The Bell Curve Controversy

The standard normal distribution

Variation in human IQ, like many other psychometric traits, can be represented on a standard normal distribution, i.e. a bell curve. When tested, most (68.2%) people will fall within one standard deviation (15 IQ points) of the mean, which in the case of IQ, is a score of 100. Fewer (14.9%) fall within two standard deviations and so on, with even fewer scoring at the far extremes. IQ tests are standardized but have been shown to have different means and distributions for different groups.

The standard normal distribution. For IQ, the mean point represents a score of 100

An unpopular hypothesis

The Bell Curve attempted to explain and understand this variation. The book outlined that although intelligence was due both to nature and nurture, high intelligence seemed to be the stronger predictor of life outcomes: lower risk of criminality, divorce, poverty, unemployment or dropping out of school.

The authors argued that since greater cognitive ability predicted more socially desirable behavior, and since IQ has a heritable component, an intellectual “elite” could eventually be expected to emerge, with far-reaching implications for social organization.

Murray and Hernstein suggested that for some racial groups, lower scores on standardized IQ tests are not caused by disadvantaged backgrounds, but vice versa: that such backgrounds are themselves caused by lower intelligence. Furthermore, they claimed that IQ tests are not culturally biased and any variation between groups reflects a valid difference.

Though the book takes pains to explain that nobody should be mistreated because of their IQ, it also suggested that higher birth rates among those with lower IQs or immigration may gradually lower the national IQ mean over time. A “cognitive elite” may steadily increase in wealth and separate from the rest of society.

These uncomfortable claims about the cause of many of the modern world’s social issues led the authors to give a bleak prediction for America’s future, where the quality of life for lower IQ groups progressively deteriorates.

Critical response: Mainstream science on intelligence and the APA’s Knowns and unknowns

Understandably, the book garnered both feverish criticism and defense. Many rushed to challenge the underlying assumptions of the authors’ arguments (that IQ is reducible to one number, immutable, genetically determined, linear etc.) whilst others criticized the statistical methods and interpretations, or else the IQ tests used to generate that data. Others rallied behind the authors and pointed out how researchers had the right and responsibility to interpret and report on their findings whatever the political implications.

A public statement signed by 52 university professors titled Mainstream science on intelligence was published in 1994 in the Wall Street Journal. It attempted to moderate the wave of media reports on the research presented in The Bell Curve, and put forward 25 conclusions, some of which were:

• "Members of all racial-ethnic groups can be found at every IQ level"
• "Differences in intelligence certainly are not the only factor affecting performance in education, training, and complex jobs ... but intelligence is often the most important"
• "Intelligence tests are not culturally biased"
• "There is no definitive answer as to why bell curves differ across racial-ethnic groups. The reasons for these IQ differences between groups may be markedly different from the reasons for why individuals differ among themselves within any particular group"

Similarly, a special task force appointed by the American Psychological Association issued a report in 1995 called Intelligence: Knowns and Unknowns. The report established itself as a dispassionate authority which would correct misunderstandings instigated largely by The Bell Curve, and separate out what was scientifically established fact from what was politically motivated opinion. The report essentially weighed in on the state of intelligence testing in the field as a whole, and came to several conclusions that went beyond the most obvious and controversial group differences.

Interestingly, these two reports only seemed to increase the degree of emotionally charged debate around the topic, with many of the claims made on both sides of the debate being separately argued. Though there has been much more written on the topic since then, today much of the argument has simmered down as researchers become more interested in cognitive ability as only one of many variables in the human experience. The convention today is largely to avoid research into IQ group difference altogether.