David Reich

And what we did is we looked at maybe a hundred or so traits where there had been genome-wide association studies for all sorts of different traits, like ones associated with immunity or autoimmunity or behavior or metabolism and basically other things.

And for each of these, we could ask, are the genetic variations that are known to affect these traits from genome-wide association studies, do they have an unusual number of genetic selection signals?

And what we found is there was a vast enrichment by about a four or five-fold for immune traits.

That is, there was a super concentration of selected signals in immune traits.

Whereas also we saw a strong enrichment for metabolic traits, things that might impact your obesity or fat traits or type 2 diabetes.

And really almost no detectable enrichment, as far as we could tell, for behavioral traits or for psychiatric traits.

Yeah, that's exactly right.

So it might seem from the results of that analysis that, in fact, immune traits are highly selected and that there's been no selection for behavior in the last 18,000 years in this part of the world.

But in fact, that's a wrong conclusion.

And in fact, we have evidence that that's a wrong conclusion.

And in fact, there's clear evidence of selection also on behavioral traits.

And the reason we think we see, and we have evidence that this is so, much weaker signals for behavioral traits is that behavioral traits we know from other studies, medical studies, are underpinned by much larger numbers of genes than genes.

immune traits, which are underpinned by relatively small numbers of genes of strong effect.

Behavioral traits are shaped genetically by very large number of genes of weak effect, and we just don't have the statistical power to detect these very weak signals there.

So when we do an analysis where we look at our very strong signals of selection,

That collection of very strong results is very effectively querying the immune traits, but is not very effectively querying the behavioral traits.

It may still be the case, and I guess it is, that immune traits are the most selected category, but it is not at all the case.

And in fact, we can prove it's not the case that behavioral traits are not selected.

Interesting.

So we think there's two reasons why natural selection has—we've been able to prove, really, that there's two reasons why—how to reconcile the previous observations with our new observations.