Scientists claim that the majority of children are overweight as a result of their genes rather than their lifestyle.
A study of more than 5,000 pairs of twins aged eight to 11 showed that genes contributed 77% of the amount by which their body mass and waist circumference varied.
Only 23% of the differences between the children was due to home environment.
Study leader Professor Jane Wardle, from University College London, said: "In today's environment, which provides unprecedented opportunities for all children to over-eat and be sedentary, it is not surprising these tendencies result in weight gain.
"It is therefore especially important to provide the best possible environment for all children to help protect those who are at higher genetic risk.
"We looked at identical pairs of twins who share all their genes and compared their measurements with non-identical pairs of twins who only share half their genes. Contrary to the widespread assumption that family environment is the key factor in determining weight gain, we found this was not the case."
The findings, published in the American Journal of Clinical Nutrition, showed that being served the same food, watching the same amount of TV, playing outdoors to the same extent, and going to the same school, did not necessarily make siblings more similar.