The average woman on the planet can expect to live about five years longer than the average man. Comparison with other species suggests that females live longer because they are primary caregivers for children. Yet, the real reason women outlive men may be that they take better care of themselves.
The caregiver theory
In every society, women do more of the child care than men. According to the caregiver theory, women live longer than men because they are more essential to the survival and well-being of children. Because motherless children had poor survival prospects, natural selection ensured that ancestral women were good at surviving. This meant avoiding unnecessary risks.
Males of some other primate species are more involved in care of offspring than men are and this gives them an advantage in terms of life expectancy (1). If care giving duties are about equal, males live as long as females in the case of siamangs (a type of ape) and titi monkeys.
The star exhibits here are owl monkeys, -- a South American species in which the male does most of the carrying from birth onwards. Fathers hand off their charges to the mother only when they are being nursed. The father is critical for survival of owl monkeys: if the male dies, the female refuses to carry the infant except when it is being nursed.
Male owl monkeys are more likely to survive than females and this survival advantage becomes noticeable at about the age they begin to raise young. The care-giver explanation thus looks convincing but it is not the only game in town. Its masculine counterpart is the notion that sexual competition favors risk-taking by males thereby reducing masculine life expectancy.
The risky lifestyle explanation
Males are generally more boisterous and reckless than females and experiments on monkeys find that the difference is explainable in terms of prenatal exposure of the brain to testosterone.
Evolutionists generally attribute male riskiness to male-male competition over mates. Analysis of human mortality statistics supports the risky lifestyle theory. Women take much better care of their own health and men are much more likely to die from violence, and accidents, and from neglecting their medical care.
According to researcher Will Courtenay (2), risky lifestyle accounts for most of the human gender difference in life expectancy, suggesting that any fixed biological differences (e.g., metabolic rate, Y-chromosomes, exposure to testosterone versus estrogens) make only minor differences.
How does one reconcile the care-giving theory and the risky lifestyle theory? Perhaps the two explanations are really just different facets of the same idea. Risk taking is elevated by sexual competition in men but riskiness is reduced by care giving for women.
Women are more risk averse than men and this risk aversion is particularly noticeable if they have young children. Married men are also much lower on risk-taking than single men and even produce less testosterone. Married men have much lower mortality rates than their single counterparts (3).
Women live longer than men mostly because they avoid a risky lifestyle. They avoid risk because they are primary care-givers. Men lead shorter lives because they take greater risks, particularly if single and dating. Specific risks are described in a future post.
1. Allman, J., Rosin, A., Kumar, R., & Hasenstaub, A. (1998). Parenting and survival in anthropoid primates: Caregivers live longer. Proceedings of the National Academy of Sciences, 95, 6866-6869.
2. Courtenay, W. H. (2000). Behavioral factors associated with disease, injury, and death, among men: Evidence and implications for prevention. The Journal of Men's Studies, 9, 81-142.
3. Waite, Linda J., and Maggie Gallagher. 2000. The case for marriage. New York: Doubleday.