We're Probably Not Getting Dumber

There's been buzz over the past few days in the idea that humans have been getting stupider for the past few millennia. That's according to Stanford's Gerald R. Crabtree -

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.

OK. How so?

Crabtree's argument in a nutshell:

Part I,

he outlines the latest evidence showing that many thousands of genes contribute to human cognitive ability, and that because mutation rates are high (higher than previously believed), any given individual probably carries harmful variants of many of these genes. This is quite possibly true ,and interesting, but by itself it's nothing to do with declining IQ.

In Part II, Crabtree says that during human evolution, all of these genes were under strong selection pressure because any human or proto-human who wasn't smart enough to hunt, fight and survive in the stone age environment, would get eaten by a predator or starve. However, after these hunter-gatherer tribes became settled farming communities (in say 6000 BC), they were no longer so vulnerable, so the less intellectually able could live... and breed... leading to ever-more unintelligence genes.

Now, there's a lot of problems here. Many have said that this is all a bit like eugenics, and indeed it is, but that doesn't necessary mean it's wrong... no, it's wrong because the argument is flawed.

For instance, it's already been pointed out that, even if it recently got easier to stay alive, that doesn't mean it's got easier to get laid lots and have lots of kids; and sexual selection is a powerful force in evolution, perhaps even stronger than survival, and it probably favours higher intelligence.

However, there are other issues.

The idea that hunter-gatherers have especially hard lives is dubious. There's good evidence that life expectancy and health fell when hunter-gatherer societies settled down and got agriculture. Today, survival doesn't exert much of a selective pressure in most parts of the world but life stayed pretty precarious (by modern standards) until at least the 19th century. You could indeed argue that

8,000-odd years of agriculture made us smarter than ever before, and that we're enjoying the benefits .

In which case we'd be smarter than someone from 1000 BC, who only had 5,000 years or so.

Crabtree does acknowledge that agriculture changed selection pressures. He suggests that it would have made it more important to be immune to the various diseases that emerged when our population density rose. But this isn't enough -

for his argument to work, intelligence would also have needed to become less important with agriculture,

and whether that's true is really not clear at all. Maybe it did. Maybe it didn't. There's a prejudice in modern culture against 'ignorant peasants' and 'dumb hicks', but farming is not easy.

Even if we do grant that cognitive evolutionary pressures have eased since 1000 BC, it's not clear that this would make us 'less intelligent'. 'Intelligence' is not one thing. To simplify, it might be that there's a trade-off between 'book smarts' and 'street smarts', and that you used to need the latter to survive.

A society in which everyone survives would then allow more people the luxury of being book-smart.

Would Einstein or Newton would have made good peasants?

Crabtree's arguments are interesting, but they're entirely speculative. There's just no hard evidence for the decline of intelligence over recent millennia. Since we can't go back in time and do IQ tests, there never will be, although he does suggest an experiment, using genetics, that might be able to check whether there's been a build-up of harmful mutations.

But until then, as he puts it,