The study found almost half of girls and a third of boys aged between seven and 12 wanted to be thinner.
And scientists blame the body shapes of skinny celebs like Geri Halliwell and Posh Spice, Victoria Beckham.
From a selection of photographs of children, they were asked to pick out which image they thought was most like their own and which one they thought was an ideal figure.
Scientists found that 48% of girls selected an ideal body image that was thinner than their own. With 36% of boys preferring a thinner body image.