Get down! Tonight we're going to party like it's 99BC

Our taste for bad behaviour at this time of year predates Christmas. It's a ritual thing. So what dark forces push us to act so weirdly? Elizabeth Heathcote peers into the unseemly murk

However resistant you consider yourself, the chances are that it will have got you before the week is out. Blame the alcohol or the sheer pressure of good cheer but few of us are destined to make it through to Boxing Day without having behaved out of character on at least one occasion.

However resistant you consider yourself, the chances are that it will have got you before the week is out. Blame the alcohol or the sheer pressure of good cheer but few of us are destined to make it through to Boxing Day without having behaved out of character on at least one occasion.

Hands up if, in the haze that surrounds Christmas, you have never done any of the following: given a fiver to the beggar you studiously ignore every other day of the year; ended up in the river/wrong bed after the works do; stopped for a jolly chat with your neighbour and thought, what a good sort, why don't I speak to them all year, in fact, why don't we go on holiday together?

See? You're only human. Besides, it would be futile to resist; the need for a festival that sanctions the turning upside down of the normal rules by which society and individual humans function is, apparently, fundamental to our well-being.

"It is amazing the way that this sort of celebration cuts across cultures and times," says Chris Humphrey, author of The Politics of Carnival (Manchester University Press). "I use the term 'misrule' to describe it – the way a society has certain rules and patterns but then occasions are set aside where everyone can step outside them."

Christmas itself has its origins in pagan festivals of the winter solstice, which could get pretty wild. Its forerunner in ancient Rome was the Saturnalia, during which all the usual rules were suspended. Grudges and wars were put on hold, businesses and schools closed, and law-breakers went unpunished. Cross-dressing was popular, but at the centre of the festivities was another inversion of the social order – the rich bought the poor presents, and the festival feast was served by the masters to the slaves.

"It is now the month of December, when the greatest part of the city is in a bustle," wrote Seneca the younger from Rome AD50. "Loose reins are given to public dissipation; everywhere you may hear the sound of great preparations ... should [we] eve in our usual way, or, to avoid singularity, both take a better supper and throw off the toga?"

Throwing off the toga is still a favoured form of Christmas transgression – so common in fact that some couples take the pragmatic approach and negotiate an amnesty for light petting during the festive season. Or else they cast off the toga together. "I've no idea where it came from," says Lisa Gibson, 31, a nurse. "We were in a pub on Christmas Eve with our best friends – another couple – and somehow or other I started kissing the woman. Then my boyfriend was kissing her and her boyfriend was kissing me. We ended up staggering back to our house discussing whether we should spend the night with each other's partners and we almost did it, but my boyfriend chickened out at the last minute. Thank God."

Clearly alcohol played a part in this misadventure, but this foursome – who never revisited their sexual experiment – went out drinking together most Saturday nights, so why now? "Well it's your only chance, isn't it?" says Lisa. "If you did something like this at any other time of year the fall-out would be immense. But at Christmas we could just shrug it off as a laugh."

And therein lies the key to our strange festive behaviour. Christmas is the time when society allows us, and we allow ourselves, to push our moral boundaries; when we dare to test our identity by becoming the people we spend all year struggling not to be. Bad instead of good. Friendly instead of hostile. Tart instead of spouse. Truth-teller instead of diplomat. Out of control instead of in control.

"In an ordered society with rules, people need to be able to mediate with all that they exclude – hence the inversions, of officers serving the soldiers and so on," says Humphrey. "But it is true on a personal level, too. There is work to suggest that if people are incapable of tapping into the things they define themselves against, they can literally go mad."

But while you can try wheeling this out as an excuse in an attempt to get a more sympathetic hearing from your partner or boss, you can still go too far. According to a survey by the Chartered Institute of Personnel and Development, disciplinary action at work increases by 12 per cent over the festive period, with attacks on colleagues, sexual harassment and swearing at managers all cited as problems.

Remember, too, that the months immediately after Christmas are the busiest for divorce lawyers. After a difficult year, Joe Berlin, 40, a normally mild and temperate teacher, went wild at the Christmas party and slept with a colleague. He confessed to his wife on Boxing Day and two days later broke the news to his solicitor that in fact only he would be buying the flat that the couple had exchanged contracts on. "He got on the phone to the estate agent and I could hear him muttering, 'You know, it's that Christmas thing again'," says Joe. "I really hated him at that moment."

Four years on, Joe can admit that the timing of his affair was no coincidence. "All the big confrontations in my life have happened around Christmas," he says. "It's the combination of the end-of-the-year reckoning and a sense of recklessness. I'm not the most assertive person, and it gives me the kick to take action."

Killjoys over the centuries have recognised and feared the dangers inherent in relaxing the rules. The Puritans in particular hated Christmas: it was forbidden in England by an act of Parliament in 1644 and declared "an extraeme forgetfulnesse of Christ, by giving liberty to carnall and sensual delights". But the natural inheritors of this stern line – Marxists, who have traditionally frowned on Christmas as a sop for the masses – are lightening up.

"Christmas is a few days' hard-won statutory holiday for workers to spend time with their loved ones," says a member of the Socialist Workers Party. "It's a haven in a heartless world, to misquote Marx." And will they be letting it all hang out? "You'll see some members wearing Santa outfits to sell the paper," she assures me.

Such transgressions pale beside the rituals of other cultures, particularly the extravagant, debauched carnivals that lead Latin American Catholics, unrepentant, into Lent. Germans too celebrate this period with a festival that taps into pagan fertility cults, called Fasching; one Bavarian court has refused to acknowledge adultery that happens during this time.

But everything is relative. For Michael, a cynical misanthrope who makes a virtue out of scowling at the world, a Christmas transgression means something very different. This year's slip came while waiting for the Tube on the morning after the office party. "This woman was looking at me, half-smiling, and I thought what's wrong with her, but then I realised – I was whistling "Hark! The Herald Angels Sing". "Hark! The Herald Angels Sing" for God's sake!" He shakes his head. Another moral giant felled by the season of goodwill.