This article originally appeared in the July-August 1995 issue of American Scientist.

Last year, Richard Herrnstein and Charles Murray published The Bell Curve: Intelligence and Class Structure in American Life. Although it had more graphs than a Ross Perot speech, The Bell Curve made its authors' names household words, sometimes accompanied by four-letter words. Herrnstein and Murray maintained that America is splitting into the intelligent, who will move and shake society, and the less intelligent, who will be moved and shaken. They thought that the split is inevitable, because our technological society requires intelligence to run it. Finally, they said that intelligence is largely hereditary, and that numerous government programs, especially Affirmative Action, are undesirable because they amount to discrimination against the capable.

Such thoughts are not entirely politically correct. The first reactions to The Bell Curve were expressions of public outrage. In the second round of reaction, some commentators suggested that Herrnstein and Murray were merely bringing up facts that were well known to the scientific community, but perhaps best not discussed in public. A Papua New Guinea language has a term for this, Mokita. It means "truth that we all know but agree not to talk about."

The uproar over The Bell Curve is remarkably similar to a debate in the early 1970s. The earlier debate began when Arthur Jensen (1969) wrote that the educational enrichment programs of the Great Society were inherently limited by the immutability of intelligence and when Herrnstein (1973) claimed that differences in intelligence are largely genetic. Counterattacks followed, and by the early 1980s widely read books and articles maintained that there is no such thing as general intelligence (Gardner 1983), or that if there is it is largely a statistical artifact of the way that tests are constructed (Gould 1983), and that even if IQ exists it has little to do with life outside of a few narrow academic settings (Ceci and Liker 1986). Some of these authors have recanted (Ceci and bruck 1994, pg. 79).

A central question in the debate is whether or not mental competence is a single ability, applicable in many settings, or whether competence is produced by specialized abilities, which a person may or may not possess independently. Almost equally important is the question of how cognitive skill, as evaluated by IQ tests, translates into everyday performance. Popular presentations on both sides of these questions leave the impression that these questions have simple answers. They do not. My goal in this essay is to discuss different theories of how intelligence is related to performance in modern society. The plural was chosen intentionally, Although we know a good deal about individual differences in human cognition, there is no monolithic, agreed-upon, all-purpose theory to organize these facts, nor is there likely to be one. There are a number of different theories that are neither right nor wrong, but are useful for different purposes.