From the moment of conception, males experience greater mortality compared to females. For reasons that are unclear, Y chromosome-bearing sperm are more likely to fertilize an egg than X-bearing sperm, leading to roughly 170 male zygotes for every 100 female zygotes. The spontaneous miscarriage ratio is 130/100 and at birth there are slightly more male births than female births (about 105 males per 100 females). This leads to genderparity at the beginning of reproductive age but from that point on females outnumber males. Beyond middle age, men have about 75-80 percent of the longevity remaining for comparably-aged women, although there is considerable societal diversity. For example, the gender differential in life expectancy from birth is about 6.5 years greater for women in the U.S., 5.3 years in the UK, 12 years in Russia, and about 6 months in India. This societal diversity implies that social factors have an influence on male and female longevity. While women have a biological advantage, historically certain living conditions and social customs tended to negate this benefit. For example, iron deficiency anemia due to female blood loss contributed in the past to excess female mortality. Social differences between men and women and the challenges of motherhood have further compromised women's biological advantage.
The biological basis for female longevity is clear: The female of every species outlives the male with very few exceptions. Having two "X" chromosomes provides women with a backup when a genetic mutation occurs on one of the genes, while men only have a single "X" chromosome to express all their genes, whether damaged or not. Perhaps this genetic reserve allows a greater repertoire of effective neural, endocrine, and immune responses to potentially deleterious environmental demands.
Female hormones, particularly estrogen, and the resiliency of the female body to accommodate pregnancy and breast-feeding are also thought to promote longevity. Estrogens have beneficial effects on lipids in the blood and seem to protect a woman from premature heart disease.
There are major differences in male and female behaviors in veterinary and human medicine, and sexual differences in the human brain appear very early in life. Ultrasound images of a developing fetus around 26 weeks old showed that the corpus callosum that links to communication between the right and left hemispheres of the brain was larger in girls than in boys. In adults, female brains show language activity on both sides of the brain, while male brains tend to use mainly the left side. Boys also tend to have more learning disabilities, dyslexia, and stuttering than girls. The male brain seemed favored in the areas of geometry and mathematics. The portions of the brain devoted to controlling our impulses of aggression and anger are larger in women.
Male behaviors tend to be riskier, and more men are exposed to hazards in the workplace. Men experience more motor vehicle accidents and have higher rates of alcoholism and smoking. In addition, males tend to drive more aggressively than females, and men on average smoke more cigarettes and drink more alcohol than women. These excess risks in men tend to increase the likelihood of heart disease and malignancy.
The ways the genders relate to their bodies is another potential factor explaining male and female longevity differences. Women tend to be more health conscious and attend to their bodies, seeking preventive health care and practicing healthier lifestyles compared to men. Men relate to their bodies with strength and power and expose them to risks from early ages.
Of course, other social factors could also have an influence on gender longevity. The perception of temperature sometimes generates marital tensions. The topic came up at a medical conference on aging, and after the discussion, an elderly man told me, "It is perfectly clear why women outlive men: Their wives freeze them to death."
Another way to approach the longevity differential between women and men is to look at the causes of death across the life span. Graphs of mortality ratios by gender show excess male mortality by violence that peaks at 4:1 around age 30. Gender-specific mortality rates for the top 10 causes of death shows excess male-to-female ratio for all conditions except diabetes mellitus, for which the risk is identical. Whether this gender gap relates to biological differences or lifestyle choices such as cigarette smoking is a matter of debate. Perhaps diabetes in women somehow reduces the gender-specific advantage enjoyed by women for other diseases.
Nonetheless, the consequences of atherosclerosis clearly affect the longevity differential between men and women. Roughly speaking, if all atherosclerotic causes of death were immediately eliminated, the gender mortality gap would be cut in half.