The proviso, of course, is that if your sexual experiences are unhappy ones, they will have a similarly negative impact upon your psyche. But assuming the sex is good, it is thought to improve body image, as well as reducing anxiety and the incidence of psychiatric illness, depression and suicide. A 2004 study of men from four different cultures found that sexual satisfaction was directly associated with an increased frequency in sexual intercourse, as well as being inversely related to depression.
During orgasm the body produces oxytocin, which is a hormone linked to a range of positive physical and psychological effects. Chief among these is its beneficial impact on sleep. "There's no doubt that sex is relaxing and so helps tackle insomnia," says Dr David Delvin, a GP and specialist in sexual medicine. "Lots of people use sex, whether with a partner or on their own, as a way of getting to sleep. That's down to the surge in oxytocin during arousal and orgasm, which is a natural sedative."
This view is backed up by a US study carried out in 2000, which found that 32 per cent of the 1,866 female respondents who reported masturbating in the previous three months did so to help them sleep.
One of sex's main health benefits is its positive impact on how we deal with stress. In a study published in the journal Biological Psychology, 24 women and 22 men kept records of their sexual activity. The researchers subjected them to stressful situations, such as public speaking and doing verbal arithmetic. Those who had intercourse had better responses to stressful scenarios than those who had either engaged in other sexual behaviours or abstained altogether.
According to Julia Cole, this could be down to the soothing effect another person's touch has. She says: "A great deal of research has shown that touch has a naturally calming effect on human beings, whether it's linked to sex or not. Of course, being touched by someone you care about will double the calming effect."
Apart from the obviously pleasurable sensation of being touched or stroked, it is thought to have a biochemical effect, reducing the levels of cortisol – the hormone that is secreted when you're under stress.
Having sex once or twice a week has been linked with higher levels of an antibody called immunoglobulin A, or IgA, which can protect you from colds and other sorts of infections. Scientists at Wilkes University in the US tested IgA levels in 112 college students who reported the frequency of their sexual activity. Those students in the "frequent" group had higher levels of IgA than those who were either abstinent or had sex less than once a week.
Paula Hall, a psychosexual therapist with Relate, also thinks that the impact of sex on our general wellbeing helps to boost immunity. "All the psychological benefits have an impact on your physical health, such as your immune system," she says. "We know that when you're feeling good about yourself your body fights off illness and disease better – so the healthier we are psychologically and emotionally, the healthier we are physically."
Frequent ejaculations may reduce the risk of prostate cancer for men in later life, according to a study by Australian researchers reported in the British Journal of Urology International. When they followed men diagnosed with prostate cancer and those without it, the researchers found that men who had at least five or more ejaculations weekly during their twenties reduced their risk of getting prostate cancer by a third.
"The evidence is good that men who masturbated regularly in the past are less likely to get prostate cancer," confirms Dr Delvin. "Nobody knows exactly why this is, but it does seem to be pretty cast-iron."
Research also suggests that regular sexual activity could help women to avoid breast cancer. A study conducted in 1989 examined 146 French women and found a higher risk of breast cancer in those women without sexual partner or who had sex less than once a month.
Having sex and orgasms is a key part of improving intimacy and ensuring a healthy long-term relationship – which has been linked to a longer lifespan in a number of studies. It's all down to oxytocin again. "Oxytocin, also called the 'bonding hormone', is released when women give birth, so it is part of the bonding process with their baby," says Julia Cole. "It's also released in people who are in secure or long-term relationships, as well as during sexual contact. This bonding effect is one of the reasons people continue to have a sexual relationship long after they have ceased to be fertile."
This was backed up by a study conducted by researchers from the University of Pittsburgh. They evaluated 59 premenopausal women before and after warm contact with their partners ended with hugs. The study found that the more contact the women had, the higher their oxytocin levels were.
And studies in which couples were asked to go without sex for long periods found that their general relationship declined, indicating that sex has a powerful bonding effect for couples. "There's also the slightly more indefinable feeling that you are thought to be attractive and someone your partner wants to be with and touch," adds Cole. "That's very important – often when I see couples who are in trouble they have stopped having sex, and one of them will say their partner no longer thinks of them as attractive."
Sex has been linked with a pain reduction for a wide range of conditions, including lower back pain, migraines, arthritis and premenstrual syndrome symptoms. It's all down to those hormones again. "Sex increases endorphins, the body's natural painkillers," confirms Dr Delvin. "So there is evidence that having sex eases period pain and PMS."
Oxytocin is also linked with pain relief. In a study published in the Bulletin of Experimental Biology and Medicine, 48 volunteers who inhaled oxytocin vapour and then had their fingers pricked reduced their sensitivity to pain by half.
In 2001, two studies of orgasms and migraine headaches in a woman and man found that orgasm resulted in pain relief. And an earlier study of 83 women who suffered from migraines reported that orgasm resulted in pain relief for more than half of the group. Although this form of pain relief is less reliable and effective than the use of drug therapies, the effects of orgasm as an analgesic are more rapid.
Sexual activity, like other forms of exercise, burns both calories and fat. Thirty minutes of energetic sex burns 85 calories or more. Although this may not sound like much, it does add up – 42 half-hour sessions will burn 3,570 calories, which is enough to lose a pound. "Sex does burn calories, so it's comparable to moderate exercise like doing the housework or going swimming," says Dr Delvin. And it is, clearly, a great deal more fun.
But there is something of a chicken-and-egg element here, because people who lead more active sex lives tend to exercise more regularly and physical exercise improves sexual health. A 1990 study that followed 78 men over a nine-month period found that with consistent aerobic exercise, participants had an increase in frequency of sexual activity, improvement in performance and an increased ability to reach a "satisfying" orgasm.
One of the most extensive studies into the relationship between sex and mortality was carried out in Caerphilly, South Wales, from 1979 to 1983, with a 10-year follow-up. In the study, 918 men were given a physical examination and asked about their frequency of orgasm. After 10 years it was found that the mortality risk was 50 per cent lower among men who had frequent orgasms – which was defined as two or more per week. The study also found that, even when adjusting for age and other risk factors, frequent intercourse was associated with lower incidence of cardiovascular disease and stroke.
"There has been a great deal of research into whether people in relationships live longer," adds Paula Hall. "We know that having a strong relationship is a good indicator of longevity – and a healthy sex life is a big part of that."
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