IQ and economic success - intelligence quotient

In The Bell Curve, the late Richard J. Herrnstein and I described an emerging class society in which the intellectually blessed become ever more rich and powerful and the intellectually deficient find it harder and harder to cope. We proposed that this new form of class division is substantially (though by no means completely) independent of one's socioeconomic background.

As evidence for this thesis, we explored trends in college graduation, stratification of intellectual talent within the university system, the growth of occupations screened for high IQ, and the growing dollar premium for brains in the labor market. We also examined the dark side of the picture: the relationship of IQ to outcomes such as poverty, unemployment, welfare recipiency, and crime. But we never laid out explicitly and in detail the relationship of IQ to one of the most hotly debated topics in contemporary policy studies: income inequality.

One purpose of this article is to fill in that gap. The other purpose is to make use of a powerful method, not employed in The Bell Curve, of assessing the importance of IQ independently of all other family background factors.

Income inequality and IQ

Consider the population as divided into the five cognitive classes that Herrnstein and I defined in The Bell Curve. Our point of departure is the group in the middle, those with a measured IQ somewhere from 90 through 109, whom we labeled Normal. Fifty percent of the American population falls in this category. Their intelligence easily permits them to be competent in all the core roles of family and community life and to pursue any occupation not requiring a college education. Most of them have difficulty in completing a college education (historically, the mean IQ of college graduates has been about 115), but some do so.

To their immediate right on the bell curve come the Bright, with IQs from 110 through 124, representing the 75th through 94th percentiles of the IQ distribution. Anyone with an IQ this high has the intellectual ability to get through college, though not necessarily in every major. This IQ range includes many of the most successful Americans. The Very Bright have IQs of 125 and above. They represent the top 5 percent of the IQ distribution. Having an IQ this high is not necessary to become a physician, attorney, or business executive, but extra cognitive horsepower gives an edge in any occupation that draws heavily on the verbal and visuospatial skills measured by IQ tests.

As for the left-hand side of the bell curve, those adjoining the Normals are persons with IQs from 75 through 89, whom we labeled Dull (there is no such thing as a neutral label). If the IQ score is accurate, someone in this range is unlikely to get through four years of college without special dispensations. Ordinarily, the Dulls work at anything from low-skill jobs through lower-level white-collar or technical jobs.

At the far left-hand side of the distribution are the bottom 5 percent of the IQ distribution, the Very Dull, with IQs under 75. These include the retarded, but many people with IQs in this range are neither retarded nor incapacitated. They find it difficult to cope with school but can still be productive employees at menial and semi-skilled jobs, and sometimes at skilled jobs as well if their shortfall in intellectual capacity is counterbalanced by other abilities.

Now suppose we take a large longitudinal data base with income data going back to 1978, when the subjects were ages 13 to 21 and mostly too young even to have an income. All have IQ scores. We follow their median earned income from 1978 through 1992. We split the sample according to the five cognitive classes. The resulting picture is shown below.

Through the early 1980s, the medians for all the cognitive classes are low. Especially among the Bright and Very Bright, this reflects the many subjects who are still in school and working part-time or not at all. Then fortunes begin to diverge. The median for the Very Bright, represented by the thick black line, begins to rise rapidly as the college years end and continues to rise thereafter. At the other extreme, represented by the thick gray line, is the median for the Very Dulls. It peaks in 1989 and falls steadily thereafter. By the end of the period shown in the graph, when this group of young adults has reached ages 27 to 35, those in the top cognitive class have a median earned income more than seven times those in the bottom. The other cognitive classes are also clearly separated. By 1992, the Very Brights are earning 33 percent more than the Brights, who in turn make 35 percent more than the Normals, who in turn make 61 percent more than the Dulls, who make 2.5 times more than the Very Dulls.

The graph is a vivid portrait of what Herrnstein and I meant by cognitive stratification. But is IQ per se really the explanation for these results? Many other possibilities come to mind. Perhaps education, not IQ, is the real key: A better education enhances income and IQ score. Perhaps money and influence are the key: Rich parents can procure both good educations and lucrative jobs for their lucky children while poor parents can provide neither. Perhaps more subtle dynamics are at work - for example, whether the child grew up with both parents, whether the child grew up in neighborhoods that encouraged achievement, and so on.