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Four in 10 men over 75 say they are still having sex (but only two in 10 women)

Survey reveals a startling increase in libido among the 'Viagra generation'

The feminist academic Camille Paglia once described sex at age 90 as "like trying to shoot pool with a rope". But despite her coruscating verdict sexual life expectancy is increasing, researchers say.

A study published today shows that among 75 to 85 year olds, four out of 10 men and two out of 10 women are still having sex – or they claim to, at least.

All sex surveys based on self reports are bedevilled by the accuracy of measurement. It is hard to be sure whether the gender imbalance shows the resilience of male interest in sex or the resilience of their propensity to boast about it.

But the researchers say there is a simpler explanation. Men tend to marry women younger than themselves and die sooner; more women are widowed and thus lack a partner with whom to have sex. Men can therefore look forward to longer active sex lives than women, though women seem unperturbed by it.

The study, by academics at Chicago University in the US, estimates for the first time the "sexually active life expectancy" of men and women at different ages. It shows that at the age of 30, men have a sexually active life expectancy of nearly 35 years; for women it is almost 31 years. At age 55, the expectancy changes to almost 15 years for men and 10 years for women.

The findings, published in the British Medical Journal, are based on results from two surveys, each of 3,000 men and women. One subject group was aged 25 to 74 and the other was aged 57 to 85.

The two studies offered different definitions of "sexual activity". The first, among the younger age group, defined it as "any mutually voluntary activity with another person that involved sexual contact, whether or not intercourse or orgasm occurred" during the previous six months.

This elaborate construction may reflect the confusion many Americans feel about what counts as sexual activity. When Bill Clinton famously declared of his aide Monica Lewinsky "I did not have sex with that woman", he was telling the truth in the eyes of a large percentage of his countrymen (and women) whom surveys showed agreed that oral sex did not count as "having sex".

The second survey among the 57 to 85 group used the simpler definition of "having had sex with anyone" but over a longer period of 12 months.

The researchers say the results show that, unremarkably, people in good health are almost twice as likely to be interested in sex as those in poor health. Men have more sex and more interest in sex at all ages, and the gender gap widens with advancing years.

The big factor that limits sexual activity is a man's health, not a woman's. Many illnesses, such as diabetes, heart trouble and prostate cancer, affect a man's capacity to have and maintain an erection. Though women are prey to similar illnesses, they "typically do not render a woman physically incapable of sexual intercourse", the researchers say.

The effect of the Viagra generation can be seen in the growth of interest in sex among older men over the past 10 years, but interest in sex has remained unchanged among women. The researchers say: "The difference may partly reflect the introduction of the highly effective and widely promoted male erectile dysfunction drugs to the US and European markets, beginning with sildenafil [Viagra] in 1998."

In a study in 2005, more than 14 per cent of US men said they had taken drugs or supplements in the previous year to improve sexual performance. Overall, most sexually active men report a good-quality sex life but only half of sexually active women do. This contradicts the findings of Swedish poll three years ago, which showed that female sexual satisfaction had increased while satisfaction among men had decreased.

Men are still blamed, by both sexes, for when sex ends. Peggy Kleinplatz, from the school of psychology at the University of Ottawa, Canada, said in a commentary on the Swedish study that this finding had been consistent for the past 40 years.

"Even if women are coming into their own sexually – and are more satisfied than ever in the latest cohort – years of men being in charge of making the first move in adolescent sexual encounters in the 1940s and during marriage in early adulthood in the 1950s and 1960s has led to the expectation that men remain responsible for making sex happen. Thus, attributing responsibility for the frequency or lack of sex to men continues," she added.

An editorial in the BMJ hails the latest study for its "health enhancing, health promoting perspective", because most sex research centres on problems.