Are Humans Getting Dumber?

Stanford geneticist Gerald Crabtree recently theorized that, due to that the diminishing impact of natural and sexual selection in procreation, human intelligence peaked some 2,000 to 6,000 years ago, when thousands of genes that determine smarts began to mutate. He argues:

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.

Pointing to the steady rise in average IQ over the last 100 years, known as the Flynn effect, Andrew Brown considers IQ research on people who most resemble our ancestors:

[T]he kind of reasoning that an IQ test measures is quite finely adapted to the modern, technological world in which most people who take one have grown up. It rests on assumptions that simply don't hold in different societies. Flynn illustrates this with studies conducted on Siberian hunter-gatherers in the 1930s. Their answers, apparently stupid, were highly acute once you realised that they did not trust the questioners at all. In their society, suspicion was not in the least bit stupid as a general rule, no matter how much it might fail to provide useful answers in this particular instance.

It well may be that contemporary adults are hopeless stupid and inadequate by the standards of hunter-gatherer life and that adult hunter-gatherers would die quickly in a modern city, too. But neither of these tests are measures of innate intelligence, as Flynn makes clear.

Tia Ghose summarizes Crabtree's counter-argument to the Flynn effect: that most of the increase in IQ "probably resulted from better prenatal care, better nutrition and reduced exposure to brain-stunting chemicals such as lead":

But just because humans have more mutations in their intelligence genes doesn't mean we are becoming less brainy as a species, said psychologist Thomas Hills of the University of Warwick, who was not involved in the study. Instead, removing the pressure for everyone to be a superb hunter or gatherer may have allowed us to evolve a more diverse population with different types of smarts, he said.

"You don't get Stephen Hawking 200,000 years ago. He just doesn't exist," Hills told LiveScience. "But now we have people of his intellectual capacity doing things and making insights that we would never have achieved in our environment of evolutionary adaptation."

George Dvorsky pushes back on a more general point:

[Crabtree] completely understates the importance of sexual selection — an ongoing process that most certainly has an impact on our ongoing genetic constitutions. In his study, Crabtree writes that modern Wall Street executives only have to worry about receiving a substantial bonus in order to attract a mate. "Clearly," writes Crabtree, "extreme selection is a thing of the past."

But what Crabtree is grossly under-appreciating is the degree to which intelligence brings couples together in modern society. His Wall Street executive wouldn't be a Wall Street executive without a requisite level of intelligence. The same goes for anyone else with a complex and modern job. And without the ability to survive and thrive in today's highly competitive environments, it's very unlikely that anyone would be capable of attracting a mate.