Sharron Davies shed some after Rebecca Adlington won gold in Beijing. Windsurfer Bryony Shaw couldn't hold back when she picked up bronze, and Sienna Miller's been at it while filling up at the petrol pumps. Suddenly PDTs (public displays of tears) are everywhere. But what makes us well up? And who are the biggest crybabies of all time? Read this and weep...

What makes us weep? and, why don't boys cry?

We do it when our football team loses, when we split up with a loved one, or welcome a new member of the family into the world – or, in the case of many members of Team GB this week, if we realise a long-held dream.

In other words, we cry at moments when our emotions brim over. The precise psychological mechanics of emotional tears are still debated by academics, but there is a general agreement that they perform a cathartic action relieving feelings of stress, supporting the oft-given advice from well meaning aunties that we should, "have a good cry, it'll make you feel better."

We might feel better after a bawl, but the flow of salty tears can be interpreted by onlookers in a variety of ways, says Dr Gail Kinman, a reader in occupational health psychology at the University of Bedfordshire and an expert on the subject. Tears can be infectious, she explains. "In the UK, this was seen at the funeral of Diana, Princess of Wales, in 1997. There, we saw a phenomenon known as emotional contagion. You pick up other people's emotions in the same way you would pick up a cold. Since then, it has become more acceptable for people to weep in public or show more extremes of emotion."

It is acceptable in the West for women to cry freely in times of trouble or stress (although a study earlier this year by the British Psychological Society explored how female tears in the workplace can be a source of embarrassment). But the rule that "boys don't cry" still applies, even in our more emotionally liberated times. "In certain circumstances it can be seen as a sign of weakness, especially among men," agrees Kinman. "You can get a tough bloke covered in tattoos crying because his football team has just won, which is quite acceptable in private. But it can be a whole different story in front of his mates."

From an early age most British males are taught that it is unacceptable to cry in public. Some think this could even be an attitude handed down from the battlefield. "Crying not a marshal quality," says Phillip Hodson, a fellow of the British Association for Counsellors and Psychologists and an expert on male emotions. "Showing excessive emotion on the field of battle is the last thing you want. When you are crying, your guard is down, your vision is blurred. The traditional male stereotype is of the infallible warrior who never rests. And this perception is not limited to the West – there is quite a lot of shame in appearing to be weak and womanly in Eastern cultures."

Boys need to toughen up to "be" their gender, is the message. But why do tears signal weakness? "In truth, crying isn't weakness, it's a relaxation system in the human organism," says Hodson, "In order to function, we have to function with our emotions, not against them. You cry when tension needs to be dissipated. If we don't use this system we're not being very intelligent."

To go by the outpouring of tears at the Olympics , this particular taboo might be eroding at last.

Rob Sharp

So What makes a film a tear-jerker?

"Tears, idle tears, I know not what they mean," wrote Tennyson in "The Princess" – a little disingenuously, because he knew exactly what they meant: the speaker was weeping with nostalgia for "the happy autumn-fields" and "the days that are no more". Nostalgia is seldom an emotional trigger in the arts, however. We weep in the movies, in theatres, over books, because we're moved by the predicament, or the joy, of people we don't know – but why should these people in these circumstances activate our lachrymal ducts?

Emotions connected to the family are obvious tear-jerkers. I don't know any man, however stoic he may be, who doesn't blub like a girl at the end of The Railway Children, when Jenny Agutter sees her father through the smoke on the station platform, and cries "Daddy, my daddy!" Few theatre-goers in history have been able to cope with the last act of King Lear, when the storm-ravaged monarch re-encounters the daughter he banished for not loving him enough ("Do not laugh at me;/ For, as I am a man, I think this lady/ To be my child, Cordelia"). I'm not proud of the fact that I howl like a banshee at the end of ET, when the macrocephalic alien embraces the six-year-old Gertie with the words, "Be good". But of course ET is just one of a thousand lost-brother figures (see also the elephant man, Frankenstein's monster, Kaspar Hauser) who stumble through film history, making us weep with pathos at their outsider status.

Death and departure, the severing of close bonds, make us weep because they play on our deepest childhood fears of abandonment and solitude in the scary forest. Walt Disney's Bambi is the tear-jerker with the 100 per cent success rate in this regard, just as the demise of Little Nell in The Old Curiosity Shop and of Tennyson's Lady of Shalott reduced early-Victorian readers to lakes of tears. But equally we cry like fools at returns and redemption, whether it's on a train station (with Ms Agutter) or in a snowbound American small town like Bedford Falls, in Frank Capra's It's a Wonderful Life.

Certain filmed love stories used to be called "weepies," because of their portrayal of thwarted love or female suffering bravely borne – themes that hark back, through Romeo and Juliet, to the classical stage; we may find them a little debased in the decade of Bridget Jones's Diary, but then we're dealing here with tears, not dignified emotions. Noël Coward remarked on the extraordinary potency of cheap music; we can be similarly amazed by how easily our tear ducts well up because of simple things: claspings and sunderings, parents and children, redemption and discovery, love denied and love triumphant. It's rather shocking to discover what sentimental sponges we are under our cool 21st-century carapace. But let's not cry about it, eh?

John Walsh

What happens to our bodies when we cry?

Crying is considered to be a uniquely human phenomenon. It is almost always associated with tears welling up in the eyes and running down a person's cheeks. But tears are always being produced by the lachrymal glands of the eye, which serve the essential function of protecting the delicate outer surface of the eye, the cornea. The lachrymal gland, situated behind the upper eyelid of each eye, secretes a constant flow of saline fluid that is spread across the cornea each time we blink, keeping it moist and clean.

As the lachrymal fluid is drawn towards the inner corner of the eye, it is drained down small channels – the lachrymal canaliculi – into the nasal cavity and out through the nose. When we cry, there is a build-up of lachrymal fluid in the form of tears. These can quickly flood the lachrymal canaliculi, making our noses run and causing an overflow of tears.

The tear film coating the eye consists of three layers. An outer lipid layer contains oils that retard evaporation and prevent the tear film from spilling down the cheeks. A middle, aqueous layer contains proteins and enzymes designed to fight bacterial and fungal infections. The inner, mucous layer sitting next to the corneal surface helps to spread the tear film and protect the delicate cells of this living membrane.

Pain and emotion can trigger the release of excess lachrymal fluid, which quickly forms the salty tears we know so well from childhood. The salinity reflects the salt concentration of the body's blood plasma, from which the tears are derived. Such tears must help to protect the eyes at a time of stress, but they also have the function of being a signal to others.

It is often difficult to control the flow of tears because the lachrymal glands are under the control of the "involuntary" arm of the nervous system, called the parasympathetic nerves, which is also linked to the limbic system of the brain. This is an evolutionary, ancient part of the brain, associated with raw emotions.

Steve Connor, Science Editor