Their academic antennae had been alerted by a talk on the Rugby Sevens delivered this month by Chinese University anthropologist Joseph Bosco - and their willingness to give up a Friday night to observe the phenomenon may have been fired in part by Dr Bosco's assertion that a lot of what goes on concerns a universal pursuit: sex.

'It is clear to even a casual observer that sex is a major theme of the Sevens,' Dr Bosco says in a study based upon his years of observation at the event. 'One way this can be seen is from the costumes worn by spectators. Sex-themed T-shirts are common too. One I saw read: 'Sex instructor, first lesson free. Ask wearer for details. Satisfaction guaranteed'.'

Other costumes give out less obvious signals, he found. 'Wearing Catholic and other religious habits is quite common,' he says. 'The purity and asceticism and celibacy represented by the costume is in fact meant to symbolise the inverse: debauchery and sex.'

The Rugby Sevens is an event, Dr Bosco found, where women have the freedom to be as overtly sexual as men. 'The players act as the sex objects, with women asking players to stop at the railings so the women can lean back and have their pictures taken with the 'hulks',' he says.

'These men - athletic, fit, good-looking, tough, maybe even aggressive - are defined by their physicality. It is very common for some women to talk openly about men's attractiveness, making some of the women as sexually explicit as the men are about the 'babes' in the stands.'

Dr Bosco admits he was never a rugby fan himself but says his interest in the rituals of the Sevens was stirred when his son became a keen player and took him along. Professionally, what he saw going on off the pitch fascinated him.

More than a sporting event, he concludes the Sevens is also a carnival - particularly in the way fans dress. 'There are women in sexy costumes - sexy nurses, devils and belly dancers - men dressed as women, men in animal costumes ranging from apes to Dalmatians, men in Borat-style underwear as well as a group of Borat lookalikes,' he observes.

Other fans have turned up dressed as condoms, Arabs, superheroes and even as patriot missiles, which, Dr Bosco points out, 'in addition to being powerful weapons are also phallic symbols'.

'In 2003, when the Sevens was held as the Sars [severe acute respiratory syndrome] epidemic raged in the city and some recommended that fans wear masks, some wore women's G-strings as masks as a humorous taunt against authority,' he adds.

And some of the bizarre outfits almost defy categorisation. 'Some have obvious interpretations - gorillas as animalistic males, chickens as bearing H5N1 bird flu, and superheroes of various sorts as epitomes of strength and virility, perhaps with a sense of irony,' Dr Bosco says.

However, some are 'obscure and hard to interpret', he admits, citing one group of fans he saw wearing helmets made out of scooped-out watermelons. 'Different spectators bring their own meanings and interpretations and take away multiple understandings of what the Sevens is all about,' he concludes.

What is clearer, however, is that the event has struck a chord and is the most successful one of its kind anywhere in the world. 'Many like to note that the Sevens game fits the Hong Kong pace of life and attention span. In Hong Kong, time is scarce,' Dr Bosco says. 'While everyone else in the rugby world enjoys an 80-minute game, the city has shortened it to just 14 minutes.'

The crowd, he points out, is 'overwhelmingly Caucasian'.

'Anyone familiar with Hong Kong will be struck at the surprising concentration of Caucasians in the stadium,' he says, although he points out that there are Japanese and Fijians as well as an increasing number of Chinese fans.

Good-natured rivalry is key, he says. 'All the booing is playful and not hostile, especially since the boos and cheers come from throughout the stadium,' he says. 'Booing the Australians and the French is one of the traditions of the tournament.'

One of the few areas where cultural misunderstanding can take place is in the exchanges between fans and security guards, Dr Bosco points out. 'The guards are older Chinese men and women and have no idea what is going on,' he says. 'The chaos of the carnival aspects familiar to those of British or Anglo-Saxon culture is frightening to them. They cannot distinguish between a group having fun and a group about to cause a riot.'

Like much of Hong Kong's magic, the appeal of the Sevens appears to lie in a combination of potent and unique ingredients - location, sponsorship, timing and the coming together of different nationalities with common passions. It simply isn't something that can be reduced to a formula or bottled for export.

Hong Kong's expat population is a result of its history and reflects the city's economic role, Dr Bosco argues. 'At the same time, the Sevens have been localised and made into a Hong Kong event, albeit for rugby fans and for those of Anglo-Saxon culture.

'It is difficult for other sports in Hong Kong to copy this context and recreate this history, just as it is impossible for other cities to recreate the Hong Kong Sevens in their city. Context and history do matter.'

One thing is undeniable: the Sevens is highly infectious. After being reluctantly drawn into the annual ritual by his young son, Dr Bosco went on to coach mini and colts rugby. His son went on to play rugby for the US at under-17s level. And who would bet against one or two of the academics who went along with Dr Bosco on Friday night becoming lifelong converts, too?