Overweight people are often bullied and discriminated against because ancient fears that fat humans may be diseased causes their thinner counterparts to dislike them, say scientists.
In a study, reported in the journal Evolution and Human Behaviour, the researchers have found that the mere sight of someone who is overweight can trigger feelings of disgust and nausea similar to encountering rotten food.
Since bacteria and viruses are invisible, human brains have evolved to react to outward signs of disease like rashes and wounds, and these signs also include excessive body fat, suggests the study.
"Our results reveal, for the first time, that this prejudice may be rooted in multiple, independent mechanisms," the research team added.
The study showed that feelings of disgust towards the obese were strongest in people with the greatest fear of disease.
Participants who agreed with comments like "it really bothers me when people sneeze without covering their mouths" in the questionnaires given were found to be more likely to agree with statements such as "if I were an employer looking to hire, I might avoid hiring a fat person".
"The research sounds reasonable enough I suppose; obesity is unhealthy and if there is something in us that helps us avoid ill health I can understand that," television presenter Anne Diamond, who helps people conquer obesity after briefly ballooning to 14st 9lb herself, said.
"But I don't believe that in the 21st century we can use it as an excuse for prejudice. Obesity is an illness, but something becoming so common that it will soon be the norm," she added.