The moral of our authors’ story is this: The races are different in ways that matter because, given what we know—or think we know—about population genetics and human history, they must be. Science predicts that troublesome traits should exist, and they do. We now need to ask two questions about this theory: “Is it correct?” and “Does it matter?”

Are Cochran and Harpending correct? They may be. The theory of population genetics is sound. If you believe Homo sapiens is an animal species like any other, then you must believe that it obeys the laws of population genetics. One of these laws is that populations under sufficiently different selection pressures will diverge genetically. So if, as Cochran and Harpending argue, different human populations have existed under very different selection pressures, it seems that they must have diverged enough to produce the troublesome traits suggested by standardized tests and other data.

The validity of this thesis depends on three kinds of evidence. The first is historical evidence of selection pressures sufficiently powerful to result in species divergence. Cochran and Harpending say that there were two such episodes: The Neanderthal introgression 40,000 years ago and the advent of agriculture 10,000 years ago. From an evidentiary point of view, we don’t know if the former happened at all, but we do know that the latter did. In other words, one scenario is speculation, the other is established fact. I am not qualified to evaluate Cochran and Harpending’s arguments for the Neanderthal introgression. All I can say is that it seems possible, it would have altered selection pressures if it occurred, and most experts doubt that it did. The theory is a clever way to explain the mysterious transition from primitivism to behavioral modernity, but being clever isn’t proof of anything.

I can say a bit more about the advent of agriculture. Cochran and Harpending are on solid ground when they claim that this new mode of production and everything it brought with it—namely, civilization—changed selection pressures in a whole host of ways. Humans living by agriculture need to do many things that hunter-gatherers don’t: They have to eat a lot of food that isn’t meat and isn’t tasty or terribly nutritious; they have to work very hard for that food; they have to stay in one place for a long time, and this place is mostly populated by strangers; they have to live in a hierarchal society with extensive division of labor; and they have to get used to living according to rules and laws, some quite restrictive; they have to go to school, or at least learn new skills; they have to fight without running away, sometimes in large groups such as armies. It certainly seems reasonable, then, to suggest that an environment defined by agriculture would have selected for genes and traits that were not favored on the African savannah. Moreover, since agricultural populations have been continually changing for 5,000 years, and for the most part these changes have been in the direction of increasing size and complexity, it also seems reasonable to suppose that selection pressures have continued to change both globally (across the entire human population) and locally (in specific sub populations). We all have to do things that people living in ancient Mesopotamia didn’t, and most of us have to do things that others in our population don’t. Modern selection pressures are both different from and more varied than pre modern selection pressures. The question is whether these selection pressures are different enough to cause significant genetic divergence—the kind that would produce troublesome traits—among and within populations.

The second kind of evidence is drawn from observed phenotypic differences among descent groups. If, as Cochran and Harpending claim, selection pressures changed and diversified drastically after the adoption of agriculture, then we should see many such differences. There is no doubt that we do. People of different descent groups often—though not always—look different from each other. As Cochran and Harpending indelicately put it, you would never mistake a Finn for a Zulu. But the observed differences are not confined to appearance. Some are behavioral. To use the obvious example, people of different descent groups often—though not always—perform differently on a wide variety of standardized tests. As we’ve seen, many critics doubt that these tests can be used to draw inferences about genetic distinctions. Yet the critics have yet to provide a good reason why such differences—troublesome traits among them—would not exist given what we know about population genetics and strongly suspect about human history. Changing selection pressures produce differences, not similarities. The selection pressures on humans have certainly changed. Therefore, we have every reason to expect that we would see differences like the ones we see in the test data. If we didn’t see them, that would be very surprising indeed. The question, again, is whether the changes in selection pressures were powerful enough to produce the phenotypic differences that we observe in the data.

The third kind of evidence is observed genetic differences among descent groups. If the laws of population genetics hold and Cochran and Harpending’s retelling of human history is accurate, we should see these genetic differences. There is no doubt that we do. Geneticists have identified the genetic characteristics of many descent groups; moreover, they have succeeded in measuring the genetic distance between many descent groups. Some are very different from one another, which suggests an ancient divergence, and some are very similar to one another, suggesting a more recent divergence. This is consistent with the authors’ version of events. Critics counter, however, that these genetic differences are too new and too few to manifest themselves as anything like major phenotypic differences. Cochran and Harpending meet both of these objections by showing that species can change quickly and that a few genes can make a large difference in behavior. The key question, of course, is whether these few genotypic differences actually give rise to troublesome traits.