"Yet there is a large segment of the population that wants to pretend this is not true."
No one knows how these neural differences between the sexes translate into thought and behavior — whether they might influence the way men and women perceive reality, process information, form judgments and behave socially.
Some activists fear that research like Witelson's could be used to justify discrimination based on gender differences, just as ill-conceived notions of human genetics once influenced laws codifying racial stereotypes about blacks, Asians and Jews.
Other experts argue that the physical differences Witelson observed may result not from the brain's basic design but from conditioning that begins in infancy, when the brain produces neurons at a rate of half a million a minute and reaches out to make connections 2 million times a second.
Spurred by learning, neurons and synapses are ruthlessly pruned, a process that continues in fits and starts throughout adolescence, then picks up again in middle age.
"The brain is being sculpted gradually through sets of interactions," said Anne Fausto-Sterling, a gender studies expert at Brown University. "Even when something in the brain appears biological, it may have come to be that way because of how the body has experienced the world."
As Witelson's research helped establish, however, the mental divide between the sexes is more complex and more rooted in the fundamental biology of the brain than many scientists once suspected.
In the last decade, studies of perception, cognition, memory and neural function have found apparent gender differences that often buck conventional prejudices.
Women's brains, for instance, seem to be faster and more efficient than men's.
All in all, men appear to have more gray matter, made up of active neurons, and women more of the white matter responsible for communication between different areas of the brain.
Overall, women's brains seem to be more complexly corrugated, suggesting that more complicated neural structures lie within, researchers at UCLA found in August.
Men and women appear to use different parts of the brain to encode memories, sense emotions, recognize faces, solve certain problems and make decisions. Indeed, when men and women of similar intelligence and aptitude perform equally well, their brains appear to go about it differently, as if nature had separate blueprints, researchers at UC Irvine reported this year.
"If you find that men and women have fundamentally different brain architectures while still accomplishing the same things," said neuroscientist Richard Haier, who conducted the study, "this challenges the assumption that all human brains are fundamentally the same."
Yet, for the most part, scientists have been unable to document such patterns conclusively.
No one, however, had scrutinized as many brains as Witelson.
She began by studying the corpus callosum, the cable of nerves that channels all communication and cooperation between the brain's two hemispheres.
Examining tissue samples through a microscope, she discovered that the more left-handed a person was, the bigger the corpus callosum.
To her surprise, however, she found that this held true only for men. Among women there was no difference between right-handers and left-handers.
COLUMN ONE | MAPPING THE MIND