Every so often we are told that audiences are ripe for a 'return to genre'. This is not just academic; genre-oriented production companies, such as Filmax in Spain, NoShame in Italy or Sahamongkol in Thailand, have money riding on our appetite for contemporary Euro-horror, hard-boiled Milanese crime classics or Muay Thai martial-arts kickfests.

But what is genre, and what does it do' Ten film theorists are likely to answer the question in 10 different ways. Some take a genetic view, claiming genres are embedded in our body chemistry; those of a Marxist persuasion see genres as potent instruments of social control; there are even some, such as US academic Thomas Schatz, who see genres as expressions of quasi-religious social rituals where each genre is a kind of sect with its own rules and gods.

A messy business

One of the few things they all agree on is that genre classification is a messy business. In its widest usage, the term 'genre' can refer to a film's story and setting (westerns, sci-fi), or a combination of story, setting and mood (film noir), or its audience (teen movies, arthouse), budget (blockbuster, indie), technique (animation) or provenance (Bollywood, 'world cinema'). In its narrowest sense, 'genre' means low-quality, fanbase product - in fact, in some circles it's almost become a synonym for horror.

It may not be philosophically elegant, but it makes sense to start any discussion of genres by pointing out that if such categories exist, it's because we feel they have some use. Down at our local DVD store, titles are generally listed under 'comedy', 'martial arts', 'sci-fi-fantasy' and so on, so these categories must be thought to have some sort of customer payback.

And yet, does it really work this way any more' For every undecided renter who fancies a rom-com there's a frustrated searcher like me who can't find Kiss Kiss Bang Bang because it's been filed under 'action'. When we buy, we're more likely to know what we want: that must be why some retail outlets have dispensed with categories such as the catch-all 'drama', in favour of alphabetical racks - the exception being categories with strong fan-bases, such as horror or anime.

This shelving shuffle seems to reflect a loosening of genre's grip on audiences' psyches, at least in the West. While 'ritual' genres such as horror, with cohesive demographics (insecure, young, white male) and underground appeal seem to be alive and well, other genres are attracting less brand recognition and loyalty than before - but are also becoming more fluid. Action films such as the Bourne trilogy yearn for the drama shelf; sci-fi merges with thriller and horror in Sunshine; Leatherheads mixes rom-com, sports movie and period parody.

Social fragmentation

Maybe it's something to do with the fact genres thrive on a social and moral consensus that is becoming increasingly fragmented - and the age of distinct movie species is over. Or maybe it's the fact we're waiting for the old genres to mutate into something else. After all, it sometimes takes a leap of faith, or desperation. A demand for spaghetti westerns was created by a bunch of Italian scriptwriters and directors frustrated by the increasing elitism of their national film culture and attracted by cheap Spanish locations and co-production opportunities. Arguably, the genre would never have evolved if the swords-and-sandals epics that had previously kept Italian genre crews in work had not fallen out of favour, and if the supply of US westerns had not dried up.

Producers took a risk, and audiences discovered an appetite they never knew they had - for stark, violent, cartoonish westerns that replaced heroic self-realisation with sardonic self-preservation.

It's doubtful whether a new sub-genre could be launched today with such resounding popular success. But small forays still seem possible - such as the now-exhausted Cockney gangster brand. Note, too, that economic recessions seem to give birth to genres: the vampire film in 1920s Berlin, the gangster movie in early 1930s America. If we really are heading for another crash, it could be good news for genre.