David Reich

walking pace, performance and IQ test, household wealth, all these crazy traits all seem to be governed to a substantial extent by a shared combination of genetic variants.

And let's just think about what this might mean.

So in Iceland in the last hundred years, there's been selection against this combination of variants.

And one possible interpretation is it's basically selection for two ways of investing in your children, having many kids and not investing a lot in them, or having few kids and investing more in them.

Right.

So if you invest in deferring, deferring having kids, but becoming, you know, having more wealth, having more resources and putting more into each kid, you're going to have a lower fertility and you're going to have fewer kids.

And that's going to result in lower fertility.

But those kids might survive more and do better in society.

Alternatively, you can just have as many kids as you can and invest less in them.

They might have individually less good outcomes, but in a time of plenty, which is potentially Iceland in the 20th century, it might make sense to have more kids and invest less in them.

And so there's a toggle between having more kids and investing less in them and having more kids and investing less in one's life and having fewer kids and investing more in excelling in various ways or something like this.

And so you can imagine that actually at different times and in different places in ecology, there's resource, there's different ways like mammals often invest a lot.

with a pregnancy and a small number of children, whereas fish will spawn huge numbers of offspring into the river, the great majority of whom will be eaten, but that is an effective way to produce offspring in certain conditions.

So there'll be a toggle, depending on the environmental conditions, back and forth between investing in large numbers of offspring with fewer and less investment or smaller numbers of offspring with more investment.

And maybe we're just seeing that move back and forth over different places and times.

Similarly, for schizophrenia and bipolar disease, how could this ever be advantageous?

But maybe what we're seeing with these diseases is a kind of readout of some kind of

spectrum of traits that actually, in some contexts, might be advantageous.

Maybe being anxious or being imaginative or being neurotic might be helpful in a shamanistic tradition, you know, in a religious tradition which values people who can have visions or values people who can be creative.

And maybe these are