Author’s note: I revisited this topic in 2008 and 2018 and concluded that it’s unscientific to portray race as a causal factor in aptitudes.

We hold these truths to be self-evident, that all men are created equal, that they are endowed by their Creator with certain unalienable Rights … —Declaration of Independence

Last month, James Watson, the legendary biologist, was condemned and forced into retirement after claiming that African intelligence wasn’t “the same as ours.” “Racist, vicious and unsupported by science,” said the Federation of American Scientists. “Utterly unsupported by scientific evidence,” declared the U.S. government’s supervisor of genetic research. The New York Times told readers that when Watson implied “that black Africans are less intelligent than whites, he hadn’t a scientific leg to stand on.”

I wish these assurances were true. They aren’t. Tests do show an IQ deficit, not just for Africans relative to Europeans, but for Europeans relative to Asians. Economic and cultural theories have failed to explain most of the pattern, and there’s strong preliminary evidence that part of it is genetic. It’s time to prepare for the possibility that equality of intelligence, in the sense of racial averages on tests, will turn out not to be true.

If this suggestion makes you angry—if you find the idea of genetic racial advantages outrageous, socially corrosive, and unthinkable—you’re not the first to feel that way. Many Christians are going through a similar struggle over evolution. Their faith in human dignity rests on a literal belief in Genesis. To them, evolution isn’t just another fact; it’s a threat to their whole value system. As William Jennings Bryan put it during the Scopes trial, evolution meant elevating “supposedly superior intellects,” “eliminating the weak,” “paralyzing the hope of reform,” jeopardizing “the doctrine of brotherhood,” and undermining “the sympathetic activities of a civilized society.”

The same values—equality, hope, and brotherhood—are under scientific threat today. But this time, the threat is racial genetics, and the people struggling with it are liberals.

Evolution forced Christians to bend or break. They could insist on the Bible’s literal truth and deny the facts, as Bryan did. Or they could seek a subtler account of creation and human dignity. Today, the dilemma is yours. You can try to reconcile evidence of racial differences with a more sophisticated understanding of equality and opportunity. Or you can fight the evidence and hope it doesn’t break your faith.

I’m for reconciliation. Later this week, I’ll make that case. But if you choose to fight the evidence, here’s what you’re up against. Among white Americans, the average IQ, as of a decade or so ago, was 103. Among Asian-Americans, it was 106. Among Jewish Americans, it was 113. Among Latino Americans, it was 89. Among African-Americans, it was 85. Around the world, studies find the same general pattern: whites 100, East Asians 106, sub-Sarahan Africans 70. One IQ table shows 113 in Hong Kong, 110 in Japan, and 100 in Britain. White populations in Australia, Canada, Europe, New Zealand, South Africa, and the United States score closer to one another than to the worldwide black average. It’s been that way for at least a century.

Remember, these are averages, and all groups overlap. You can’t deduce an individual’s intelligence from her ethnicity. The only thing you can reasonably infer is that anyone who presumes to rate your IQ based on the color of your skin is probably dumber than you are.

So, what should we make of the difference in averages?

We don’t like to think IQ is mostly inherited. But we’ve all known families who are smarter than others. Twin and sibling studies, which can sort genetic from environmental factors, suggest more than half the variation in IQ scores is genetic. A task force report from the American Psychological Association indicates it might be even higher. The report doesn’t conclude that genes explain racial gaps in IQ. But the tests on which racial gaps are biggest happen to be the tests on which genes, as measured by comparative sibling performance, exert the biggest influence.

How could genes cause an IQ advantage? The simplest pathway is head size. I thought head measurement had been discredited as Eurocentric pseudoscience. I was wrong. In fact, it’s been bolstered by MRI. On average, Asian-American kids have bigger brains than white American kids, who in turn have bigger brains than black American kids. This is true even though the order of body size and weight runs in the other direction. The pattern holds true throughout the world and persists at death, as measured by brain weight.

According to twin studies, 50 percent to 90 percent of variation in head size and brain volume is genetic. And when it comes to IQ, size matters. The old science of head measurements found a 20 percent correlation of head size with IQ. The new science of MRI finds at least a 40 percent correlation of brain size with IQ. One analysis calculates that brain size could easily account for five points of the black-white IQ gap.

I know, it sounds crazy. But if you approach the data from other directions, you get the same results. The more black and white scores differ on a test, the more performance on that test correlates with head size and “g,” a measure of the test’s emphasis on general intelligence. You can debate the reality of g, but you can’t debate the reality of head size. And when you compare black and white kids who score the same on IQ tests, their average difference in head circumference is zero.

Scientists have already identified genes that influence brain size and vary by continent. Whether these play a role in racial IQ gaps, nobody knows. But we should welcome this research, because any genetic hypothesis about intelligence ought to be clarified and tested.

Critics think IQ tests are relative—i.e., they measure fitness for success in our society, not in other societies. “In a hunter-gatherer society, IQ will still be important, but if a hunter cannot shoot straight, IQ will not bring food to the table,” argues psychologist Robert Sternberg. “In a warrior society … physical prowess may be equally necessary to stay alive.” It’s a good point, but it bolsters the case for a genetic theory. Nature isn’t stupid. If Africans, Asians, and Europeans evolved different genes, the reason is that their respective genes were suited to their respective environments.

In fact, there’s a mountain of evidence that differential evolution has left each population with a balance of traits that could be advantageous or disadvantageous, depending on circumstances. The list of differences is long and intricate. On average, compared with whites, blacks mature more quickly in the womb, are born earlier, and develop teeth, strength, and dexterity earlier. They sit, crawl, walk, and dress themselves earlier. They reach sexual maturity faster, and they have better eyesight. On each of these measures, East Asians lag whites and blacks. In exchange, East Asians get longer lives and bigger brains.

How this happened isn’t clear. Everyone agrees that the three populations separated 40,000 to 100,000 years ago. Even critics of racial IQ genetics accept the idea that through natural selection, environmental differences may have caused abilities such as distance running to become more common in some populations than in others. Possibly, genes for cognitive complexity became so crucial in some places that nature favored them over genes for developmental speed and vision. If so, fitness for today’s world is mostly dumb luck. If we lived in a savannah, kids programmed to mature slowly and grow big brains would be toast. Instead, we live in a world of zoos, supermarkets, pediatricians, pharmaceuticals, and information technology. Genetic advantages, in other words, are culturally created.

Not that that’s much consolation if you’re stuck in the 21st century with a low IQ. Tomorrow we’ll look at some of the arguments against the genetic theory.