Turkheimer's findings make perfect sense once you recognize that IQ scores reflect some varying combination of differences in native ability and differences in opportunities. Among rich kids, good opportunities for developing the relevant cognitive skills are plentiful, so IQ differences are driven primarily by genetic factors. For less advantaged kids, though, test scores say more about the environmental deficits they face than they do about native ability.

This, then, shows the limits to IQ tests: Though the tests are good measures of skills relevant to success in American society, the scores are only a good indicator of relative intellectual ability for people who have been exposed to equivalent opportunities for developing those skills - and who actually have the motivation to try hard on the test. IQ tests are good measures of innate intelligence--if all other factors are held steady. But if IQ tests are being used to compare individuals of wildly different backgrounds, then the variable of innate intelligence is not being tested in isolation. Instead, the scores will reflect some impossible-to-sort-out combination of ability and differences in opportunities and motivations. Let's take a look at why that might be the case.

Comparisons of IQ scores across ethnic groups, cultures, countries, or time periods founder on this basic problem: The cognitive skills that IQ tests assess are not used or valued to the same extent in all times and places. Indeed, the widespread usefulness of these skills is emphatically not the norm in human history. After all, IQ tests put great stress on reading ability and vocabulary, yet writing was invented only about 6,000 years ago - rather late in the day given that anatomically modern humans have been around for over 100,000 years. And as recently as two hundred years ago, only about 15 percent of people could read or write at all.

More generally, IQ tests reward the possession of abstract theoretical knowledge and a facility for formal analytical rigor. But for most people throughout history, intelligence would have taken the form of concrete practical knowledge of the resources and dangers present in the local environment. To grasp how culturally contingent our current conception of intelligence is, just imagine how well you might do on an IQ test devised by Amazonian hunter-gatherers or medieval European peasants.

The mass development of highly abstract thinking skills represents a cultural adaptation to the mind-boggling complexity of modern technological society. But the complexity of contemporary life is not evenly distributed, and neither is the demand for written language fluency or analytical dexterity. Such skills are used more intensively in the most advanced economies than they are in the rest of the world. And within advanced societies, they are put to much greater use by the managers and professionals of the socioeconomic elite than by everybody else. As a result, American kids generally will have better opportunities to develop these skills than kids in, say, Mexico or Guatemala. And in America, the children of college-educated parents will have much better opportunities than working-class kids.