Are heritable individual differences just genetic noise? What the architecture of quantitative traits says about their evolution

The evolution of heritable individual differences (for example in personality, cognition, and the risk for psychopathology) is the subject of a long-running debate between proponents of adaptive and non-adaptive explanations. Newly available genomic data show that most quantitative traits conform to what I label the “default genetic architecture,” characterized by extreme polygenicity with contributions from both common and rare variants, with large-effect variants that tend to be rarer and younger than small-effect ones. Furthermore, targeted tests of balancing selection return largely null or negative results. These findings indicate widespread purifying selection at the genetic level; they have led some scholars to argue that heritable individual differences are essentially non-adaptive or maladaptive, and that evolutionary hypotheses that invoke balancing selection are inconsistent with the data. Here I show that this strong interpretation is not warranted. I distinguish between four questions about the evolution of heritable individual differences, and explain why the data do not support sweeping inferences about their adaptive function (or lack thereof). I also discuss why tests of balancing selection are much less informative than is often believed. While the pervasive role of purifying selection is beyond dispute, the default architecture of complex traits is potentially compatible with a broad range of evolutionary scenarios, including scenarios in which heritable individual differences can be adaptive and functional rather than just manifestations of neutral/maladaptive noise.