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The Sicker Sex

M. Zuk

Published 2009 in PLoS Pathogens

ABSTRACT

Arguments about the weaker sex notwithstanding, there is no contest about the identity of the sicker sex—it is males, almost every time. Everyone knows that old age homes have more widows than widowers, but the disparity extends far beyond the elderly. Fewer women than men died in the 1917–1918 influenza epidemic; the differential mortality was not related to World War I, as originally thought, but was global and widespread among ages. Kruger and Nesse [1] compared men's and women's mortality rates for 11 causes of death in men and women from 20 countries, including accidents and homicide as well as infectious and non-infectious diseases, and found that men virtually always die earlier. They concluded, “Being male is now the single largest demographic risk factor for early mortality in developed countries”. Furthermore, in many free-living mammals, males are more likely than females to harbor parasites or to suffer more intensely from their effects. During the mid-20th century, a virtual cottage industry developed in which investigators experimentally infested laboratory rodents with parasites and documented any resulting sex differences in the prevalence or intensity of the infection that developed [2]. Males usually developed higher parasitemia, with castration removing the sex difference. The persistence of these patterns in the laboratory suggests that the sex difference is not merely due to differences in exposure to parasites, with males and females behaving differently in the field and hence incurring different risks of infection, but to an inherent sex difference in vulnerability. What causes this disparity between the sexes in longevity and parasite susceptibility? Most research has focused on the proximate mechanisms, such as endocrine or immunological pathways, that are immediately responsible for any one cause. Here, I take a different approach. Sex differences in infection rates or mortality may come about for the same reasons as other differences between males and females, such as morphology: selection acts differently on the sexes because they maximize their fitness in different ways. Below I discuss an evolutionary approach to the question of why males so often die sooner and develop more diseases than females [3],[4]. Some researchers are hopeful that the gap between men's and women's lifespans will close as we develop better medical care and education about health risks, but I will argue instead that the disparity is not going away any time soon.

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