Every now and then, I notice someone, often an anthropologist, saying that human cognitive capability just has to be the same in all populations. According to Loring Brace, “Human cognitive capacity , founded on the ability to learn a language, is of equal survival value to all human groups, and consequently there is no valid reason to expect that there should be average differences in intellectual ability among living human populations. ”

There are a lot of ideas and assumptions in that quote, and as far as I can tell, all of them are wrong. First, you really need to note that populations today sure look as if they differ in average intellectual ability. They vary a lot in measured IQ: almost three standard deviations from lowest to highest. Some pairs of populations show big differences in scholastic results, and interventions to the tune of tens of billions of dollars haven’t had much effect.

Populations vary tremendously in the fraction that contributes original work in science and technology – and that variation mostly agrees with the distribution of IQ. Which is what you would expect, really – the fraction that exceeds a high threshold drops rapidly as the population mean decreases.

Nobody knows exactly what drove the evolution of human intelligence. That includes Loring Brace. In particular, nobody knows that it was just one factor, and certainly nobody knows that it was just one factor that was effectively uniform worldwide. Mind you, even that wouldn’t be enough. Fitness counts both costs and and benefits. A big brain is costly – it uses up a lot of calories and greatly complicates birth. For the net selective effect – the sum of payoffs and costs – to be almost exactly the same everywhere, both payoff and costs would have to be constant. In principle they could both vary in a way that left their sum always the same, but that’s too silly to even talk about…

And even that might not be enough, since the course of evolution depends on the existing genetic background, not just the local selective pressures. People outside sub-Saharan Africa picked up noticeable amounts of archaic human ancestry: maybe that changed the adaptive landscape. People inside sub-Saharan Africa didn’t pick up Neanderthal genes, but they apparently picked up a few from other, unknown archaic hominids.

I mentioned earlier that intelligence is correlated with brain size – and that it pretty much has to be, if our ideas about natural selection are correct. In Brace’s world, average brain size would have to be almost exactly the same in every population. It’s not.

Human populations been living in significantly different environments for 70,000 years (maybe more), with very little gene flow until quite recently. On top of that, populations have experienced wild changes over the last 10,000 years – agriculture, civilization, and all that. Some populations changed more than others during the Neolithic, and some hardly at all.

Surely most anthropologists agree with Loring Brace, one way or another. Most don’t know much about psychometrics, or genetics, or natural selection. Mainly, they know what they want to hear.

I was wondering what it would take to get me to swallow this nonsense. First thought was four Black Russians over an hour or so, but then I remembered the time I re-derived stationary phase on a napkin while in that state.

More like six, if I really want to think like an anthropologist.