David Reich

And we see very strong natural selection for this combination of genetic variants that predicts people's performance on IQ tests and also is highly correlated to the predictor that predicts the number of years of school or the household wealth of people.

All crazy traits in the past because there was no wealth in the past.

There was no school in the past.

But if you look at the predictors today, there is a strong...

there is a strong movement in a systematic direction, a large effect about a standard deviation on the scale of modern variation.

And then we can do this trick of looking to see whether there's periods of time when this natural selection has occurred more intensely or less intensely,

What we do is we drag a 2,000 year window through our data and we repeat our whole analysis, not on 18,000 years, but just on a short 2,000 year window.

And we can measure the strength of selection in each of these 2,000 year windows.

And what you see when you look at intelligence is you see that this maxes out

in the Bronze Age, between 5,000, 4,000, 3,000, 2,000 years ago.

And the impact in the last 2,000 years is almost nothing.

There's no evidence of natural selection at all.

You might think your bias coming into this, my bias perhaps, if there's any signal of natural selection on this trade at all, might be that it would be unusually strong in the last 2,000 years.

Maybe this is a time of

industrialization.

Maybe this is a time of greater need for this particular trait.

But in fact, there's no evidence of natural selection at all in the last 2,000 years, but there's very strong evidence in between 2,000 and 4,000 years ago, where instead of a one standard deviation strength of selection, it's a two standard deviation strength sort of averaged over this time period.

How much the polygenic score trait moves over a 10,000 year period within a population that is held constant in terms of its ancestry.

Because what's actually we're doing is we're looking in our data set at a kind of heterogeneous group of people.

There's, you know, Southern Europeans and Northern Europeans and hunter-gatherers and farmers.