Girls as young as nine see exercising as a way of keeping fit and controlling weight, while boys of the same age are more likely to associate exercise with fun.
Joint research by academics at Luton, Birmingham and City universities shows that young girls and boys have markedly different reasons for exercising.
Carol Burrows, acting head of sport and exercise science at Luton, said:
"The fact that about a third of nine-year-old girls are already preoccupied with the cosmetic outcomes of exercise may have implications for their future eating and exercise patterns. To me it looks as if adults are influencing children with negative stereotypes, which seem to have a far greater impact on young girls than on young boys."