More than a quarter of infants and a third of junior pupils in four Swansea primary schools told Jacqueline Blissett of Birmingham University's school of psychology about their concerns with body size.
The findings, presented last weekend at a British Psychological Society conference, showed how most of the 100 children questioned were normal or under-weight, and yet a significant number still wanted to be thinner.
Almost 31 per cent of infants who wanted to be thinner felt they were overweight, of whom 71 per cent were girls. Ms Blisset said: "Those children who knew what dieting meant knew they had to cut out sweets and chocolate.
"They were primarily concerned with their appearance, health, attractiveness and fitness. Where the perception has come from I don't know, but they were very convinced that if you are thinner you are more attractive."
However, girls would appear to cope earlier and better with everday problems than boys, according to psychologists Gill Burdett and Harriet Gross from Loughborough University.
By eight, girls had developed a wider range of strategies to deal with situations such as bullying and falling out with friends.