Why Humans Stopped Evolving Smarter 2,000 Years Ago - David Reich
David Reich discusses genetic evidence showing that natural selection for cognitive performance peaked during the Bronze Age (2,000–4,000 years ago) and has effectively ceased in the last 2,000 years. Contrary to intuitive expectations, the industrialization era shows no detectable selective pressure on intelligence-related genetic variants. The strength of selection during the Bronze Age period was notably strong at two standard deviations.
Summary
In this excerpt, geneticist David Reich discusses findings related to the evolutionary history of cognitive performance as measured through genetic variants associated with intelligence test scores in white British populations. He acknowledges the methodological strangeness of projecting a modern construct like intelligence testing onto ancient populations, but argues that these genetic variants still serve as meaningful proxies for cognitive traits across time.
Reich presents a surprising finding: natural selection acting on these cognitive-performance-associated genetic variants appears to have peaked during the Bronze Age, roughly 2,000 to 4,000 years ago, and shows virtually no signal in the last 2,000 years. This runs counter to the intuitive expectation — which Reich admits was his own prior bias — that the recent era of industrialization and increasing cognitive demands on society would show the strongest selection pressure on intelligence-related traits.
Instead, the data reveals that the Bronze Age period exhibited exceptionally strong natural selection on these variants, measured at approximately two standard deviations in strength on average across the period, compared to a baseline of one standard deviation. This suggests that whatever environmental or social pressures were driving cognitive selection were more intense in prehistoric and early historic times than in the modern era, challenging assumptions about the relationship between societal complexity and cognitive evolution.
Key Insights
- Reich finds that genetic variants associated with cognitive performance in white British people today peaked in their selective pressure during the Bronze Age, roughly 2,000 to 4,000 years ago, and show almost no change in the last 2,000 years.
- Reich acknowledges that measuring intelligence-related genetic traits in ancient populations is methodologically unusual, since intelligence tests and formal schooling did not exist, yet argues the genetic variants still have predictive value.
- Reich admits his own prior bias was that natural selection on cognitive traits would be strongest in the last 2,000 years due to industrialization and increased cognitive demands — but the data directly contradicts this expectation.
- The strength of natural selection on cognitive-performance variants between 2,000 and 4,000 years ago was approximately two standard deviations, roughly double the baseline one standard deviation strength used for comparison.
- There is no detectable evidence of natural selection acting on intelligence-associated genetic variants in the last 2,000 years, suggesting that modern societal complexity and industrialization did not meaningfully drive cognitive evolution.
Topics
Transcript
[0:00] If you look at genetic variance that affect measures of cognitive performance such as performance on intelligence tests in white British people today, this is of course a very strange trait to measure in the past because there were no intelligence tests and there was no school. But it is a predictor today and you could look at how it's changed in the past. 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 [0:31] think your bias coming into this,…
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