It makes nine out of 10 people feel better, reduces stress, and may help to keep the body healthy. It's also free, available to almost everyone, and has no known side effects, other than wet tissues, red eyes and runny makeup. Crying may not be a blockbuster drug, but the latest research suggests it's highly effective at healing, and that it improves the mood of 88.8 per cent of weepers, with only 8.4 per cent feeling worse. So beneficial is it that the researchers suggest there may be a case for inducing crying in those who find it difficult to let go.

But while almost all of us shed emotional tears at some time – at least 47 times a year for women, and seven for men – exactly why we cry, and much about what happens when we do, remains a mystery. For crying, a uniquely human form of emotional expression, to have survived evolution, it should have a practical purpose and give some kind of survival advantage. Laughter and anger are both well known to have advantages. Laughter, for example, has been shown to promote healing, increase blood flow, reduce levels of stress hormones, boost the immune system and produce more disease-fighting compounds.

But what of crying? Emotional tears come from the same tear glands that produce the fluid that forms a protective film over the eyeballs to keep them free of irritants, and which also releases extra fluid when the eye becomes irritated, or is invaded by a foreign body.

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A clue to the purpose of crying may lie in the experimental finding that emotional tears contain different compounds from regular eye watering, such as that triggered by chopping onions.

The phenomenon supports the so-called recovery theory, that emotional tears, and their contents, may be a way of getting the body back in balance after a stressful event. "I have suggested that we may feel better after crying because we are literally crying it out. Chemicals that build up during emotional stress may be removed in our tears when we cry,'' says William Frey, professor of pharmaceutics at the University of Minnesota. "Because unalleviated stress can increase our risk for heart attack and damage certain areas of our brain, the human ability to cry has survival value.''

Other evidence backs up the theory. It's been shown that tears associated with emotion have higher levels of some proteins, and of manganese and potassium, and hormones, including prolactin than mere eye watering. Manganese is an essential nutrient, and too little can lead to slowed blood clotting, skin problems, and lowered cholesterol levels. Too much can also cause health problems. Potassium is involved in nerve working, muscle control and blood pressure.

Prolactin is a hormone involved in stress and plays a role in the immune system and other body functions. Its involvement in tears may help to explain why women cry more than men. Women have more prolactin than men, and levels rise during pregnancy, when the frequency of crying among women also increases.

There have also been some claims that crying can reduce pain, although there has been little research into this area. The phenomenon, if verified, may be an indirect effect – in that crying may trigger physical contact with another individual and touch has been linked to improved wellbeing.

A counter theory is that crying doesn't so much help the body recover from whatever triggered the tears, but that it increases arousal to encourage behaviours to see off the threat. In support of this theory, some research shows that skin sensitivity increases during and after crying, and that breathing deepens. Some argue that crying could perform both these functions: "It is possible that crying is both an arousing distress signal and a means to restore psychological and physiological balance," say researchers at the University of South Florida. Others suggest that emotional tears signal distress and encourage group behaviour, as well as improve social support and inhibit aggression.

A study at Tilburg University in The Netherlands shows that both men and women would give more emotional support to someone who was crying, although they judged less positively someone who wept. Another study showed men were liked best when they cried and women when they did not. "Overall, results support the theory that crying is an attachment behaviour designed to elicit help from others,'' say the Dutch researchers.

In the latest study, at the University of South Florida, researchers found that almost everyone feels better after a cry and that personality has a big effect on how often we cry. Neurotics were more frequent criers and were more easily and quickly moved to tears. The American researchers suggest that the beneficial effects of crying may make induced weeping a useful therapy for some people. In, particular, they propose that it may be suitable for people who have difficulty expressing their emotions.

"The overwhelming majority of our participants reported mood improvement after crying,'' they say. "Our results may have also implications for clinical interventions. Currently there is only anecdotal evidence that learning how to cry and how to derive positive effects from it could help people who are having difficulty expressing sadness or crying.

"Our findings support the idea that people with alexithymic [a deficiency in feeling emotions] or anhedonic [the inability to derive pleasure from pleasurable experiences] tenden-cies may profit from therapeutic interventions that encourage crying.''

Like other researchers, the Florida psychologists suggest more work is needed to understand the origins, nature, and function of crying. New research is under way, including teams of brain mappers using scans to locate the areas of the brain involved in crying. Some of it supports the recovery theory, while other work backs up the arousal idea. More support has also been shown for the social role of crying.

Some studies are giving intriguing new insights into shedding tears. When researchers at Bunka Women's University and Nagano College in Japan, set out to investigate what they call the passive facial feedback hypothesis, they produced a surprise finding. In an experiment, they simulated the experience of tears by dropping 0.2 ml of water on to the tear duct of both eyes. They report that 53.8 per cent of the 100 or so men and women felt sad when the water ran down their cheek, compared with 28.6 per cent who were cheerful.

The increasing research into crying and its beneficial health effects may also make shedding tears less of a taboo behaviour. As Professor Frey, author of Crying: the Mystery of tears, points out, it is no accident that crying has survived evolutionary pressures. Humans are the only animals to evolve this ability to shed tears in response to emotional stress, and it is likely that crying survived the pressures of natural selection because it has some survival value,'' he says. "It is one of the things that makes us human.''

Not a dry eye: Weeping by numbers

20% of bouts of crying last longer than 30 minutes

8% go on for longer than one hour

70% of criers make no attempt to hide their crying

77% of crying takes place at home

15% at work or in the car

40% of people weep alone

39% of crying occurs in the evening, the most popular time compared with morning, afternoon, and night (16, 29 and 17 per cent respectively)

6-8pm is the most common time for crying

88.8% feel better after a cry

47: average number of times a woman cries each year

7: annual number of crying episodes for a man

Sob story: The science of tears

Three types of tear are produced by the lachrymal gland above the eye.

Continuous or basal tears, produced to keep the eye surface permanently moist and protected contain water, lipids or fats and proteins. They also contain compounds that protect against infections. Each blink of the eyelid spreads tears.

Reflex tears have a similar make-up and are a reaction to irritants or foreign objects.

Emotional tears have a different make-up including enkephalin, an endorphin and natural painkiller.

"Emotional tears contain higher concentrations of proteins, manganese, and the hormone prolactin which is produced during stress-induced danger or arousal,'' says Dr Carrie Lane of the University of Texas .

A question of sex: Why big boys boo hoo

* While women cry more than men, tearful males are becoming increasingly acceptable in society.

* A moist eye, perhaps a tear or two, at the right time, and in the right place, are now viewed more kindly, say researchers.

* Until relatively recently, crying was associated with sensitive, weak men, while now it is linked to strong, powerful men. One theory is that a driving force behind the change has been powerful and emotional events such as 9/11.

* Norms for men and crying are changing. Certain types of expressions that were proscribed for men are now becoming more acceptable. "It may be that certain types of tears are no longer associated with powerlessness, and thus no longer conflict with assertions of masculinity,'' says Professor Stephanie Shields of Pennsylvania State University.

* In the research, Professor Shields and colleagues quizzed men and women about reactions to crying by men and women. The results showed that crying at serious events by both men and women was rated positively.

* The results also show that men were rated more highly when they cried out of sadness than anger. The reverse was the case for women. Men who cried in sadness were more positively rated than women who cried because they were sad. The results also show that men who have a wet eye and a tear or two are rated more highly than men who weep.