Two papers in which Gerald Crabtree argues that human intelligence has declined since a peak thousands of years ago (Part I and Part II) have been the subject of the popular science media rounds over the last week (such as this piece in The Independent).

Crabtree’s argument has two components. The first is that intelligence is fragile and vulnerable to genetic load. He estimates that within the last three thousand years, the average person would have accumulated at least two mutations that have harmed our intelligence.

The second component is that selection pressure on intelligence eased when humans started to live in supportive societies where failures of judgement are no longer fatal. Adaptations relating to immunity would have been more important. Without this selection pressure, the negative mutations are no longer eliminated from the population.

I’m relatively sympathetic to the first limb, and the idea that thousands of genes influence intelligence (although I’m not convinced that Crabtree has put together the best case for it). For the second, however, Crabtree overestimates the existence of a social support web over recent millennia and underestimates the need for intelligence to survive in agricultural societies. I am not sure what level of social security existed in medieval Europe, but I expect it was minimal and probably little different from the web of family support received in a hunter-gatherer group. Crabtree appears to be projecting today’s supportive conditions back further than they existed.

Crabtree anticipates one response to his argument, that intelligence can still be favoured by sexual selection:

Intellectual capacity and emotional stability have mating advantages that would reduce the rate at which mutations affecting these traits become fixed in our genome. This is true, but I fear does not take into account the extreme selection required to maintain traits dependent upon thousands of genes with reduced heritability. A hunter–gatherer who did not correctly conceive a solution to providing food or shelter probably died, along with his/her progeny, whereas a modern Wall Street executive that made a similar conceptual mistake would receive a substantial bonus and be a more attractive mate. Clearly, extreme selection is a thing of the past.

The problem is that Crabtree does not see sexual selection as an “extreme” selective force, when it is. Consider Wade and Shuster’s estimate that sexual selection accounts for 55 per cent of total selection in homo sapiens. Or take Greg Clark’s data from A Farewell to Alms, with the rich having twice the children of the poor. The link between resources and reproductive success is strong across societies, and assuming a link between resources and intelligence (which if anything appears to be getting stronger), the intelligent have been reaping a reproductive bounty for some time. For those less fortunate, survival without reproduction is still a genetic dead-end.

At the opening of the first paper, Crabtree offers a wager on his hypothesis.

I would wager that if an average citizen from Athens of 1000 BC were to appear suddenly among us, he or she would be among the brightest and most intellectually alive of our colleagues and companions, with a good memory, a broad range of ideas, and a clear-sighted view of important issues. Furthermore, I would guess that he or she would be among the most emotionally stable of our friends and colleagues. I would also make this wager for the ancient inhabitants of Africa, Asia, India, or the Americas, of perhaps 2000–6000 years ago.

Crabtree does not seem to realise that this experiment has been run before. Over the last couple of hundred years, members of previously isolated hunter-gatherer tribes have been incorporated into modern societies. Those hunter-gatherers' lives of day-to-day danger, by Crabtree’s theory, should have kept them out of the downward intelligence spiral that the rest of us has been subject to. We have a reference point that we can use.

Part of the problem with Crabtree’s argument is that he has a different conception of intelligence to that in psychometry. He talks of spatial abilities, which are hard to replicate using robots, as being of a different order to playing “superficially intellectual” chess. I’d be more sympathetic to Crabtree’s argument if he limited his argument to spatial abilities, and we could look at the spatial abilities of hunter-gatherers. Different skills were almost certainly required post-agriculture. But trying to turn questions around the need for spatial abilities into an argument of general intellectual decline is stretching it too far.

Crabtree also suggests that genomic sequencing may also shed some light on this question. I agree, but I expect that the increasing evidence of accelerating adaptive evolution of humans will only grow stronger with that genomic evidence, and that intelligence is likely to be one of those selected traits.