Gray matter, made up of the bodies of nerve cells and their
connecting dendrites, is where the brain's heavy lifting is done. The
female brain is more densely packed with neurons and dendrites, providing
concentrated processing power -- and more thought-linking
capability.
The larger male cranium is filled with more white matter and
cerebrospinal fluid. "That fluid is probably helpful," says Gur, director
of the Brain Behavior Laboratory at the University of Pennsylvania. "It
cushions the brain, and men are more likely to get their heads banged
about."
White matter, made of the long arms of neurons encased in a
protective film of fat, helps distribute processing throughout the brain.
It gives males superiority at spatial reasoning. White matter also
carries fibers that inhibit "information spread" in the cortex. That
allows a single-mindedness that spatial problems require, especially
difficult ones. The harder a spatial task, Gur finds, the more
circumscribed the right-sided brain activation in males, but not in
females. The white matter advantage of males, he believes, suppresses
activation of areas that could interfere with work.
The white matter in women's brains is concentrated in the corpus
callosum, which links the brain's hemispheres, and enables the right side
of the brain to pitch in on language tasks. The more difficult the verbal
task, the more global the neural participation required -- a response
that's stronger in females.
Women have another heady advantage -- faster blood flow to the brain,
which offsets the cognitive effects of aging. Men lose more brain tissue
with age, especially in the left frontal cortex, the part of the brain
that thinks about consequences and provides self-control.
"You can see the tissue loss by age 45, and that may explain why
midlife crisis is harder on men," says Gur. "Men have the same impulses
but they lose the ability to consider long-term consequences." Now,
there's a fact someone's grandmother may have figured out already.
Minds of Their Own
The difference between the sexes may boil down to this: dividing
the tasks of processing experience. Male and female minds are innately
drawn to different aspects of the world around them. And there's new
evidence that testosterone may be calling some surprising shots.
Women's perceptual skills are oriented to quick -- call it
intuitive -- people reading. Females are gifted at detecting the feelings
and thoughts of others, inferring intentions, absorbing contextual clues
and responding in emotionally appropriate ways. They empathize. Tuned to
others, they more readily see alternate sides of an argument. Such
empathy fosters communication and primes females for attachment.
Women, in other words, seem to be hard-wired for a top-down,
big-picture take. Men might be programmed to look at things from the
bottom up (no surprise there).
Men focus first on minute detail, and operate most easily with a
certain detachment. They construct rules-based analyses of the natural
world, inanimate objects and events. In the coinage of Cambridge
University psychologist Simon Baron-Cohen, Ph.D., they systemize.
The superiority of males at spatial cognition and females' talent
for language probably subserve the more basic difference of systemizing
versus empathizing. The two mental styles manifest in the toys kids
prefer (humanlike dolls versus mechanical trucks); verbal impatience in
males (ordering rather than negotiating); and navigation (women
personalize space by finding landmarks; men see a geometric system,
taking directional cues in the layout of routes).
Almost everyone has some mix of both types of skills, although
males and females differ in the degree to which one set predominates,
contends Baron-Cohen. In his work as director of Cambridge's Autism
Research Centre, he finds that children and adults with autism, and its
less severe variant Asperger syndrome, are unusual in both dimensions of
perception. Its victims are "mindblind," unable to recognize people's
feelings. They also have a peculiar talent for systemizing, obsessively
focusing on, say, light switches or sink faucets.
Autism overwhelmingly strikes males; the ratio is ten to one for
Asperger. In his new book, The Essential Difference: The Truth About the
Male and Female Brain, Baron-Cohen argues that autism is a magnifying
mirror of maleness.
The brain basis of empathizing and systemizing is not well
understood, although there seems to be a "social brain," nerve circuitry
dedicated to person perception. Its key components lie on the left side
of the brain, along with language centers generally more developed in
females.
Baron-Cohen's work supports a view that neuroscientists have
flirted with for years: Early in development, the male hormone
testosterone slows the growth of the brain's left hemisphere and
accelerates growth of the right.
Testosterone may even have a profound influence on eye contact.
Baron-Cohen's team filmed year-old children at play and measured the
amount of eye contact they made with their mothers, all of whom had
undergone amniocentesis during pregnancy. The researchers looked at
various social factors -- birth order, parental education, among others -- as
well as the level of testosterone the child had been exposed to in fetal
life.
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