If you could take an immense group snapshot of everyone in the United States today, it would contain six million more females than males. In this country, women outlast men by about seven years. Throughout the modern world, cultures are different, diets are different, ways of life and causes of death are different, but one thing is the same -- women outlive men.

It starts before birth. At conception, male fetuses outnumber female by about 110 to 100; at birth the ratio has already fallen to about 105 boys to every 100 girls. By age 30, there are only enough men left to match the number of women. Then women start building a lead. Beyond age 80, there are nearly twice as many women as men.

"If you lookat the top ten or 12 causes of death," says Deborah Wingard, an epidemiologist at the University of California at San Diego, "every single one kills more men." She rattles off one melancholy fate after another - heart disease, lung cancer, homicide, cirrhosis of the liver and pneumonia. Each kills men at roughly twice the rate it does women.

A century ago American men outnumbered and outlived the women. But in the 20th century,women began living longer, primarily because pregnancy and children had become less dangerous. The gap grew steadily. In 1946, for the first time ever in the United States, females outnumbered males.

Part of the reason are self-inflicted. Men smoke more than women, drink more and take more life-threatening chances. Men are murdered (usually by other men) three times as often as women are.They commit suicide at a higher rate and have more than twice as many fatal car accidents as women do. Men are likely to be involved in alcohol-related fatalities. Men drivers!

But behavior doesn't explain away the longevity gap. Nor is stress the answer. In the 1950s, as heart disease claimed more and more male victims, pressure in the corporate boardroom was blamed. Let women venture out of the home and into the line of fire, doctors said, and they would begin duingat the same rate as men. But a funny thing happened on the way to the funeral. Between 1950 and 1985, the percentage of employed women in the United Stated nearly doubled. Those working women, several studies have found, are as healthy as women a home.

Today, some scientists studying the gender gap believe that the data point to one conclusion: Mother Nature may be partial to women.

Every living thing is assembled according to instructions on its chromosomes, and humans have 23 pair of them. But in males, one of these is a vulnerable non- matching pair, denoted by "xy".The corresponding pair in females is "xx", and its genetic "backup" power is sometimes cited as a clue to woman's superior resilience. If the male's single "x" chromosome is defective, it is possible for a serious genetic disorder to appear. Hemophilia and certain types of muscular dystrophy, for instance, are diseases caused by a defect in a single gene on the "x" chromosome,They are far more common in males than females.

The single "x"theory has problems, though. There just aren't enough cases of the most feared genetic diseases to account for more than a tiny bit of the longevity gap between men and women. And some researchers pin the blame directly on the male "y" chromosome.

The answer may rather be hormones. Before age 40, when virtually all women are still producing estrogen, heart disease kills three men for every woman. But from that point onward, the odds in favor of women drop steadily. For both sexes, heart disease is the leading cause of death. But women have an extra decade before their mortality rate for heart disease approaches that of men.

If estrogen is the heroine of story, testosterone, the male sex hormone, may be the villain. Until puberty, boys and girls have the same cholesterol levels. But when boys and girls have the same cholesterol levels. But when boys hit adolescence and testosterone kicks in, their level of HDL cholesterol, "good cholesterol,"plunges. In girls, HDL levels hold steady. In both sexes, LDL, "bad cholesterol" levels rise in late adolescence. But the increase is somewhat steeper in men.

Not every difference between the sexes favors women. On average, men are taller than women and have heavier bones and bigger muscles. Men will die sooner, but we'll have hit more home runs by the time we go.

While women turn out to be less vulnerable than men to life-threatening diseases, they are more vulnerable to everyday sicknesses and pain. In 1676 one diarist noted, "I have heard physicians say they have two women patients to one man." Women still make more visit to the doctor than men do, take more prescription and non-prescription drugs and spend more days in bed. They are plagued by arthritis, bunions, bladder, infections, corns, hemorrhoids, menstrual woes, migraines and varicose veins.

In the meantime, men get heart attacks and strokes. Women are sick, but men are dead.

Mental health? Depression is more common in women than in men. But schizophrenia, perhaps the most devastating mental illness, often affects men more severely.

After a spouse dies, men seem to fare worse than women. They are more depressed, more likely to fall ill and more likely to die. As a result, nearly 80 per cent of the population over 65 years old and living along are women. Men fare poorly, it seems, because in many cases their wives were their sole confidantes. Without a spouse, new widowers fall and sink. Women who lose a husband, in contrast, often have a circle of close friends to confide in and count on.

But behavior changes, so the health gap between men and women isn't a fixed feature of the landscape. In recent decades, the gap between men and woman's life-spans has narrowed from 7.6 years in 1970 to an estimated 6.8 years in 1990. The explanation is not that women's health is deterioration. Women's health is improving, but men's is improving faster.

Men are smoking less, drinking less and eating better. "The gap isn't shrinking because women are acting like men," says epidemiologist Wingard. "It's shrinking because men are behaving more like women."