Last month, Ben Stiller's comedy Tropic Thunder was the subject of an America-wide Rally for Respect, organised by a coalition of disability groups. Participants were urged to picket cinemas, leaflet audiences, lobby newspapers and protest to the film's producers. The complaint was that the word "retard", used 17 times in the film to denote a person with learning difficulties, was unacceptable. "Hate speech" of this kind was accused of inspiring "discrimination, abuse, negative stereotypes, disenfranchisement and violence".

The arrival in Britain of Tropic Thunder has prompted few protests of this kind. For we, of course, are so much more sophisticated than our literal-minded transatlantic cousins. The film's a satire. We, at least, can see perfectly well that it isn't belittling disadvantage: it's attacking the crass attitudes to disadvantage of people who perhaps themselves deserve to be considered emotionally retarded. In the process, it's doing their victims a favour. Shouldn't that be enough to dispose of any objections?

To dismiss the issue so glibly, you perhaps need to be, well, intellectually retarded. Airing abusive language under the guise of mocking it cannot but beget collateral damage. Whether well-intentioned or not, it provides cover for indulgence in the very attitudes it purports to condemn. Ben Stiller may be a paragon of concern for humanity's less privileged, but some cinemagoers won't be. Many of them may take a sneaking delight in hearing a word they might use themselves, or like to use, acknowledged if not approved. And for all we know, this might conceivably precipitate some of the ill-effects of which America's protesters have warned.

So we can't just ignore the possibility that a film like Tropic Thunder will do harm. The real question is whether or not any harm done can be justified by benefits gained in return. And evaluating this requires more than simply counting the number of times that a particular word is used.

Stiller's film is, ostensibly at least, about Hollywood. The point it's making about people with learning difficulties is that the movies sanitise their plight. The character Stiller plays is a fading film-star who has himself just portrayed a gibbering, goofy dimwit in a film called Simple Jack. According to a colleague, this has ruined his career. His mistake was to go "full-retard". To win an Oscar you should only go "part-retard", like Dustin Hoffman in Rain Man or Tom Hanks in Forrest Gump.

This isn't just accurate and amusing; it's important. For decades, Hollywood colluded in the dismissal of disability by ignoring it. Its current practice of glamorising the subject is perhaps even more pernicious. Understanding of dementia was set back, rather than advanced, by its rosy misrepresentation in Away From Her. The movies' insistence that manic depression and autism come accompanied by good looks, unusual charm and near-magical powers hasn't endeared people with these conditions to the rest of us. It's increased the burden on them, by arousing unrealistic expectations of their capacities.

By using the word "retard", Stiller relocates those to whom it's applied back in the real world. By acknowledging the distaste they may inspire, he does them the service of taking their situation seriously. And he reminds audiences that cinema's reluctance to engage honestly with them is ultimately the fault of cinemagoers themselves, not the studios, which must work within the parameters of acceptability.

"Retard" isn't the only potentially upsetting term that Tropic Thunder deploys. It also cheerfully trots out "idiot", "moron" and "imbecile". In doing so, it invites us to reconsider the whole issue of the euphemistic relabelling of disadvantaged groups. Is it really better to insist that people like Simple Jack should be called what busybodies say they must be called, rather than whatever everyday usage dictates?

Maybe it is, but the question is worth asking. Euphemisms don't work for long. And in one way, it's worse when children taunt a disadvantaged playmate with the cry of "special needs", than it would be if they shouted "retard". They're making the point that their victim isn't only handicapped, but is also incapable of handling his or her condition without recourse to ineffectual protection from witless officialdom.

Disability lobbyists haven't been the only complainants that Tropic Thunder has attracted. Some anti-racists haven't been too happy to see Robert Downey Jr blacking up to play the archetypal celluloid person of colour, saintly but none too bright, and not all that black either. However, this device, too, does a useful job.

It may well be that some war veterans will feel that the film derides their sacrifice. Perhaps a few Jewish studio honchos will even be hurt by Tom Cruise's savage representation of one of their kind. If so, in all cases, too bad.