David Reich

subclinical versions of schizophrenia or bipolar disease that in certain times may be advantageous and in other times may be disadvantageous.

Maybe you're just seeing selection from different types of creativity or other thinking that can be valuable in different contexts.

I'm waving my hands here, but my sense is that these complex traits

have not pushed in one direction because there are spectrums where there's advantages to both ends of the spectrum and there's multi-dimensional impacts of these different traits.

You're being asked to believe in visions.

You know, and even today, I think, you know, there's valuation in some religious communities and, you know, communicating with God and having visions and having supernatural communions.

And so I just don't know.

But I think it's super interesting to ask the question why certain traits are not always advantageous.

For schizophrenia and bipolar disease, there is a sense in which most of the mutations are disadvantageous.

We can see that from the patterns of variation where the variants that are risk factors tend to be low frequency and they tend to be small effects.

So this is what you see as a reduction in the combination of genetic mutations that make you at risk for obesity, body mass index.

And similarly, and very correlated to it, higher fat mass, higher waist to hip ratio, higher type 2 diabetes risk.

And so...

There is clear selection by about a standard deviation on the scale of modern variation for these traits, reducing about 10,000 over the last 10,000 years in this part of the world.

So what can be going on there?

Why was there not selection for this combination of traits before?

There's a longstanding idea known as the thrifty genes hypothesis.

The idea is that once you have hunter-gatherer populations that move into a farming environment where there's plentiful food, there is no longer a need to the same extent to be able to build up body fat to sort of survive in times of stress because there's more constant stores of food.

And so as a result, there will be natural selection against body fat, which can be once you move into an agricultural environment and to periods of food plenty.

And so maybe what you're seeing is that this...