Women live longer than men because their bodies are better at repairing themselves. In an article in November's Scientific American, Professor Tom Kirkwood of Newcastle University argues that women have to be better at fixing the wear and tear on their cells in order to have healthy offspring.

In interviews last week, Kirkwood said that it made biological sense for men to be more disposable and therefore die younger. The body is disposable, argues Kirkwood, because the genes are passed to the next generation.

"This theory is widely accepted now," Kirkwood says. "Ageing is not driven by a clock."

Ageing is regulated by genes that specify the levels of maintenance that repair the damage to cells. Much of this damage is inevitable and caused by free radicals, a byproduct of the chemical reactions in cells that produce energy. This damage may injure the cells' DNA or their membrane, which need to be repaired. Some of the damage is caused by external factors, such as smoking. But in Kirkwood's theory, ageing is not pre-programmed because the genes that repair cells can be modified.

The gap between men's and women's life expectancy has actually been closing in this country and is now 4.2 years (a narrowing from six years over the past 27 years). A boy born today lives to an average 77.7 years, compared with a girl, who would be expected to reach 81.9. The differential is thought to be due to higher rates of heart disease and risk-taking in men; oestrogens have protected women from heart disease.

But this would still fit with Kirkwood's theory because these factors could influence how the maintenance genes are expressed. "It's important to understand what influences longevity – how much is societal and could it disappear?" says Kirkwood, who believes biology dictates it can never disappear. "This theory matters when we are discussing things such as when women should start their pensions. For a long time, we've had a paternalistic attitude that women are the weaker sex."