There's a Peruvian tribe where the men fancy pear-shaped women, in contrast to the Kate Moss shape that we favour. Shown pictures of the Moss variety, the witty Peruvians suggested she probably suffered from diarrhoea.

That men's tastes vary culturally is hardly news, but less well known is the extent to which the vogue for thin models is related to female emancipation. This was demonstrated by American psychologist Brett Silverstein. He established that in magazine pictures of women in the mid-Twenties, and from the mid-Sixties onwards, standards became very thin. Bustier women were in vogue between 1900 and 1920, and 1930 to 1960.

Silverstein pointed to the fact that men have been more likely in most societies to be the high achievers. At the same time, curvaceous women are perceived to be less competent and intelligent than non-curvaceous ones, and therefore women who want to be successful might minimise their visible femininity. Could female aspirations to male achievements be linked to varying popularity in the 'thin standard'?

First of all, he established that thinness and achievement are connected in women. Responding to silhouettes of female figures, women who preferred smaller breasts and buttocks were also more likely to choose 'masculine' careers and desire high academic achievement.

Disordered eating in modern girls is particularly common among high achievers. Girls from fee-paying schools are more at risk than those at state schools, and upper-class girls are more likely to desire to be slimmer than working-class ones.

This is also related to parental expectations. Silverstein showed that women undergraduates were more likely to binge eat if they felt their mothers were dissatisfied with their career performance. For women with brothers, bingeing was more likely if the father felt his son was more intelligent than his daughter.

A further study proved that those who had male sex-role aspirations were more at risk of eating disorders. In short, women who want to have 'male' achievements are more likely to aspire to a male body shape.

Silverstein's coup de grace was to correlate the bust-to-waist ratios of models in magazines since the beginning of the century, with the proportion of women working as professionals at the beginning of each decade up to 1980. When women were pushing harder to be taken seriously academically and professionally, a thinner body shape, closer to the male one, was in vogue.

Although girls have outperformed boys at school for some time now, it really does seem that pressure to do so increases the risk of eating disorders and the general tendency of girls to criticise their shape. While other factors are also important (notably, adverts), it seems we still have a long way to go before the equation 'successful = male' ceases to underpin our thinking.

You ask the questions

Is there anything about why we are as we are you'd like to ask Oliver James? Email him at oliver.james@observer.co.uk and his answers will be published in a special Ask the Experts edition of OM at the end of July