprint. The meta-analysis included 31 studies and more than 30,000 twins in total.
Two other findings from the paper are noteworthy. First, as with many traits, the shared family environment had no effect on self-control - in other words, twins who grew up in the same home were no more similar in self-control than those who grew up apart. This doesn’t mean that self-control is all down to genes; we’ve already seen that genes account for only 60% of the variance in self-control. What it does mean, though, is that whatever the environmental contributors to self-control turn out to be, the things that people share if they grow up together aren’t major players. The finding also suggests that the environment mainly makes people who grow up together different rather than similar.
Second, the heritability of self-control was the same for men and women. As Willems and co-authors note,
monozygotic and dizygotic twin correlations did not differ for males and females, indicating no gender differences in the heritability of self-control. This is consistent with earlier research illustrating that gender differences in heritability for a broad range of behavioral, psychiatric, and health related phenotypes are rare.
To be clear, this doesn’t mean that men and women are identical in self-control or any other trait, or that any differences between them are purely environmental. It just means that the balance between genes and non-genetic factors in shaping men and women’s (somewhat different) levels of self-control is the same. (See this previous post for an exception to the rule that heritability is the same for both sexes.)
The paper is open access, so you can read it for free here. You can also check out the preprint here.
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Related Reading
On self-control, check out this post from the Archive (no paywall):
Self-Control as a Performance-Enhancing Drug
Just a quick note today to share with you one of my all-time favorite graphs. It’s from a classic paper by the psychologist Terrie Moffit and colleagues, and it shows how self-control measured in childhood correlates with a range of important adult life outcomes. Long story short, self-control is an all-purpose good like IQ: It predicts health, wealth, …
On behavior genetics, check out my five-part series on the Four Laws of Behavior Genetics and why they matter. You can access all the posts to date here.
The First Law of Behavior Genetics
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.
Finally, on sex differences in heritability, see this earlier post (no paywall):
Sex Differences in the Heritability of Autism
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Since you have talked a lot about behavioral genetics, twin studies, and the heritability of psychological traits, I would be very interested in your opinion on the "missing heritability problem," which I was introduced to in particular by Alexander Gusev in his highly interesting and complex Substack essays:
- https://theinfinitesimal.substack.com/p/twin-heritability-models-can-tell
- https://theinfinitesimal.substack.com/p/no-intelligence-is-not-like-height
- https://theinfinitesimal.substack.com/p/some-notes-on-assortative-mating
I think he really challenges everything we know from twin studies except that traits are heritable.