What we know and don’t know about intelligence

iStock_000014316766XSmallThe American Psychological Association (APA) recently issued a report on the state of our knowledge about intelligence. Following the heated debate sparked by the publication of Herrnstein and Murray’s book The Bell Curve, the APA formed a task force to distinguish scientific issues from political ones in the discussion about intelligence.

Because there are many ways to be intelligent, there are many conceptualizations of intelligence. We know least about the kinds of intelligence that tests do not easily measure, such as wisdom, creativity, practical knowledge and social skill. Tests, however, have successfully measured a range of abilities related to intelligence.

Standardized intelligence test scores (IQs), which reflect a person’s standing in relation to others his or her age, are based on tests that tap a number of different abilities. Intelligence test scores predict individual differences in school achievement moderately well — both school grades and the number of years of education a person will complete. Despite this, school achievement is not determined solely or even primarily by intelligence. The fact that children in Japan or Taiwan learn more mathematics than than U.S. students is not due to differences in IQ scores but rather to differences in culture and schooling.

Occupational status is also correlated with test scores, probably through school achievement, which serves as a gatekeeper for certain professions. A significant correlation between intelligence test scores and occupation remains, even when family background and education are statistically controlled. However, these correlations are low, and the APA stresses that achievement is the result of a complex interaction of causes, of which intellectual skills are only one.

Intelligence is the product of genetic and environmental factors. We know that both biological and social factors in the environment are important for intelligence, but we do not know how they exert their effects. One environmental variable with clear-cut importance is the presence of formal schooling.

Schools affect intelligence in many ways, not only by transmitting information but by developing certain intellectual skills and attitudes. Failure to attend school or attendance at very poor schools has a clear negative effect on intelligence test scores. Preschool programs have positive effects, but in most cases the gains tend to fade when the program is over. Several conditions of the biological environment have negative consequences for intellectual development. Complications during birth, exposure to lead, prenatal exposure to high blood levels of alcohol and severe malnutrition in childhood all negatively effect IQ.

Worlwide rise in IQ

One striking phenomenon is the steady, worldwide rise in IQ scores over the last 50 years. Mean scores have risen about 15 points — a full standard deviation — and the rate of gain seems to be increasing. These gains may be the result of improved nutrition, cultural changes, experience with testing, shifts in schools or child-rearing practices or other unknown factors.

Although there are no important sex differences in overall intelligence test scores, substantial differences are apparent in specific abilities. Males typically score higher on visual-spatial and mathematical skills (beginning in middle childhood); females excel on a number of verbal measures. Sex hormone levels are related to some of these differences, but social factors probably play a role as well. As for all groups reviewed here, the range of performance within each group is much larger than the average difference between groups.

Because ethnic differences in test scores reflect complex patterns, no overall generalization about them is appropriate. The mean IQ scores of Chinese- and Japanese-Americans differ little from those of whites, although the spatial ability tends to be higher. Their outstanding school and occupational performance evidently reflects cultural factors. The mean intelligence score of Hispanic Americans is somewhat lower than whites’, in part because Hispanics are often less familiar with English. African-American IQ scores average about 15 points below those of whites, with correspondingly lower scores on academic-achievement tests. In recent years, however, the academic achievement score gap has narrowed appreciably and it is possible the IQ difference is narrowing as well.

Many questions about intelligence are still unanswered. Here are a few of the issues the APA will address in the future:

1. Differences in genetic endowment contribute substantially to individual differences in intelligence, but the way genes produce effects is still unknown. The impact of genetic differences appears to increase with age, but we do not know why.

2. Environmental factors also contribute substantially to the development of intelligence, but we do not clearly understand what those factors are or how they work. Attendance at school is certainly important, for example, but we do not know what aspects of schooling are critical.

3. The role of nutrition remains obscure. Severe childhood malnutrition has clear negative effects, but theories that specific nutrients may affect intelligence, in otherwise adequately nourished populations, have not been convincingly demonstrated.

4. Information-processing speed is related to tested intelligence, but the overall pattern of these findings leads to no easy interpretation.

5. Mean scores on intelligence tests are rising steadily. No one is sure why these gains are happening or what they mean.

6. The differential between the mean intelligence scores of blacks and whites does not result from any obvious biases in test construction and administration, nor does it simply reflect differences in socioeconomic status. Explanations based on factors of caste and culture may be appropriate, but so far have little direct empirical support. There is certainly no support for a genetic interpretation. At present no one knows what causes this differential.

The APA task force concludes that many issues related to intelligence are unresolved and the confident tone that characterizes most of the debate on these topics is clearly out of place. There is no reason to think that these questions are unanswerable, but they will require a great deal more research and sustained reflection.

“Intelligence: Knowns and Unknowns” American Psychologist Volume 51, Number 2, February 1996 pp. 77-99.

Published in ERN May/June 1996 Volume 9 Number 3

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