Men are smarter than women, according to a controversial new study that adds another cinder to the fiery debate over whether gender impacts general intelligence.

"For 100 years there's been a consensus among psychologists that there is no sex difference in intelligence," said J. Philippe Rushton, a psychologist at the University of Western Ontario, Canada.

Recent studies, however, have raised questions about the validity of this claim, he said. One such study showed that men have larger brains than women, a 100 gram difference after correcting for body size. Rushton found similar results in a study of gender and brain size. 

To determine if there was a link between gender and intelligence, and perhaps between brain size and intelligence, Rushton and a colleague analyzed the Scholastic Aptitude Test (SAT) scores from 100,000 17- and 18-year-olds.


When Rushton and colleagues weighted each SAT question by an established general intelligence factor called the g-factor, they discovered that males surpassed females by an average of 3.6 IQ points.

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The g-factor works like this. "If I tell you the last four digits of my telephone number and ask you to repeat them back to me, that's a low g-loaded memory test," Rushton explained. "But if I then ask you to repeat them back to me in the reverse order, that suddenly requires a tremendous amount more cognitive processing. It is a very high loaded g-item.”

So the g-factor "is really the active ingredient of the test," Rushton said. "It's the single best, most predictive part of the test."

Rushton suspects that the results are due to males having more brain tissue than females on average. "It's a very reasonable hypothesis that you just need more brain tissue dedicated to processing high ‘g' information," Rushton said.

The study, which Rushton co-wrote with Douglas Jackson, also of the University of Western Ontario, is detailed in the current issue of the journal Intelligence.

Flawed conclusion

Bruce Bracken, a psychologist at the College of William and Mary in Williamsburg, Virginia, who was not involved in the new study, said he thinks Rushton and Jackson make a convincing argument for the sample they used.

"The difference appears to be real," Bracken said.

But he questions the team's conclusions. "I believe that the differences probably lie in the variables they hadn't considered," Bracken said.

One plausible explanation is that more females than males decide to go to college and thus take the SAT test. The study did in fact include about 10,000 more females than males.

"This suggests that more males are deciding to do something else," Bracken said. "It may be that the males who would not have scored as high on the SAT chose not to take it, and they chose another route."

A more reliable study, he said, would be to match each male with a very similar female and then compare the results. 

The debate goes on

The findings add fuel to a still smoldering debate ignited by former Harvard President Lawrence Summers, who stated early last year that males have a higher intrinsic aptitude in science and engineering.

In a letter from Summers days after his controversial statements, he wrote: "Despite reports to the contrary, I did not say, and I do not believe, that girls are intellectually less able than boys, or that women lack the ability to succeed at the highest levels of science. As the careers of a great many distinguished women scientists make plain, the human potential to excel in science is not somehow the province of one gender or another.”

While Rushton called his results significant, he doesn't think they are a basis for uprooting the field of education.

"I don't think it has any real implications for education policy or schoolwork," he said. "In fact, females actually get better grades than males.”

Plus, he doesn't think the IQ difference would show up in everyday activities. "For the vast majority of people in the vast majority of jobs, it really doesn't translate into very much," he said.

But when it comes to Nobel Prize winners, he said that men could outnumber women 10-to-1.  "Where it will really show up is at the very high end of the distribution," Rushton said.

Rushton has left the door open for opposing views and findings that might contradict his new study.

"I wouldn't say it's the last word. We really do need more research on it before we can be absolutely certain," Rushton said.