If you're watching your waistline, beware of skinny friends who pack in the food. That's the upshot of a new study from the University of British Columbia tracking how our eating choices are influenced by the plates and body types of those around us.

Participants were brought into a lab ostensibly to help with an experiment on movie viewing, with each volunteer paired up with another person who was a "confederate" working with the researchers - unbeknownst to the volunteer. Half the time, the confederate was a thin size-0 woman and the other half the time, it was the same woman wearing a fat suit that made her appear to be a size 16.

Each pair was offered a snack of granola or M&M's and the confederate always went first and took a big helping of the snack. When she was thin, the participants heaped their plates too, taking even more food than people did when they were in the room alone with no one influencing them. When the confederate was obese, the volunteers took a smaller amount of food, though still more than they did when they were alone.


"You see this thin person take a lot of food and you're like, 'Cool, I can, too. She's thin, she's heaping it on, sign me up,'" says Brent McFerran, an assistant professor of marketing at the university and lead author on the paper. "If you see that same portion choice being made by someone who's obese, you're like, 'Well, maybe I should pare back, not take quite as much, or else I could become like them.'"


The difference was dramatic, with people taking 61 per cent more M&M's and 64 per cent more granola when they observed a thin person loading up their plates.


"The magnitude of the effects were surprising; there are very powerful effects," says McFerran.


People use the eating choices of others as an "anchor" that influences their own consumption, he says, but they adjust that based on other people's body type, modelling the behaviour of thin people because that's what most aspire to.


"The most dangerous people to eat with are not heavy people, as a lot of other research suggests - it's thin people who eat a lot of food," he says.

A second study showed the effect also works in reverse: When the thin confederate took a tiny amount of food, people scaled back their own eating, but when the obese person took only a little food, people indulged a little more.

"You see a thin person take a very small portion, you think, 'If I'm going to be thin, I better do likewise,'" McFerran says. "But you see someone who's obese take a small portion, you think, 'Well, maybe she's on a diet, she's not like me, I can take a little extra,' so you end up taking more."

The researchers also found that people who are dissatisfied with their appearance are more strongly influenced by this effect than those who are more confident. "It's not really driven by whether you're heavy or thin yourself, but rather how you feel about your own body image," McFerran says.

But regardless of body type or confidence, other people's choices exert a powerful influence on our own consumption, he adds. "If you go for ice cream and three people in front of you get the extra-large, you're not going to get the small." The paper will be published in the April issue of the Journal of Consumer Research.