The average man and the average woman have about the same level of intelligence, but men account for a higher proportion of both geniuses and the mentally deficient, according to a new study of IQ test results.
Seven of every eight people in the top 1% on IQ tests are men, accounting for their overrepresentation in the elite ranks of the sciences and mathematics where such genius is crucial, University of Chicago researchers report today in the journal Science.
However, men represent an almost equally large proportion of the mentally disadvantaged, especially in terms of reading and writing skills.
The test scores imply "that men are, on average, at a rather profound disadvantage in the performance of certain basic intelligence skills," said Chicago education professor Larry V. Hedges.
"The U.S. has a larger number of men who can barely read, write or do arithmetic than is currently assumed," Hedges said. Increased attention from schools may be necessary to bring these men into the work force, he concluded.
The finding that there are a high proportion of men in the top IQ levels conforms with previous studies, but "we didn't know there were more males at the low end," said neuroscientist Richard Haier of UC Irvine.
Studies like this, Haier added, are "very important in documenting the kinds of differences that seem to be real so that other research can continue to figure out where these differences come from. . . .
"We need to understand the mechanisms that make some brains high-functioning brains and others low-functioning brains so that we can fix [those that don't work well]," Haier said.
Differences between the brains of men and women have been the subject of increasingly intense study in recent years, and it has become clear that there are structural distinctions between the two. Women's brains are, on average, smaller than men's--a fact that has long been recognized.
Nonetheless, recent studies by neuroscientist Sandra Witelson of MacMaster University in Hamilton, Canada, show that women's brains contain an average of about 11% more brain cells, suggesting that they work more efficiently.
Other studies have also shown that the corpus callosum, a band of tissue that connects the two sides of the brain, is larger in women, perhaps allowing the two halves to work together more effectively.
Recent studies using sophisticated techniques to observe brain cell activity have shown that male and female brains function differently when performing the same tasks. It has become increasingly clear that men and women do not think alike, and the new study by Hedges and graduate student Amy Nowell illuminate one facet of that difference.
Most previous studies of IQ differences, Hedges said, have used relatively small groups of individuals, most of them recruited from college communities. It is easy to argue that societal biases in favor of men could have influenced the outcomes of these studies, he said, and the total number of individuals involved did not allow close examination of the small numbers of individuals at the IQ extremes.
In the new study, Hedges and Nowell combined data from six large population-based studies--the smallest of which contained 10,000 people--that represented a true cross-section of American society. For the first time, he said, the sample populations were large enough to reveal discrepancies between the numbers of men and women at those extremes.
"This is stronger than any study that has ever been done," Hedges said.
Among the conclusions:
* In math and science skills, boys outnumbered girls three to one in the top 10% of test scores and seven to one in the top 1%. In some science and vocational aptitude tests, no girls scored in the top 1% to 3%.
* In reading comprehension and writing skills, boys outnumbered girls two to one at the bottom of the scale, and there were fewer boys than girls in the top 5%.
The reasons for the differences are not clear. Neuroscientist Roger Gorski of UCLA noted that brains, like sex organs, are basically feminine at conception and that male organs have to be molded from female "clay." "That molding could explain the variability," he said.
Haier noted that the efficiency of the brain, the fundamental basis of IQ, is regulated by "neural pruning," the elimination of redundant brain cells and pathways between the ages of 5 and 20, and that this pruning could be affected by hormone levels.
Hedges, however, argues that the differences lie in the opportunities boys are exposed to, the encouragement they receive in technical areas, and the socialization involved in their upbringing. "I think little boys have more opportunities to become engaged with science and technical kinds of activities," he said.
Hedges also noted that the differences between men and women have remained relatively constant over the 30 years covered by the six studies, despite efforts to equalize the educational achievements of boys and girls. "We haven't seen the gender differences going away," he said.
But that lack of change is not surprising, added Michael Timpane, vice president of the Carnegie Foundation for the Advancement of Teaching. "It doesn't surprise me that we haven't made more progress because, until recent years, we really haven't tried very hard."