Why women live longer than men
Hong Kong women have the longest lifespan in the world. Why do women live longer than men? asks Richard Lord
If there's one area that women clearly have the upper hand over men, it's longevity. Globally, females live about six years longer than males - roughly 73 years versus 67 - and women outlive men in all but a few countries.
In Hong Kong, of nearly 40,000 people in private elderly homes, about three in five residents are female, according to figures released by the Census and Statistics Department a fortnight ago. As the age group goes up, so does the ratio of women to men: there are 16,600 people aged 85 and above, of whom 70 per cent are female.
In fact, Hong Kong women live longer than anyone else in the world at 90.8 years, according to department statistics released last year. It increased from 86.7 years in 2011, and in the process Hong Kong women overtook Japanese women, who had held the global longevity crown since 1985.
Figures for longevity in Japan were pushed down in part as a result of the 2011 earthquake and tsunami. Hong Kong men also saw their life expectancy leap last year - but it still lags far behind women's, at 84.4 years.
Health Department figures show that age-standardised death rates for the eight leading causes of death are higher for men than women.
There are many supposedly humorous answers to the question of why women live longer than men, most of which score higher on sexism than scientific credibility. But there is a range of solid evolutionary, biological and lifestyle-related factors that explain why men, despite appearances, are such terribly fragile creatures.
The reason may be biological. Females live longer than males in most species. There is evidence in animals and humans that neutered males live longer than their fully intact peers.
Scientists in Japan have created mice with entirely female DNA - effectively, two mothers - and these so-called "super-females" lived a third longer than ordinary females. Recent research has focused on the biological mechanism by which this happens.
Scientists at Melbourne's Monash University found that the DNA of mitochondria - tiny units within our cells that generate energy - may have an important role to play. A study of fruit flies found that variations in mitochondria could accurately predict life expectancy - but only in males.
"Mitochondria are involved in life's most essential function, energy production, and they have their own set of genes," explains evolutionary biologist Damian Dowling, a Monash research fellow, who led the study.
"Unlike the vast majority of genes in our DNA, we only get them from our mothers. That has implications: if nasty mutations start to turn up in these mitochondrial genes, the body's quality-control mechanism weeds them out in women.
"But this doesn't take effect in men, so they build up over the generations. DNA is a hotbed for harmful mutations. The mutations affect the way men age, but not the way women do," comments Dowling.
The main problem for human males is that they are first and foremost hunters rather than caregivers. There is an evolutionary advantage in women living longer so they can look after children, who would be largely helpless without them. Also, a woman's role in the reproductive process lasts for a longer time, and that makes it more important for their bodies to be in good shape. Women have to be fit to withstand the stresses of pregnancy, childbirth and breastfeeding.
Men, by contrast, play a short role in reproduction as, the most important thing for them is to mate. After that, their work is done, and there is less reason for them to live to an old age. But in certain species of ape, such as siamang gibbons and titi monkeys, males are as involved in childcare as females - and life expectancy of the sexes is equal.
Married men live a lot longer than single men, but married women only live a little bit longer than single women. This bodes well for the future longevity of the vulnerable human male, provided men do their fair share of late-night nappy changing.
On a broader level, early human evolution seems to have favoured strength during the reproductive phase of human life over longevity.
This theory was first put forward in the 1970s by Tom Kirkwood, now associate dean for ageing at Newcastle University's Initiative on Changing Age, in Britain.
Effectively, our genes regarded our physical bodies as short-term carriers, so recreating and thereby improving them was more important than preserving individual examples for a long time. Hunger was a bigger priority, so we adapted to be as strong as possible during the more evolutionarily important phase of our lives.
"Biological differences are of considerable importance but the main issue here is concerned with hormones rather than DNA," says Kirkwood, who describes the Monash study as "interesting but probably only a minor factor at best".
"A key difference between men and women relates to the biology of reproduction. The female contribution to successful reproduction is very direct, through nurturing the fetus inside her body and through breastfeeding afterwards. Therefore, any physiological deterioration in the female body, or soma, will have adverse consequences for her evolutionary fitness.
"For the male, reproductive success is determined much more by the capacity to achieve mating. Across the animal kingdom it is very clear that males generally are driven by their hormones to be highly competitive in ways that are not necessarily compatible with long-term somatic well-being. Therefore ... in important respects the female soma is less disposable than the male soma."
Behaviour that is "not necessarily compatible with long-term somatic well-being", of course, is another way of saying men tend to do stupid stuff. Smoking is the biggest cause of preventable death; Hong Kong government figures suggest that about 20 per cent of men smoke, but only five per cent of women indulge. Men are also far more likely to be overweight, and to drink alcohol to excess: globally, 6.2 per cent of all male deaths are alcohol-related, compared to only 1.1 per cent of female deaths.
Men are also far more likely to be involved in violence, including wars. "Men are more likely to indulge in risky behaviour such as drink-driving and dangerous sports," says Linda Hui, chief physician at Hong Kong's Matilda Medical Centre in Tsim Sha Tsui.
"A higher proportion of men sustain injuries or die from road traffic accidents. They also take up more dangerous work roles compared to women and therefore place themselves at a higher risk of injury and death."
When they do get ill, men are less likely to go to a doctor. According to a 2009 Hong Kong government Thematic Household Survey Report, 19.2 per cent of women have regular medical check-ups but only 14 per cent of men do. The same is true of psychological conditions: "Women are more likely to seek help for mental health problems from their primary physician," says Hui.
But the lifestyle decisions men make, argues Dowling, are partly influenced by their biology, so they could be the effect as much as the cause. "Some of the discrepancy is caused by lifestyle factors. Men are more likely to get into fights, to drive dangerously, to drink alcohol, and those decisions are regulated by the male hormone testosterone."
Kirkwood agrees. "But there are also sociocultural factors at play. It is sometimes suggested that differences in work, leisure, and smoking and drinking habits, are the cause of the lifespan difference. There might be something in this, but the gap between male and female sociocultural factors is narrowing," he says.
But he feels there is some hope for the male species. "One might argue that in a more 'civilised' world, males won't need to compete for access to mates in quite the old ways, as caricatured by the traditional caveman image of the guy with a big club, or the bar-room brawl.
"A significant factor will be what biologists call sexual selection. If women call the tune in mate selection, and if they go for non-aggressive types, then there might be selection for some reduction in levels of the hormonal difference," Kirkwood says.
So there we have it: ladies, our future is in your hands.