IT IS a central tenet of advertising that thin models sell more products. The only trouble is, it may not be true.

In the first empirical research into the question undertaken in Australia, health psychologists have found young people's response to an ad, and their willingness to consider buying what it promotes, is exactly the same whether the featured model is catwalk-slender or of a more average body shape.

Phillippa Diedrichs created a series of mock ads, using regular models - typically size eight - and so-called "plus size" models, about size 12. She then presented three ads - for a hair-care product, a party dress and underwear - to 400 young people. She found there was no difference between their responses, with those who viewed the larger models reporting themselves just as interested in buying the goods as those who were presented with the skinnier women.

But on another question the responses did differ: when asked further questions designed to assess their own body satisfaction immediately after viewing the draft ads, the women completing the survey, who were aged 18 to 25, felt better about themselves if they had been shown the images of larger models, compared with those who saw the slim models.

"For anything to change, research has to be convincing not just to government and health researchers but also to people in advertising who actually make the decisions," says Diedrichs, whose study is part of her PhD work at the University of Queensland's Health Psychology Research Unit. "Often people make the argument that thinness sells, and that's why they use [slim models].