Last week Heritage Foundation scholar Jason Richwine, coauthor of a hotly disputed new study on the fiscal costs of comprehensive immigration reform, resigned his position in a hail of controversy over his 2009 Harvard Ph.D. dissertation. In that dissertation Richwine had argued, among other things, that American "Hispanics" are less intelligent than native-born whites as evidenced by their lower average scores on IQ tests. Richwine then attributed Hispanics' alleged intellectual inferiority at least partly to genetic factors.
The Richwine affair is just the latest flap in a long-running dispute over the significance of IQ tests and group differences in IQ scores. It's easy enough to shut down that debate with cries of racism, but stigmatizing a point of view as morally tainted isn't the same thing as demonstrating that it's untrue. Here I want to explain why Richwine's position is intellectually as well as morally unsound.
Let's start with the fact that there is no such thing as a direct test of general mental ability. What IQ tests measure directly is the test-taker's display of particular cognitive skills: size of vocabulary, degree of reading comprehension, facility with analogies, and so on. Any conclusions about general mental ability are inferences drawn from the test-taker's relative mastery of those various skills.
How justified are such inferences? Well, it depends. Without a doubt, the skills assessed on modern IQ tests are widely applicable and highly valued in contemporary American society. Accordingly, considered just as a measure of skills rather than as a proxy for underlying ability, IQ scores clearly tell us something of genuine importance. They are a reasonably good predictor not only of performance in the classroom but of income, health, and other important life outcomes.
But what about innate mental ability? Does such a thing even exist? Evidence from IQ tests provides strong support that it does. First of all, scores on the various IQ subtests are highly correlated with each other, suggesting the presence of a general underlying factor. Furthermore, IQ scores tend to stabilize around age eight and are resistant to moving around much thereafter, in keeping with a relatively fixed level of innate intellectual capacity. And studies of twins and adoptees offer substantial evidence that this capacity has a strong genetic component. The scores of twins (who are genetically identical, more or less) are much more highly correlated than those of regular siblings (who share only about half the same genes). Meanwhile, the scores of regular siblings are in turn much more highly correlated than the scores of adopted and biological children raised together.
So what's the problem? These studies typically assume that the similarity of twins' shared environment is the same as that of regular siblings (highly unlikely) and that adoptive families are as diverse as families generally (in fact, parents that adopt tend to be better off and better educated). When these assumptions are relaxed, environmental factors start to loom larger. In this regard, consider a pair of French adoption studies that controlled for the socioeconomic status of birth and adoptive parents. They found that being raised by high-SES (socioeconomic status) parents led to an IQ boost of between 12 and 16 points - a huge improvement that testifies to the powerful influence that upbringing can have.
A study of twins by psychologist Eric Turkheimer and colleagues that similarly tracked parents' education, occupation, and income yielded especially striking results. Specifically, they found that the "heritability" of IQ - the degree to which IQ variations can be explained by genes - varies dramatically by socioeconomic class. Heritability among high-SES (socioeconomic status) kids was 0.72; in other words, genetic factors accounted for 72 percent of the variations in IQ, while shared environment accounted for only 15 percent. For low-SES kids, on the other hand, the relative influence of genes and environment was inverted: Estimated heritability was only 0.10, while shared environment explained 58 percent of IQ variations.
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.