Sex secret
Question: Why do women live longer than men?
Answer: The diagram shows the probability of men and women dying within a year, up to age 40. The figures are taken from tables for the population of England and Wales in the period 1990-92, but similar patterns are seen in other industrialised nations and for other periods.
Men are more likely than women to die at any age throughout the whole of their lives. The shape of the graph and, in particular, the surge of male deaths around the time of adolescence, points to some of the key factors involved in the first 40 years. Beyond that age, the two curves continue to rise smoothly, while diverging slightly further down the line. The key reasons behind the differences in the two curves are:
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1. Baby girls are less susceptible to almost the whole range of childhood illnesses, although improvements in medical science this century have lessened the impact of this.
2. Young men-especially around age 17-die far more frequently in road, and, to some extent, other accidents. The mortality rates for boys in their late teens are nearly three times as high as those for girls of the same age, and the difference is due almost solely to road accidents.
3. Men spend more time out of the house and, in particular doing manual work, and so fall prey to industrial injuries more often, although the effect has decreased over the past century.
4. Men are more susceptible than women to heart attacks and other cardiovascular diseases. This is now the main cause of the mortality differential between the sexes.
Important general indicators of life expectancy are smoking, marital status and social class. Smokers die earlier and married people live longer than people who are single and divorced. At age 15, in England, those in the top social class can expect to live almost four years longer than those in the lowest social class.
Andrew Wells Fellow of the Institute of Actuaries, Rochester, Kent
Answer: There are four main reasons: men have higher accident rates especially when they are aged under 25; higher smoking rates for men; higher suicide rates, again especially for younger men; and a greater tendency to work in more stressful jobs. I expect that these differences should even out over time, and that the life expectancy rates will get closer.
Richard Turle Gloucester, Ontario
Answer: A family group in which older women play a part as mothers and grandmothers in rearing children is likely to produce good survival rates. Natural selection will favour that group. The men may hunt and bring in some food, but the very fact that they are hunters means that they are not in contact with the children. If a man dies while hunting, the group as a whole can still survive. While we do not need older men, older women aid survival, so they themselves survive longer.
Geoffrey Watson Winchester, Hampshire
The first two answers focus on the reasons why men in industrialised countries are more likely to die than women. The last raises a deeper evolutionary issue and leads naturally to the question of how widespread the difference between males and females is across cultures and across species.
Life expectancy figures show that women live longer than men in almost all regions of the world, with the differences being biggest in the industrialised nations. In the few small areas where men live longer than women, the difference is caused by a higher mortality rate among female infants and is linked to social systems that value male offspring over females.
The answers above show that many of the differences in life expectancy are caused by behaviour: young men take more risks, men have generally worked in more dangerous environments and they suffer more stress. Would the difference disappear if men adopted more similar lifestyles to women? We don鈥檛 really know, but as some of the risk-taking behaviour of males is linked to a surge in male hormones at adolescence, it is interesting that there is evidence that eunuchs have slightly longer life expectancies than normal males.
Among higher animals, females generally live longer than males, although there are exceptions. In his recent book, Time of Our Lives: the Science of Human Ageing, (Weidenfeld & Nicolson, p 184), Tom Kirkwood points out that male wolves live at least as long as females. The reasons are not known.
A general evolutionary explanation for why females live longer than males might be as given in Geoffrey Watson鈥檚 answer. If males lived longer, they would be unlikely to pass on more of their genes or sufficiently help to ensure the survival of those who carry their genes. Females might do the latter and so longer-lived females have been at a selective advantage. Or, as Kirkwood puts it, 鈥淚n a mammal, the male鈥檚 contribution to child raising is often much less than the female鈥檚. This makes the female intrinsically much less disposable than the male.鈥� This theory has not been tested, as far as we know-Ed
This week鈥檚 question
Plant life: My garden contains a number of day lilies which, as their name suggests, flower for only a very short time. Other flowers keep going for weeks. How are different plants able to adopt such different strategies? Surely any flower should have evolved to keep going until pollination has taken place and then wither?
David Henry Washington DC