That friend who stays thin despite eating anything and everything is not just annoying. She might also wreck your diet, new research suggests.

Researchers found that when they had college students watch a movie and snack with either a skinny or overweight companion, the students typically followed the thin friend's lead when she overindulged.

In contrast, study participants used more self-control when snacking with a heavier companion who overate.

The findings, published in the Journal of Consumer Research, suggest that seeing a thin friend devour a big meal gives us implicit permission to do the same.

"We think 'if she can eat like that and stay thin, so can I,' or 'she is having cake, then I can too,'" explained Dr. Brent McFerran, an assistant professor of marketing at the University of British Columbia in Kelowna, Canada.

"In other words," he told Reuters Health in an email, "the most dangerous person to eat with is not someone who is obese, but a thin friend with a large appetite."

For their study, McFerran and his colleagues recruited 210 female college students for what the participants believed was a study on movie viewing; each woman watched a movie with a companion, who was actually a member of the research team.

In some cases, the researcher showed up as her normal 105-pound self, while in others she donned padding that made her appear to be obese.

During the movie, the pairs were offered snacks, with the undercover researcher taking her portion first.

In general, McFerran's team found, the students ate more when their thin companion took a large portion, versus cases where the "obese" companion took a similarly large portion.

For example, when the skinny researcher ate a lot (30 candies), the participants ate an average of 10 candies. When the researcher was "obese" and ate a lot, the kids ate about 6 candies.

"Eating involves much social pressure," McFerran said, noting that social influences may, in fact, be the strongest predictor of what we eat.

But the current findings do not mean that we need to cancel all future dinner plans with our overindulgent skinny friends.

"If we think about what we are doing in advance," McFerran said, "we are less likely to overconsume."

He suggested that instead of looking at a thin friend who is gorging on dessert and feeling permission to do the same, we should remind ourselves that individuals are different — with different metabolisms and exercise habits, for example.

Focus on your own goal to eat healthfully, McFerran said, rather than automatically mimicking a friend.

He pointed out, however, that there is a "flip side": Friends who choose smaller portion sizes and healthier foods can encourage us to do so, especially if those friends are thin.