If you walk down a US city street and don't think a lot of people are overweight, you probably are. Likewise, thin people will increasingly be regarded as an anomaly to be eliminated out of concern as people get heavier. In a culture of obesity, thin is like a cancer.

Research by economists at the University of Warwick, Dartmouth College, and the University of Leuven, finds that people are subconsciously influenced by the weight of those around them - human beings keep up with the weight of the society they live in, which can lead to a spiral of imitative obesity.

The researchers will present their results on Friday July 25th at a National Bureau of Economic Research conference in Cambridge Massachusetts in a paper entitled Imitative Obesity and Relative Utility at the NBER Summer Institute on Health Economics.

Using data on 27,000 Europeans from 29 countries, the researchers find that nearly half of European women feel overweight. Less than a third of males feel overweight.

The authors suggest that whether for reasons of job promotions or finding a mate it is someone's weight relative to others that matters. They show that overweight perceptions and dieting decisions are influenced by people's comparisons with others of the same age and gender.

Highly educated Europeans hold themselves to a particularly tough standard, the research shows. For any given level of Body Mass Index (BMI), somebody with a university degree feels much fatter than someone with low educational qualifications.

Overall, the researchers believe that a person's "utility" (an economic term roughly meaning satisfaction levels) depends on their own weight relative to the weight of those around them. They suggest that it is easier to be fat in a society that is fat.

However, the authors also found a significant gender split. Females were much more prone, for any given BMI value, to feel overweight. For European women, weight dissatisfaction and overweight perceptions depended crucially upon not just their own absolute BMI, but also upon their BMI relative to other women of exactly the same age in their country. Conversely, being overweight tended not to be a significant issue for men if many of those around them were as overweight as they were.

Professor Andrew Oswald at the University of Warwick, one of the researchers, 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 emphasized 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."

However, the authors found a significant gender split. Females were much more prone, for any given BMI value, to feel overweight. For European women, weight dissatisfaction and overweight perceptions depended crucially upon not just their own absolute BMI, but also upon their BMI relative to other women of exactly the same age in their country.