PARTIES were invented by human beings for the sole purpose of boring each other, according to H L Mencken, the American humourist, who put them in the same category as epic poems and the science of metaphysic. Many would agree, but thousands will nevertheless troop off on new year's eve - the biggest party night of the year - for trial by cashew nuts and lukewarm wine.

Why do we do it? Wouldn't we be better off at home with our embroidery, or curled up in bed with an improving book? Unattached, predatory males obviously go out looking for improving females to curl up in bed with, but why do attached couples with no such apparent aim do it?

Julian Boon, lecturer in personality at Leicester University, says: 'We are all rampant hedonists at heart. Parties allow the hedonist in people to come out and express itself. Unfortunately for people with a highly developed conscience, it will come back the next morning and swipe them. But for extroverts (and the definition of an extrovert is someone who enjoys parties), any outrageous behaviour while under the influence of drink is more likely to provoke glee than regret, because they don't give a monkey's what anyone thinks of them.

'The sole aim of some parties is to get totally drunk, to have an orgy of indulgence, because it is so far away from the norm. Such parties are more fun for being over the top; for people doing outrageous things like dancing with their trousers on their heads. They produce memories that last for years afterwards.'

To enjoy parties you need to be a risk-taker, someone who does not mind getting it wrong, Dr Boon adds. Those least suited are introverts who like to control the situation and hate the unpredictable. Shakespeare said that drink provokes 'nose-painting, urine and sleep', so people who don't like having their noses painted are probably better off at home.

According to Dr Boon's theory, we go to parties because inside all of us is a hedonist trying to escape, in some cases against Colditz-type odds. His theories are echoed by Martin Lloyd-Elliott, a London psychotherapist who sees a child trapped inside our everyday straitjacketed lives.

'Having fun is good for you,' he says. 'A good deal of the time as adults we are expected to suppress the child within us. A party is one of the few times when everyone thinks it is OK to be really silly, to play, and break the rules.

'Some people thrive in the company of others and thrive off the feedback. They love parties because they can hold the floor and act as a catalyst in breaking the ice. Of course, there is the party bore, who goes on and on at the top of his voice and thinks he is being the life and soul of the party, when everyone else is wishing he wasn't there. But there are also genuine entertainers who are talented, either in telling jokes, or dancing, or making music.'

Dancing in particular can be therapeutic, he believes. Whereas Dr Boon believes that modern dancing is intricately bound up with sex - 'It does not have to be the lambada, even the humble twist has been known to get people going' - Mr Lloyd-Elliott believes there is more to it: 'Dancing is very ritualistic and tribal. It is very, very primitive. In a world more and more obsessed with individualistic thinking and selfish one-upmanship, the shared experience of dancing is amazingly uplifting. To immerse yourself in sound can be all-absorbing and to function at a non-thinking level is very therapeutic. There is an unscrambling of your thoughts, perhaps similar to what happens when you are asleep.'

Dancing, like other forms of intense physical activity, can release endorphins into the blood system and brain, which produce a 'high', similar to a runner's high, so that after an ideal party, one should feel happy, calm and relaxed, the experts say.

But these experiences clearly apply only to party animals. To some people the idea of setting foot on the dance floor, let alone functioning at a 'non-thinking level' is only marginally worse than being caught with your trousers down by Jeremy Beadle. 'Some people suffer fear: fear of being themselves; fear of discovering they are not miserable; fear of losing control; fear of the intensity of feeling they might experience if they let themselves go,' Mr Lloyd-Elliott says.

Generally such people are characterised as being shy, a heavily researched psychological condition. Peter Harris, senior lecturer in social psychology at the University of Hertfordshire, says: 'The prevailing assumption is that shy people do not go to parties. But actually they do not find parties any worse than meeting people on a one-to-one basis or in a small group.'

Shy people hate complaining in restaurants, get nervous before parties and are terrified of making speeches to large groups - but so are many people. The difference is that they do not feel they can share their problems or seek professional help to resolve them.

Dr Harris explains: 'They do not feel they have a legitimate problem. They think shyness is a childhood problem that you should grow out of. Consequently, they try to tackle the problem themselves by going to evening classes or dating agencies, usually with poor results.'

According to Ray Crozier, lecturer in the psychology of education at the University of Wales, Cardiff, there are two types of shyness: fearful shyness, which appears early; and self-conscious shyness, which appears later. In the first category are children who appear wary and timid from the time they are born; they stay close to their mothers and do not like new situations.

In the second category are those who are self-conscious - who hate speaking up in class or in front of a group. This type of shyness does not usually appear before the age of seven or eight, when children develop a more elaborate sense of self. Both types of shyness can co-exist in adults, but whereas the first seems to be innate, the second is thought to be acquired and is tied up with early relationships.

Are parties good even for the very shy? Duncan Cramer, senior lecturer in social psychology at Loughborough University, says: 'To the extent that a party distracts you from the problems of everyday life, you will feel better off. But anything that is different from the routine and absorbs your attention will have the same effect, whether it is reading a good book or watching a good play or film.

'There is an expectation that you should go out to parties at this time of year, but you should not feel pressurised into doing so and you can get satiated if you go to too many.'