Researchers from the University of Liverpool asked participants to look at photographs of male models and categorise whether they were healthy weight, overweight or obese according to World Health Organisation (WHO) Body Mass Index (BMI) guidelines.
They found that the majority of participants were unable to correctly identify whether they were a healthy weight, overweight or obese person.
Participants instead underestimated weight, often believing that overweight men had a healthy weight.
In a related study of healthcare professionals, the researchers also found that GPs (General Practitioners) and trainee GPs were unable to visually identify if a person was overweight or obese.
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Their findings suggested that exposure to heavier body weights may influence what people see as a normal and healthy weight and causes people to underestimate a person's weight.
"We wanted to find out if people can identify a healthy, overweight or obese person just by looking at them," said Psychologist, Dr Eric Robinson, who conducted the research.
"Primarily we found that people were often very inaccurate and this included trainee doctors and qualified doctors too. Moreover, we found that participants systematically underestimated when a person was overweight or obese," said Robinson.
Recent studies have found that parents underestimate their overweight or obese child's weight and this could also act as a barrier to intervention.
"Over the last 30 years we have seen changes to population body weight, so examining how this has affected how we view our own and other people's body sizes is an interesting area of research," Robinson said.