Top 10 Most Replicable Findings from Behavior Genetics, Part 1: Findings #1-5
The nature of nature and nurture
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Psychology has been having something of an identity crisis lately.
Over the past few decades, a growing number of classic findings have failed to replicate. Indeed, one of the most replicable findings in psychology, ironically enough, is that only around 50% of psychology findings are replicable.
Psychology isn’t unique in this respect; similar concerns have been raised in many other fields. But psychology is a particularly popular field, and one with clear implications for policy, so its potential to mislead is great.
That being the case, it’s important to ask: Which findings in psychology do replicate?
One area with a particularly good track record is behavior genetics, which studies the nature and nurture of individual differences in mind and behavior. In a classic paper drawing on decades of twin, adoption, and genomic research, Robert Plomin and colleagues highlighted ten findings from the field that have proven remarkably robust. These aren’t trivial or esoteric findings. They’re large and often surprising, and have far‑reaching implications for our understanding of psychology.
In this post, I’ll outline the first five of Plomin and co.’s Top 10 most replicable findings from behavior genetics, covering everything from selectively breeding the minds of other animals to the increasing importance of genes with age. I’ll tackle findings #6-10 next weekend.
1. All Psychological Traits Show Genetic Influence
The first finding is sometimes known as the First Law of Behavior Genetics: Individual differences in all psychological traits are shaped in part by individual differences in genes.
This is true of intelligence, personality, mental illness, substance use, political attitudes, religiosity - even food preferences and social media habits. Across thousands of studies, identical twins are consistently more similar than fraternal twins on these traits, and adopted children are more similar to their biological parents than their adoptive ones. Twin and adoption studies have very different underlying logics, yet they converge on the same conclusion: Genes matter.
And often they matter a lot. For intelligence, heritability averages around 50% across diverse countries and cultures. For personality, it usually falls between 30% and 50%. For mental-health problems such as schizophrenia and autism, it clocks in at 80% or more: on a par with the heritability of height.
To be clear, heritability isn’t static; it varies from place to place and time to time. But we’ve yet to find a place or time where it’s negligible for any major psychological trait.
The First Law of Behavior Genetics wasn’t always widely accepted. Indeed, in some fields, such as education, it’s still not. However, within psychology and allied disciplines, it’s now recognized as one of the best-established findings we have.
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2. No Psychological Traits Are 100% Heritable
The first finding emphasizes genes; the second restores balance to the force. Genes matter a lot, but so do non-genetic influences. No psychological trait is 100% heritable, and most are nowhere near it.
This isn’t due to limitations in the methods of behavior genetics. Some physical traits have extremely high heritability levels - up to 90% for height, for example. Thus, the methods could give higher heritability estimates if heritability were actually higher.
Admittedly, psychological measures are usually noisier than physical ones, which depresses heritability estimates to some degree. Even so, the estimates are far enough away from 100% that improved measurement is unlikely to bring them anywhere close to that ceiling.
This finding matters more than it might first appear. It’s natural to assume that behavior genetics tells us about genes rather than the environment. In fact, though, it tells us about both. More than that, behavior genetics provides the strongest evidence we have that the environment plays a crucial role in shaping individual differences. That’s because the methods of behavior genetics control for the effects of genes, whereas other methods don’t. Given how substantial genetic effects are, the failure of most socialization research to do this is a profound oversight. Ironically, then, the clearest demonstration of the importance of the environment comes from a field often assumed to downplay it.