People are subconsciously influenced by the weight of those around them - so fat friends can cause someone to put on weight too, researchers suggest.
An international team, including University of Warwick experts, dubbed it "imitative obesity" - or "keeping up with the Joneses" on calories.
Their study, presented to a conference in the US, looked at data on 27,000 people from across Europe.
But one expert said the causes for the rise in obesity were much more complex.
The work, by scientists at the University of Warwick, Dartmouth College, and the University of Leuven, is being presented to an economics conference in Cambridge Massachusetts.
They suggest choices about appearance, on which decisions such as job offers or being deemed attractive are based, are determined by the choices others around you make.
So, if people around you are fat, it is permissible for you to be fat too.
It found nearly half of European women feel overweight, while just under a third of men felt the same.
Professor Andrew Oswald at the University of Warwick, who worked on the study, said: "Consumption of calories has gone up but that does not tell us why people are eating more.
"Some have argued that obesity has been produced by cheaper food, but if fatness is a response to greater purchasing power, why do we routinely observe that rich people are thinner than poor people?"
He said: "A lot of research into obesity, which has emphasised sedentary lifestyles or human biology or fast-food, has missed the key point.
"Rising obesity needs to be thought of as a sociological phenomenon not a physiological one.
"People are influenced by relative comparisons, and norms have changed and are still changing."
But Dr David Haslam, clinical director of the National Obesity Forum, said: "It's a bit cheeky to pin it on sociological influences - there's more to it than that.
"If you are surrounded by people, whether that's friends or within the family home, who are overweight, you are sharing the same environment where there is likely to be an abundance of the wrong kind of foods."