The Heritability of IQ Increases With Age
A counterintuitive but highly replicable finding
The graph below shows one of the most counterintuitive discoveries of behavior genetics: The heritability of IQ increases with age, while the effect of the shared family environment dwindles. In other words, when it comes to cognitive ability, nature becomes more important as we get older, nurture less.
Why does the heritability of IQ increase with age? It’s not completely known. One possibility, however, is that as people get older, they gain more and more control over their environments. They therefore begin selecting, modifying, and creating environments that align with their preferences and talents - preferences and talents shaped to a significant degree by their genes. This creates a feedback loop, where the environment increasingly reinforces genetically influenced traits.
The graph comes from a classic paper by Robert Plomin and colleagues titled “Top 10 Replicated Findings From Behavioral Genetics.” Here’s the abstract:
In the context of current concerns about replication in psychological science, we describe 10 findings from behavioral genetic research that have replicated robustly. These are “big” findings, both in terms of effect size and potential impact on psychological science, such as linearly increasing heritability of intelligence from infancy (20%) through adulthood (60%). Four of our top 10 findings involve the environment, discoveries that could have been found only with genetically sensitive research designs. We also consider reasons specific to behavioral genetics that might explain why these findings replicate.
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The rise of heritable IQ with age makes sense when we consider the constraints posed by family and siblings. These no longer serve a purpose. I wonder if the rise of heritable IQ correlates with loss of family members. Due to a genetic disease I lost 2 brothers in early adult years and I expanded into "their" family territory. I stepped up and out. I also see this as a desire to keep their spirit alive, but I cultivated competencies that felt familiar but had been dormant in me.