The idea that intelligence can differ between populations has made headlines again, but the rules of evolution make it implausible

The idea that there may be genetic differences in intelligence between one population and another has resurfaced recently, notably in the form of a New York Times op-ed by the Harvard geneticist David Reich. In the article, Reich emphasises the arbitrary nature of traditional racial groupings, but still argues that long periods of ancestry on separate continents have left their genetic marks on modern populations. These are most evident for physical traits like skin and hair colour, where genetic causation is entirely uncontroversial. However, Reich asserts that all genetic traits, including those that affect behaviour and cognition, are expected to differ between populations or races.

This extrapolation from the genetics of physical traits to how our brains work brings back memories of an argument made by the US researchers Charles Murray and Richard J Herrnstein in their 1994 book The Bell Curve, recently resurrected by Murray in conversations with the US neuroscientist and author Sam Harris. In the book, Murray and Herrnstein claim that observed differences in the mean IQ scores of ethnic groups are “highly likely” to be due to both environmental and genetic factors. This sounds quite reasonable at first: the argument concedes that environmental and cultural factors play a big part in any differences seen in the mean IQ scores of various groups. But it also suggests that since genetic variation will contribute to higher or lower IQ in any given population, the genetic differences between one group and another will also underpin mean differences in IQ.

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