They found, for some people, the sight of obesity sparked strong subconscious reactions, such as disgust.
The Evolution and Human Behaviour study suggests this is part of a deep-seated behavioural reponse designed to help detect and avoid those with infections.
"Our results suggest that it might be ameliorated through interventions that focus on individual's often irrational concerns about infectious disease."
Obesity 'contagious', experts say
Having a friend, sibling or spouse who is overweight raises a person's risk of being obese too, US researchers say.
"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."