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 University 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.
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