If God created man first, He or She apparently took advantage of hindsight when it came to woman. Except for the moment of conception (when 13 to 15 males are conceived for every 10 females), the distaff side simply has a better chance at survival. Spontaneous abortions of boys outnumber those of girls. More males than females die during infancy, youth and adulthood. In every country in the world in which childbirth itself no longer poses mortal danger to women, the life expectancy of females exceeds that of males. And in the United States the gap is growing. A girl born today can look forward to nearly 79 years, seven more than a boy.
Why? Some of the answers seem to lie deep in the genes. Others undoubtedly float in the hormones that carry messages from organ to organ, even, some researchers believe, ”imprinting” each human brain with patterns that can affect the ways it responds to injury and disease.
The research suggests that females start out with some distinct biological advantages. Among them:
– Genesis was wrong. Women came first, embryologically speaking, at least. Genetically, the female is the basic pattern of the species; maleness is superimposed on that. And this peculiarity of nature has the side effect of making males more vulnerable to a number of inherited disorders.
The reason lies in the way our genes determine who is a male and who is a female. A normal embryo inherits 23 chromosomes from the mother and 23 from the father. One of these chromosome pairs, the 23d, determines what sex the baby will be. From the mother, the embryo always receives an X chromosome. From the father, it receives either an X, creating a female, or a Y, creating a male.
The Y chromosome carries little more than the genetic signal that, in the sixth week of development, first defeminizes the embryo, then starts the masculinization process. In a female, the X chromosome supplied by the father duplicates much of the genetic information supplied by the mother. Thus, if there are potentially deadly genetic anomalies on one of the female`s X chromosomes, the other may cancel their effects.
But the male embryo has no such protection: What is written on his sole X chromosome rules the day. Among the X-linked troubles he is more likely to inherit: colorblindness, hemophilia, leukemia and dyslexia.
The main task of the female sex hormones, or estrogens, is to keep the female body prepared to carry and care for offspring. But as it turns out, what is good for female reproduction also is good for the arteries. One effect of estrogens, for example, is to keep blood vessels pliable to accommodate extra blood volume during pregnancy.
That also reduces the risk of atherosclerosis. And because a developing fetus needs lots of carbohydrates but is unable to use much fat, the mother`s body must be able to break down the extra fat left behind after the fetus`
demands are met. Estrogen makes this happen by stimulating the liver to produce high-density lipoproteins (HDL), which allow the body to make more efficient use of fat, and help to keep arteries cleared of cholesterol.
The male hormone testosterone, by contrast, causes men to have a far higher concentration of low-density lipoprotein.
”LDL forms and fixes in large amounts to the lining of the blood vessels,” explains endocrinologist Estelle Ramey. ”They become narrower and more fragile.”
That didn`t matter 2 million years ago, when men were far more physically active: Exercise lowers the LDL count.
Long after menopause, when estrogen production drops dramatically, women maintain the cardiovascular advantages built up during their childbearing years. The Framingham study, a 24-year examination of the health of almost 6,000 men and women between the ages of 30 and 59, found approximately twice the incidence of coronary heart disease in men as in women, even in the upper age range. And in an analysis of the health patterns of 122,000 U.S. nurses, Graham Colditz, assistant professor at Harvard Medical School, has found that women who use estrogen supplements after menopause cut their risk of heart attacks by a third.
So would men live longer if they took doses of estrogens? So far, the answer is a resounding no. In experiments in which men received estrogen supplements, ”they dropped like flies” from heart attacks, says Elaine Eaker, an epidemiologist at the National Institutes of Health. Eaker speculates that men don`t have the proper receptor sites for estrogen.
But there may be hope for greater longevity in highly experimental work on macrophages, cells that form part of the immune system. Macrophages, Ramey explains, ”gobble up” unmetabolized glucose that randomly affixes itself to DNA and eventually would cause damage. As people age, the macrophage system slows, and the damage gets worse.
”Macrophage activity in females, because of estrogen, is much higher,”
Ramey says. It is the hope of researchers that they can find a way to increase and prolong that activity in both sexes.
– ”Women,” Ramey declares flatly, ”respond better to stress.”
Although the evidence on how stress hurts the human body still is equivocal, there are two main hypotheses. The first is mechanical: Elevation of heart rate and blood pressure caused by stress promotes damage to the inner lining of the artery wall, laying the groundwork for heart disease. The second is chemical: Increased production of stress hormones promotes arterial damage.
Ramey is one scientist who is convinced that stress does damage. And she puts the blame squarely on testosterone, and the fact that while the world has changed substantially, men`s bodies have not.
In the world of primitive man, ”testosterone is the perfect hormone.”
In effect, it orders neuroreceptors in the brain to drop everything else and react as quickly as possible to a release of stress hormones. This greater reaction to stress may be damaging in the long run, but the short-term benefits were much more important in an age when ”the life expectancy was about 23,” Ramey says. Now, when the average man is less likely to be threatened by a saber-toothed tiger than by a corporate barracuda, his stress reaction is exactly the same, and just as damaging to long-term health.
Perhaps because testosterone isn`t egging them on, women seem to respond to stressful situations more slowly and with less of a surge of blood pressure and stress hormones. Some researchers suspect that psychosocial factors also play a big role.
Dr. Kathleen Light, a specialist in behavioral medicine at the University of North Carolina, thinks women may have a different perception of just what situations are threatening. Women show much less stress than men, for instance, when asked to solve an arithmetic problem. But Light`s preliminary data in a study of public speaking show that women experience about the same surge in blood pressure as men.
”Women may respond more selectively than men,” she suggests. ”We think this reflects learned experience.”
But Dr. Karen Matthews, of the University of Pittsburgh, is not so sure. Matthews, associate professor of psychiatry, epidemiology and psychology at the university, says postmenopausal women show higher heart rates and produce more stress hormones than women who are still menstruating. This leads her back to the reproductive hormones. One possible conclusion: Estrogens may be better adapted than testosterone for the flight-or-fight situations of modern life.
– Men`s and women`s brains really are different. Over the last decade researchers have discovered that in women, functions such as language ability appear to be more evenly divided between the left and right halves of the brain; in men, they are much more localized in the left half. After strokes or injuries to the left hemisphere, women are three times less likely than men to suffer language deficits.
What accounts for these differences in brain organization? One clue: The central section of the corpus callosum, a nerve cable connecting the left and right halves of the brain, seems to be thicker in women than in men, perhaps allowing more right-brain left-brain communication.
Many researchers think that sex hormones produced early in fetal development as well as after birth literally ”sex” the brain. In young animals, says neuroendocrinologist Bruce McEwen, of New York`s Rockefeller University, ”the brain cells respond to testosterone by becoming larger and developing different kinds of connections.”
These changes add up to big behavioral differences. Inject a female rat pup with testicular hormones, for instance, and it will mount other females just like a male. And it is not just a matter of mating. Male rat pups deprived of testicular hormones perform more poorly on maze tests than normal males; young females injected with testicular hormones do better. Many researchers are convinced that hormones have similar effects on human brains. Such findings are controversial. Feminist scholars in particular fear that they will give new life to the notion that biology is destiny, and that females just aren`t the equal of men at certain tasks. But biodeterminists tend to ignore a critical difference between humans and other animals: The hugely complex human brain is not simply the sum of its synapses. There are other factors at play.
