Diners who choose unhealthy dishes from a menu can influence the food choices of those they are eating with, according the study by psychologists.
They found that people unconsciously mirrored the eating habits of the people they were with even if they were trying to diet, the Daily Telegraph reported.
The researchers believe it may help to explain why groups of friends often put on weight at the same time and why some women complain that they get heavier when they find a boyfriend, they start to copy his eating habits.
Dr Eric Robinson, a psychologist at the University of Birmingham, said: "There is a lot of evidence that points towards this idea that your friends make you fat. If you have got friends or people you know who have put on weight recently, then you will put on weight too."
"We found that pairing people with unhealthy eating partners reduced the amount of healthy food they were eating. They tended to pick the foods that were high in calories," Robinson said.
"Recognising that this effect is real could help people who are trying to watch what they eat. They could even look at a menu and try to decide what they are eating before those they are with to help avoid being influenced," he added
The research, which is published in the British Journal of Nutrition, asked 100 female volunteers to select from a choice of food consisting of healthy fruit and vegetables or unhealthy foods including crisps, pastries and cocktail sausages.
They found that when they were eating with someone who had been asked to pick unhealthy options, the volunteers also picked far more of these compared to when they ate alone or with someone who chose healthier foods.
Dr Suzanne Higgs, a reader in the psychobiology of appetite who also took part in the study, added: "This research underlines the social nature of eating and how this influences our behaviour."
