Evolutionary
forces explain why women live longer than men
ANN ARBOR, Mich.---Despite research efforts to find modern factors that would
explain the different life expectancies of men and women, the gap is
actually ancient and universal, according to University of Michigan
researchers.
"Women live longer in almost every country, and the sex
difference in lifespan has been recognized since at least the
mid-18th century," said Daniel J. Kruger, a research scientist in
the U-M School of Public Health and the Institute for Social
Research. "It isn't a recent trend; it originates from our deep
evolutionary history."
This skewed mortality isn't even unique to our species; the
men come up short in common chimps and many other species, Kruger
added.
Kruger and co-author Randolph Nesse, a professor of
psychology and psychiatry and director of the Evolution and Human
Adaptation Program, argue that the difference in life expectancy
stems from the biological imperative of attracting mates.
"This whole pattern is a result of sexual selection and the
roles that males and females play in reproduction," Kruger said,
"Females generally invest more in offspring than males and are more
limited in offspring quantity, thus males typically compete with
each other to attract and retain female partners."
For example, in common chimps, the greatest difference in
mortality rates for males and females occurs at about 13 years of
age, when the males are just entering the breeding scene and
competing aggressively for social status and females.
From the tail of the peacock to the blinged-out SUV, males
compete aggressively for female attention, and that costs them
something. In nature, it means riskier physiology and behavior for
the males, such as putting more resources into flashy plumage or
engaging in physical sparring.
And even in modern life, where most dueling is a form of
entertainment, male behavior and physiology is shortening their
lifespans relative to women, Kruger said. In fact, modern lifestyles
are actually exacerbating the gap between male and female life
expectancies.
Male physiology, shaped by eons of sexual competition, is
putting the guys at a disadvantage in longevity. Male immune systems
are somewhat weaker, and their bodies are less able to process the
fat they eat, Kruger said. And behavioral causes---smoking,
overeating, reckless driving, violence---set men apart from most
women. "Because mortality rates in general are going down,
behavioral causes of death are ever more prevalent," Kruger said.
Looking at human mortality rates sliced by socioeconomic
status shows that the gender gap is affected by social standing.
Human males in lower socio-economic levels tend to have higher
mortality rates than their higher-status peers. The impact of social
standing is greater on male mortality than on female mortality,
Kruger noted, partially because males who have a relatively lower
status or lack a mate engage in a riskier pattern of behaviors in an
attempt to get ahead, he said.
###
The paper "An evolutionary life-history framework for
understanding sex differences in human mortality rates," appears in
the spring 2006 edition of Human Nature.