Those who take beta-blockers to control their blood pressure can often put on 3lb. Similarly, studies have shown that going on the Pill can add 11lb to a woman's weight.

Mother Nature may also be to blame, with our body shape being partly inherited. The overweight are also more likely to settle with partners of a similar size, and their children are more likely to be obese.

And the use of pollutants is on the rise, say the researchers. The manmade chemicals in pesticides and plastics can interfere with our hormones and lead to weight gain.

Your mother's age and your weight at birth are also important, with older women more likely to have obese children and underweight babies having a bigger chance of being obese in later life.

We also tend to put on weight as we get older. Finally, giving up smoking can also help pile on the pounds.

The scientists from Yale, Cornell and Johns Hopkins said there was only 'circumstantial' evidence supporting poor diet and lack of exercise as the main causes of obesity.

Ten other factors behind the flab:

Sleep - Too little increases appetite

Medicine - Many modern medicines lead to us putting on weight

Mother's age - Older women are more likely to have obese children

Choice of partner - Overweight people tend to pair with each other - and have overweight children

Heating and air conditioning - 'Comfortable' temperatures stop us from burning off fat