If your Valentine doesn't send you flowers on Sunday, don't despair. He or she may be prolonging your life. But it all depends on which side of the sex divide you are, how much you 'care and share' and whether you are employed.

Scientific evidence that married people live longer has been accumulating since the 19th century. Recent data shows that married people have lower incidences of cancer, heart disease, stroke and many other ailments than never married, widowed or divorced people.

Those with a supportive partner also, on average, recover more quickly from serious illness. A study from Duke University in Durham, North Carolina, that followed the progress of heart surgery patients found that after five years those who didn't have a close confidant were more than three times as likely to have died than those with intimate companions.

'Being in a secure relationship that provides a haven from the stresses of the day seems to improve immune and endocrine system functioning,' says Linda Waite, a sociologist at the University of Chicago who has studied numerous aspects of the benefits of marriage. 'Married people may be healthier because their bodies are functioning better.' No one knows exactly why married folk come out better in the health stakes. The statistics might be explained because healthier people, being more desirable, are more likely to marry and stay married. But a growing number of scientists and medical professionals theorise that the emotional support of a good marriage serves as a protective umbrella.

Dr Dean Ornish, professor of medicine at the University of California, who is renowned for his programme to reverse heart disease by adopting an ultra-healthy lifestyle, believes that intimate relationships are an essential aspect of healthy living. 'I am not aware of any other factor in medicine - not diet, not smoking, not exercise, not stress, not genetics, not drugs, not surgery - that has such a major impact on our quality of life, incidence of illness, and premature death from all causes,' he writes in his new book, Love And Survival.

Ornish defines emotional support more broadly than marriage or partnership to include religious affiliation, friends and basically anything that 'takes you out of the experience of being separate.' He cites numerous studies in which cancer and heart disease patients recovered faster and survived longer if they were married or had a close confidant.

Since the 1970s, when feminists expounded the idea that marriage boosted men's health but boded ill for women, it's been widely accepted that marriage is best for men. While it's still true that men probably gain more than women in both psychological and physical terms, newer research suggests that each gender benefits in different ways.

It's a cliche, but women look after their mates. 'Being married encourages men to lead healthier lives,' Waite says. 'They sleep more, eat better, they don't drink as much and don't drink-and-drive as much.' Three times as many divorced men as married men report drinking in excess of 50 units of alcohol per week, which is two and half times the recommended levels, according to One plus One, the marriage and partnership research organisation.

Divorced men's rate of unprotected sex is so high that the Health Education Authority is running a safe sex campaign directed specifically at divorced men. More than 17 per cent of divorced men report having more than one sexual partner in the previous year and not using condoms, compared to 10 per cent of never-married men and 2 per cent of married men, says Sharon Breen, of One Plus One.

Marital advantages for women tend to be more practical. One thing that hasn't changed much since the 1800s is that women's material circumstances improve when they marry, which can be a ticket to better health. Married women also report feeling happier than their unattached sisters, despite the stresses of looking after a family.

It's less clear how marriage might physically benefit women. 'The data that married people live longer is less consistent for women than for men,' says Ingrid Waldron, a biologist at the University of Pennsylvania. 'Some studies show the 'marriage effect' and others don't.' Waldron and her colleagues have just finished studying 3,000 women over 10 years and concluded that marriage improved the physical health of women who didn't work outside the home but not of employed women.

Waldron also found employment had more beneficial effects on the health of unmarried women, which suggests that the social support and income benefits of marriage and employment might be interchangeable. In the study, unmarried, unemployed women had the poorest health.

Marriage and job are less likely to be interchangeable for men because they usually have narrower social networks. 'For men the benefits of marriage are much more to do with emotional support - men tend to depend on their wives for social support, whereas women have a larger network so they are not so dependent on their spouse,' Waldron says. 'There is a lot of evidence that social support is good for your health, particularly regarding longevity.' But married people shouldn't be too quick to congratulate themselves. The quality of the relationship is key to whether or not you get benefits. 'People in a marriage filled with conflict are much worse off than those who are happily married,' Waldron says.

• Love and Survival by Dr Dean Ornish is published by Vermilion, price £9.99.

Passion killers

Beware women: Although being married might help you live longer, your husband's occupation might influence how long you live and how you die. An analysis of mortality figures since 1959 shows a strong link between a man's occupation, his cause of death, and his wife's cause of death.

Occupational hazards: Medical practitioners have a high rate of suicide and so do their wives; chefs and their wives are prone to accidents, and musicians and their mates are more likely to die of respiratory ailments, according to Professor Ben Fletcher, dean of the Business School, University of Hertfordshire. Fletcher also reports that farmers and their wives have an unusually high incidence of multiple sclerosis.

'What you do has profound influences on your specific disease risk and life expectancy. Husbands, but seemingly not wives, transfer the risks psychologically to their spouses,' Fletcher says. 'Work can be a way of death as well as a way of life.'