COVER

Boys, girls and brainpower

The sexes differ in more than appearance

MARK NICHOLS January 22 1996
COVER

Boys, girls and brainpower

The sexes differ in more than appearance

MARK NICHOLS January 22 1996

Boys, girls and brainpower

COVER

The sexes differ in more than appearance

It began almost by accident. In an effort to uncover the causes of dyslexia, psychologist Sandra Witelson decided in 1970 to conduct an experiment involving dyslexic and other children at a Hamilton grade school. Because dyslexia affects mostly males, Witelson planned to use boys only. “But the girls wanted to join in, so they could get to miss some of their regular classes too,” says Witelson.

“So we included girls.” The purpose of the experiment was to see whether some mental functions in dyslexic children— such as language skills or spatial perception—favor one or the other of the two brain hemispheres, as they do in most people. What Witelson found was that dyslexic children have fewer right-left brain differences than other kids—and that, where they exist, the differences were far more pronounced in boys than in girls. That discovery, published in 1976, sent a tremor through the world of brain research. The reason: Witelson’s finding suggested that differences between male and female behavior might not be due simply to social conditioning, but rather to biological differences in the brains of men and women.

Certainly there is ample anecdotal evidence to suggest that conclu. sion. And studies have shown that women often do possess superior verbal skills, while men are frequently better at things like mathematics and map reading. But now, the 55-year-old Witelson and a handful of other researchers have begun to produce concrete and mounting evidence of physical differences in the brains of men and women, as well as in the brains of heterosexuals and homosexuals. Typically, in a study published last May, Witelson, a professor in the psychiatry department at Hamilton’s McMaster University, reported that in a part of the temporal lobe associated with language skills, women’s brains contained up to 11-per-cent more brain cells than men’s brains.

That does not necessarily mean that women are smarter than men—but it does show that they are different. As Witelson notes, her findings challenge the politically correct dogma that “except for anatomical differences in men’s and women’s bodies, everything else is supposed to be the same, except where things have been distorted by social forces.” If there are physical brain differences between the sexes, adds Witelson, “it may be better to recognize this and deal with it, rather than pretending that we are all the same.”

A native of Montreal, Witelson studied psychology at McGill University, where she became interested in childhood learning disabilities. When American researchers in the late 1960s reported that brain regions in the left and right hemispheres often varied in size, Witelson wondered whether the disparities were related to the distribution of brain functions in the two hemispheres. Officials at the U.S. National Institutes of Health in Bethesda, Md., were interested in the same question and, in 1976, they offered to fund scientists to investigate it. Witelson and a McMaster colleague bid on the multimillion-dollar NIH contract—and won.

The three-year grant set the stage for a series of studies by Witelson and her scientific partners that have shed light on the differences between brain hemispheres—and between male and female brains. From the male point of view, one of Witelson’s most disturbing discoveries was reported in 1991. She found that as men grow older, the corpus callosum—a brain region that provides communications between the hemispheres—begins shrinking. The biggest surprise was that the study, based on post-mortem examinations of 23 male and 39 female brains, showed virtually no shrinkage of the female corpus callosum. In a current study, Witelson is trying to determine what effect the shrinkage has. “Clearly, whatever is happening in the corpus callosum is not of great consequence for most men,” she says. “Lots of men do very well in their later decades.”

Witelson and her research partners illustrated another dimension of brain differences in November, 1994. In a study involving 21 people, they showed that part of the corpus callosum in the brains

of some homosexual men was 13-percent larger than in the heterosexual men. That might explain why earlier studies, including some by Witelson, have found differences in the cognitive abilities of gay and straight men, including lower scores by gay men on tests of spatial perception. Witelson’s findings involving the corpus callosum followed a 1991 U.S. study that reported physical differences between gay and straight men in the hypothalamus, a brain region associated with sexual behavior. Other American researchers have suggested that genetic factors may play a role in homosexuality.

Working in an area of science that is fraught with political implications, Witelson insists that she is interested only in the truth. “I think of myself as a scientist,” she says, “not as a male or female scientist.” Witelson, who has been married for 35 years to Hamilton eye doctor Henry Witelson, thinks there now is persuasive evidence that men’s and women’s brains “are actually different in some of the ways they are put together, anatomically and chemically.” That will upset some people, she says, “because they assume that biology means things are immutable.” But, she adds, “the fact is that upbringing and other environmental factors play a tremendously important role” in shaping the mind—a reminder that biology alone is not destiny.

MARK NICHOLS