Why Women Live Longer than Men
By William J. Cromie
Gazette Staff
Studying people who live 100 years and more leads Harvard researchers
to conclude that menopause is a major determinant of the life spans of both
women and men.
Women's life span depends on the balance of two forces, according to
Thomas Perls, a geriatrician at Harvard Medical School. One is the evolutionary
drive to pass on her genes, the other is the need to stay healthy enough
to rear as many children as possible. "Menopause draws the line between
the two," Perls says. It protects older women from the risks of bearing
children late in life, and lets them live long enough to take care of their
children and grandchildren.
As for men, Perls believes "their purpose is simply to carry genes
that ensure longevity and pass them on to their daughters. Thus, female
longevity becomes the force that determines the natural life span of both
men and women."
"Most animals do not undergo menopause," adds Ruth Fretts,
an obstetrician-gynecologist at Harvard-affiliated Beth Israel Deaconess
Medical Center in Boston. "It seems that menopause evolved in part
as a response to the amount of time that the young remain dependent on adults
to ensure their survival."
Pilot whales, for example, suckle their young until age 14, and they,
along with humans, are two of the few species that menstruate.
Human females eventually become so frail that bearing children involves
a high risk of death. Earlier in evolution, that was as young as 35 to 40
years old. "Anyone who developed a genetic alteration that caused infertility,
i.e., menopause, obtained a survival advantage over females who continued
to be fertile and died bearing children," Perls says.
The Gender Gap
This reasoning, however, does not explain why women live so much longer
than men. "In all developed countries and most undeveloped ones, women
outlive men, sometimes by a margin of 10 years," Perls and Fretts note.
"In the U.S., average life expectancy at birth is about 79 years for
women and about 72 years for men."
The gender gap is most pronounced in those who live 100 years or more.
Among centenarians worldwide, women outnumber males nine to one. Perls and
Fretts are studying all centenarians from eight cities and towns around
Boston, 100 people in all. Eighty-five are women.
The mortality gap varies during other stages of life. Between ages 15
and 24 years, men are four to five times more likely to die than women.
This time frame coincides with the onset of puberty and an increase in reckless
and violent behavior in males. Researchers refer to it as a "testosterone
storm." Most deaths in this male group come from motor vehicle accidents,
followed by homicide, suicide, cancer, and drownings.
After age 24, the difference between male and female mortality narrows
until late middle age. In the 55- to 64-year-old range, more men than women
die, due mainly to heart disease, suicide, car accidents, and illnesses
related to smoking and alcohol use. Heart disease kills five of every 1,000
men in this age group.
"It seems likely that women have been outliving men for centuries
and perhaps longer," say Perls and Fretts. Even with the sizable risk
conferred by childbirth, women have outsurvived men at least since the 1500s.
Although, in the United States between 1900 and the 1930s, the death risk
for women of childbearing age was as high as that for men. Since then, improved
health care, particularly in childbirth, has put women ahead of men again
in the survival struggle, as well as raising life expectancy for both sexes.
A longer life doesn't necessarily mean a healthier life, however. While
men succumb to fatal illnesses like heart disease, stroke, and cancer, women
live on with non-fatal conditions such as arthritis, osteoporosis, and diabetes.
"While men die from their diseases, women live with them," Perls
comments.
One contributor to the gender difference in life span is the influence
of sex hormones. The male hormone testosterone not only increases aggressive
and competitive behavior in young men, it increases levels of harmful cholesterol
(low-density lipoprotein), raising a male's chances of getting heart disease
or stroke.
On the other hand, the female hormone estrogen lowers harmful cholesterol
and raises "good" cholesterol (high-density lipoprotein). Emerging
evidence suggests estrogen treatment after menopause reduces the risk of
dying from heart disease and stroke, as well as of dying in general.
Perls and Fretts believe that longer life means survival of the fittest,
and women, evolutionarily speaking, are more fit than men. The longer a
woman lives and the more slowly she ages, the more offspring she can produce
and rear to adulthood. Therefore, evolution would naturally select the genes
of such women over those who die young.
Long-lived men would also have an evolutionary advantage over their shorter-lived
brethren. However, says Perls, "studies of chimps, gorillas, and other
species closely related to humans suggest that a male's reproductive capacity
is actually limited more by access to females than by life span. And because
men have not been involved in child care as much as females, survival of
a man's offspring, and thus his genes, depended not so much on how long
he lived, but on how long the mother of his children lived."
In their studies of centenarians, Perls and Fretts found that a surprising
number of women who lived to be 100 or more gave birth in their forties.
These 100-year-old women were four times as likely to have given birth in
their forties as women born in the same year who died at age 73. A study
of centenarians in Europe by the Max Planck Institute of Demography in Germany
found the same relationship between longevity and fecundity.
This does not mean that having a child in middle age makes a woman live
longer. Rather, Perls says, "the factors that allow certain older women
to bear children -- a slow rate of aging and decreased susceptibility to
disease -- also improve a woman's chances of living a long time. Extending
that idea, we argue that the driving force of human life span is maximizing
the time during which woman can bear children. The age at which menopause
eliminates the threat of female survival by ending further reproduction
may therefore be the determinant of subsequent life span."
Closing the Gap
If this is true, then the genes of female centenarians hold the secrets
of a longer, healthier life. And these are no ordinary genes. Whether the
average person drinks, smokes, exercises, or eats her vegetables adds or
subtracts five to ten years to or from her life. But to live an additional
30 years requires the kind of genes that slow down aging and reduce susceptibility
to conditions such as Alzheimer's disease, stroke, heart disease, and cancer.
Clues about what those genes are and how they work could come from studying
those who survive 100 years or more, Perls believes. The New England Centenarian
Study he runs is the only scientific investigation of the oldest oldsters
being done in the United States. He has now expanded it to include all centenarians
in the city of Boston, about 100 more people.
"We think that centenarians are a tremendous resource for the discovery
of genes responsible for aging and the ways in which aging occurs,"
says Perls. "Finding these genes could lead to testing people and determining
who might be disposed to accelerated aging via diseases such as Alzheimer's,
cancer, heart disease, and stroke. Such individuals might eventually be
treated to extend the prospect of their living longer."
The oldest person for which reliable records exist was a woman who recently
died in France at the age of 123. "Reaching such an age is like winning
the lottery," Perls comments. "The odds are about one in 6 billion.
From a practical point of view, we can consider 100 years as the average
maximum of human life. We're not there yet, of course. At present, average
life expectancy for those born after 1960 is about 85 years."
Although women can expect to live longer than men, the gap is closing.
Death rates have begun to converge in the past 20 years. Some researchers
attribute the convergence to women taking on the behaviors and stresses
formerly considered the domain of males -- smoking, drinking, and working
outside the home.
For example, Perls and Fretts point out that deaths from lung cancer
have almost tripled in women in the past 20 years. One study concluded that,
on average, middle-aged female smokers live no longer than male smokers.
"Smoking," Perls and Fretts conclude, "seems to be the
'great equalizer.'"
Copyright
1998 President and Fellows of Harvard College
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