Get
out the spittoon. Men produce twice as much saliva as women. Women, for their
part, learn to speak earlier, know more words, recall them better, pause less
and glide through tongue twisters.
Put
aside Simone de Beauvoir's famous dictum, "One is not born a woman but rather
becomes one." Science suggests otherwise, and it's driving a whole new view
of who and what we are. Males and females, it turns out, are different from the
moment of conception, and the difference shows itself in every system of body
and brain.
It's
safe to talk about sex differences again. Of course, it's the oldest story in
the world. And the newest. But for a while it was also the most treacherous. Now
it may be the most urgent. The next stage of progress against disorders as disabling
as depression and heart disease rests on cracking the binary code of biology.
Most common conditions are marked by pronounced gender differences in incidence
or appearance.
Although
sex differences in brain and body take their inspiration from the central agenda
of reproduction, they don't end there. "We've practiced medicine as though
only a woman's breasts, uterus and ovaries made her unique--and as though her
heart, brain and every other part of her body were identical to those of a man,"
says Marianne J. Legato, M.D., a cardiologist at Columbia University who spearheads
the new push on gender differences. Legato notes that women live longer but break
down more.
Do
we need to explain that difference doesn't imply superiority or inferiority? Although
sex differences may provide ammunition for David Letterman or the Simpsons, they
unfold in the most private recesses of our lives, surreptitiously molding our
responses to everything from stress to space to speech. Yet there are some ways
the sexes are becoming more alike--they are now both engaging in the same kind
of infidelity, one that is equally threatening to their marriages.
Everyone
gains from the new imperative to explore sex differences. When we know why depression
favors women two to one, or why the symptoms of heart disease literally hit women
in the gut, it will change our understanding of how our bodies and our minds work.
The
Gene Scene
Whatever
sets men and women apart, it all starts with a single chromosome: the male-making
Y, a puny thread bearing a paltry 25 genes, compared with the lavish female X,
studded with 1,000 to 1,500 genes. But the Y guy trumps. He has a gene dubbed
Sry, which, if all goes well, instigates an Olympic relay of development. It commands
primitive fetal tissue to become testes, and they then spread word of masculinity
out to the provinces via their chief product, testosterone. The circulating hormone
not only masculinizes the body but affects the developing brain, influencing the
size of specific structures and the wiring of nerve cells.
But
sex genes themselves don't cede everything to hormones. Over the past few years,
scientists have come to believe that they too play ongoing roles in gender-flavoring
the brain and behavior.
Females,
it turns out, appear to have backup genes that protect their brains from big trouble.
To level the genetic playing field between men and women, nature normally shuts
off one of the two X chromosomes in every cell in females. But about 19 percent
of genes escape inactivation; cells get a double dose of some X genes. Having
fall-back genes may explain why females are far less subject than males to mental
disorders from autism to schizophrenia.
What's
more, which X gene of a pair is inactivated makes a difference in the way female
and male brains respond to things, says neurophysiologist Arthur P. Arnold, Ph.D.,
of the Uni-versity of California at Los Angeles. In some cases, the X gene donated
by Dad is nullified; in other cases it's the X from Mom. The parent from whom
a woman gets her working genes determines how robust her genes are. Paternal genes
ramp up the genetic volume, maternal genes tune it down. This is known as genomic
imprinting of the chromosome.
For
many functions, it doesn't matter which sex genes you have or from whom you get
them. But the Y chromosome itself spurs the brain to grow extra dopamine neurons,
Arnold says. These nerve cells are involved in reward and motivation, and dopamine
release underlies the pleasure of addiction and novelty seeking. Dopamine neurons
also affect motor skills and go awry in Parkinson's disease, a disorder that afflicts
twice as many males as females.
XY
makeup also boosts the density of vasopressin fibers in the brain. Vasopressin
is a hormone that both abets and minimizes sex differences; in some circuits it
fosters parental behavior in males; in others it may spur aggression.
Sex
on the Brain
Ruben
Gur, Ph.D., always wanted to do the kind of psychological research that when he
found something new, no one could say his grandmother already knew it. Well, "My
grandmother couldn't tell you that women have a higher percentage of gray matter
in their brains," he says. Nor could she explain how that discovery resolves
a long-standing puzzle.
Gur's
discovery that females have about 15 to 20 percent more gray matter than males
suddenly made sense of another major sex difference: Men, overall, have larger
brains than women (their heads and bodies are larger), but the sexes score equally
well on tests of intelligence.
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.
Baron-Cohen
was "bowled over" by the results. The more testosterone the children
had been exposed to in the womb, the less able they were to make eye contact at
1 year of age. "Who would have thought that a behavior like eye contact,
which is so intrinsically social, could be in part shaped by a biological factor?"
he asks. What's more, the testosterone level during fetal life also influenced
language skills. The higher the prenatal testosterone level, the smaller a child's
vocabulary at 18 months and again at 24 months.
Lack
of eye contact and poor language aptitude are early hallmarks of autism. "Being
strongly attracted to systems, together with a lack of empathy, may be the core
characteristics of individuals on the autistic spectrum," says Baron-Cohen.
"Maybe testosterone does more than affect spatial ability and language. Maybe
it also affects social ability." And perhaps autism represents an "extreme
form" of the male brain.
Depression:
Pink--and Blue, Blue, Blue
This
year, 19 million Americans will suffer a serious depression. Two out of three
will be female. Over the course of their lives, 21.3 percent of women and 12.7
percent of men experience at least one bout of major depression.
The
female preponderance in depression is virtually universal. And it's specific to
unipolar depression. Males and females suffer equally from bipolar, or manic,
depression. However, once depression occurs, the clinical course is identical
in men and women.
The
gender difference in susceptibility to depression emerges at 13. Before that age,
boys, if anything, are a bit more likely than girls to be depressed. The gender
difference seems to wind down four decades later, making depression mostly a disorder
of women in the child-bearing years.
As
director of the Virginia Institute for Psychiatric and Behavioral Genetics at
Virginia Commonwealth University, Kenneth S. Kendler, M.D., presides over "the
best natural experiment that God has given us to study gender differences"--thousands
of pairs of opposite-sex twins. He finds a significant difference between men
and women in their response to low levels of adversity. He says, "Women have
the capacity to be precipitated into depressive episodes at lower levels of stress."
Adding
injury to insult, women's bodies respond to stress differently than do men's.
They pour out higher levels of stress hormones and fail to shut off production
readily. The female sex hormone progesterone blocks the normal ability of the
stress hormone system to turn itself off. Sustained exposure to stress hormones
kills brain cells, especially in the hippocampus, which is crucial to memory.
It's
bad enough that females are set up biologically to internally amplify their negative
life experiences. They are prone to it psychologically as well, finds University
of Michigan psychologist Susan Nolen-Hoeksema, Ph.D.
Women
ruminate over upsetting situations, going over and over negative thoughts and
feelings, especially if they have to do with relationships. Too often they get
caught in downward spirals of hopelessness and despair.
It's
entirely possible that women are biologically primed to be highly sensitive to
relationships. Eons ago it might have helped alert them to the possibility of
abandonment while they were busy raising the children. Today, however, there's
a clear downside. Ruminators are unpleasant to be around, with their oversize
need for reassurance. Of course, men have their own ways of inadvertently fending
off people. As pronounced as the female tilt to depression is the male excess
of alcoholism, drug abuse and antisocial behaviors.
The
Incredible Shrinking Double Standard
Nothing
unites men and women better than sex. Yet nothing divides us more either. Males
and females differ most in mating psychology because our minds are shaped by and
for our reproductive mandates. That sets up men for sex on the side and a more
casual attitude toward it.
Twenty-five
percent of wives and 44 percent of husbands have had extramarital intercourse,
reports Baltimore psychologist Shirley Glass, Ph.D. Traditionally for men, love
is one thing and sex is...well, sex.
In
what may be a shift of epic proportions, sexual infidelity is mutating before
our very eyes. Increasingly, men as well as women are forming deep emotional attachments
before they even slip into an extramarital bed together. It often happens as they
work long hours together in the office.
"The
sex differences in infidelity are disappearing," says Glass, the doyenne
of infidelity research. "In my original 1980 study, there was a high proportion
of men who had intercourse with almost no emotional involvement at all--nonrelational
sex. Today, more men are getting emotionally involved."
One
consequence of the growing parity in affairs is greater devastation of the betrayed
spouse. The old-style strictly sexual affair never impacted men's marital satisfaction.
"You could be in a good marriage and still cheat," reports Glass.
Liaisons
born of the new infidelity are much more disruptive--much more likely to end in
divorce. "You can move away from just a sexual relationship but it's very
difficult to break an attachment," says Rutgers University anthropologist
Helen Fisher, Ph.D. "The betrayed partner can probably provide more exciting
sex but not a different kind of friendship."
It's
not that today's adulterers start out unhappy or looking for love. Says Glass:
"The work relationship becomes so rich and the stuff at home is pressurized
and child-centered. People get involved insidiously without planning to betray."
Any
way it happens, the combined sexual-emotional affair delivers a fatal blow not
just to marriages but to the traditional male code. "The double standard
for adultery is disappearing," Fisher emphasizes. "It's been around
for 5,000 years and it's changing in our lifetime. It's quite striking. Men used
to feel that they had the right. They don't feel that anymore."
By:
Hara Marano Originally published by Psychology Today:Jul/Aug 2003
