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Both biological and social factors affect life expectancy. Biology strikes first: during the 12 months of infancy (in the absence of any outside influence), male mortality is typically 25 to 30 percent greater than female mortality. Some 105 males are born for every 100 females, ensuring that the number of men and women will be about the same at reproductive age. Hormones also play a role in longevity. The female hormone estrogen helps to eliminate "bad" cholesterol (LDL) and thus may offer some protection against heart disease. In contrast, testosterone, found in greater amounts in males, may make men more likely to engage in violence and risk-taking behavior. The female body's ability to adapt to pregnancy and breast-feeding appears to help women manage excess calories more easily than men do. Finally, women gain an additional biological advantage because of their two X chromosomes. If a gene mutation occurs on one X, women's second X chromosome can compensate. In comparison, all the genes on men's sole X chromosome are expressed, even if they are deleterious.

Biology is not the whole story, however: social factors contribute a great deal to longevity. Although male and female life habits have been converging in the industrial world, this convergence is not absolute. Females tend to smoke fewer cigarettes, drink less alcohol and drive more carefully. On average, their professional activities are less prejudicial to their health.