Male sex hormones may play a crucial role in the development of certain human intellectual skills, according to an unusual new study with potentially controversial implications.
The study is likely to fuel an ongoing debate about the degree to which intellectual differences between men and women are biologically based. Men traditionally score higher than women in measures of spatial and mathematical ability, while women outperform men on verbal tests.
However, both the scientists who published the research in this week's New England Journal of Medicine and an accompanying editorial sought to mimimize any conclusions about sex differences, maintaining that the new evidence does not resolve the longstanding "nature versus nurture" debate in this area.
The study found that men afflicted with a disorder that dramatically lowers the levels of male sex hormones during the teen-age years have poorer spatial reasoning ability than men who entered puberty with normal hormone levels.
It concluded that androgens, the sex hormones found in higher levels in men, "exert a permanent organizing influence on the brain before or at puberty in boys."
Spatial ability--the ability to visualize objects in space and mentally rotate or manipulate these objects--is thought to be correlated with skills needed in such fields as mathematics, engineering, physics, architecture and design.
"This study is one of the first to clearly demonstrate that a deficiency of sex hormones can influence a human intellectual trait," said Dr. Daniel Hier, a neurologist at Chicago's Michael Reese Hospital. He conducted the study with Dr. William F. Crowley Jr., a colleague at Boston's Massachusetts General Hospital.
Hier suggested that the results might help explain the differences between "someone who does engineering or physics versus someone who does writing"--for example, an "Einstein versus a Shakespeare."
But he shied away from using his study to explain the differences between men and women. "You have to be very cautious in drawing conclusions about sex differences. There clearly are differences. . . . Sex hormones could be one factor that plays a role in sex differences. . . ."
"It's true that women have lower levels of androgens than men, but you can't jump to the conclusion that this is the explanation" for measured differences on intellectual tests, said Hier.
Harvard psychologist Jerome Kagan, in an editorial in the journal, also cautioned against using the new study as "proof" of inborn differences between the sexes in spatial ability, saying that expectations about the abilities of women in our culture may play a far greater role.
This view was seconded by Georgetown University physiologist Estelle Ramey, a well-known researcher and feminist. "There are physiological differences between men and women that relate to hormones. Those differences are largely reproductive, but there are some differences that relate to brain organization as well as muscle performance. But these differences are small as compared to the effect of environmental differences. I think that's the critical point."
In the Hier study, 19 men with a disorder known as "idiopathic hypogonadotropic hypogonadism"--who fail to undergo normal puberty and have significantly reduced levels of male hormones--were compared with a control group of normal men and men who had acquired the disorder later in life.
As a group, the 19 men were found to have "markedly impaired spatial ability" in comparison to the other two groups. The researchers found that receiving hormone treatment after puberty did not affect the spatial reasoning ability. There was no difference between the three groups in verbal ability.
Hier said that the men with the disorder offered "an almost ideal 'experiment of nature' in which to examine the effects of sex hormones on reasoning ability. But the mechanism by which the sex hormones affect the brain is not yet known."