A new report claims the families of obese people have an increased chance of being overweight themselves.
The study in the New England Journal of Medicine found that obesity is "socially contagious", spreading from person to person in a social network.
This means that even the friends of overweight people have a risk of gaining excess pounds as well.
Scientists found the children of obese people have a 40% greater chance of being obese themselves, while their spouse has a 37% increased risk.
The researchers looked at 12,067 adults over 32 years who underwent numerous medical assessments as part of a heart study.
Nicholas Christakis, co-author of the study and a professor at Harvard Medical School's department of healthcare policy, said: "It's not that obese or nonobese people simply find other similar people to hang out with. Rather, there is a direct, causal relationship.
"What appears to be happening is that a person becoming obese most likely causes a change of norms about what counts as an appropriate body size.
"People come to think that it is ok to be bigger since those around them are bigger, and this sensibility spreads."