THIS SOUNDS like the clumsy set-up to a bad joke, so sorry to disappoint, but have you ever wondered why women outlive men? In Ireland, the life expectancy from birth for a man is 78 years; for a woman it’s 82.5 years – very impressive in a global context. Yet as the life expectancy has gone up (a decade ago it was 75 and 80, respectively), the gender gap has narrowed a little.

So why do women have the edge, and why has it been blunted a little? The answer is still widely debated. There are some who come very firmly down on the side of biology. In the natural world, the female of the species tends to outlive the male, so humans are the norm in that regard. One biologist, Thomas Kirkwood, has suggested that it may be evolution developing along logical lines: male bodies are “more disposable” than those of females, who have a childbearing role. So men degrade more quickly.

There are many who argue that other factors are at work – most notably lifestyle. Men die earlier, they say, because they drink more, smoke more, work in more dangerous environments, get stressed, are more likely to get murdered, join the military and so forth. They’re more likely to die from heart disease and violent death and are more at risk from cancer.

There are great variations in this, depending on where you live and where you work. However, in a week when rising lung-cancer rates among Irish women were in the news, it’s notable that much research has identified one particular factor in the gender gap: smoking.

Cigarettes kill more men than women, but women are increasingly making up ground. Tobacco is being identified as a significant factor both in the gender gap as it stands but also in a narrowing that is occurring in Europe.

Traditionally, more men than women have smoked. In many parts of the world, the gap is stark: the ratio is as much as 5:1 in some parts of the developing world. But in Ireland that ratio is far closer. More men (25 per cent, according to the Office of Tobacco Control) than women (22.2 per cent) smoke. This compares with global rates of 48 per cent and 12 per cent.

Why is the gap so small? Among the theories is one published earlier this year by the World Health Organisation, which suggested that high smoking rates among women go hand in hand with the increase in “female empowerment”. In countries where equality has been hard won, smoking becomes a way in which women express their freedoms. In developing or underdeveloped countries, women, for cultural and financial reasons, are far less likely to smoke. That study does not suggest that less freedom means better lives for women, or that equality should be curtailed simply to prevent women becoming addicted to tobacco – it is at pains to say that – but this statistical study is joined by other recent research focusing on women, smoking and the consequences for them and society.

Earlier this month, the Lancet published research that said that women who smoke have double the chance of developing lung cancer than men. It seems that women suck in and store more of the carcinogens, but, whatever the physiological reason, the research now appears to be borne out by Irish cancer rates.

According to the National Cancer Registry figures released this week, while male lung cancer rates have decreased as smoking rates have declined, the disease has become a bigger killer of women than breast cancer. Between 1994 and 2008, lung-cancer rates among women increased by more than 2 per cent per year.

Ninety per cent of all lung cancer – the third-most-common cancer – is attributable to smoking. Overall, smoking in Ireland is declining, although it is clearly a major problem that almost a quarter of the entire population still smokes. Meanwhile, social class continues to jump out of the statistics: the poorer the background, the more likely a person is to smoke. But while smoking is down over the past decade across every age group, there remains this quirk about gender.

Younger female smokers have been identified as the reason behind the growth in female-lung-cancer statistics, and one Department of Health study a few years ago showed that, among 15- to 17-year-olds, girls outpaced boys by 8 per cent. Increasingly, education about tobacco needs to be gender- specific, because, even as life expectancy in Ireland grows, this statistical tumour is growing.

shegarty@irishtimes.com

Twitter: @shanehegarty