Many deciding factors come into play when you sit down to enjoy a meal. Even if you make a conscious effort to put healthy foods onto your plate, everything from your dish color and size to the lighting above your kitchen table can sneakily sabotage your efforts.

Now, a study published in the journal Appetite reveals another unexpected factor that can lead you to subconsciously eat much unhealthier than you intended to: your dining partner.

The study, co-authored by Katie Johnson of Mayo Medical School and Brian Wansink, Ph.D., director of Cornell University’s Food and Brand Lab and author of the new book “Slim by Design: Mindless Eating Solutions for Everyday Life,” found that you’re more likely to chow down on unhealthier foods when you’re eating with or near someone who is overweight. The reason? Eating with a less health-conscious pal makes it harder to remember—and stick to—your own health goals.

To test how another person’s body size and food choices alter our own decisions, researchers had an actress wear a prosthesis that added 50 pounds to her average-weight frame. The participants then dined with the actress, who served herself first, in one of four scenarios: either with/without the prosthesis and either choosing the healthier/unhealthier food option.

They found that participants took and ate over 31 percent more when the actress had the prosthesis on. And interestingly, when she appeared overweight and filled her plate with the salad choice, the unknowing dining companions helped themselves to even less salad and more of the unhealthy pasta dish.Past research has shown that others’ food choices can subtly affect our own. That’s why people tend to eat significantly more when they’re dining in a group—and why it’s sensible to order first to avoid being swayed by the selections of your friends. Whether you’re with one person (of any size) or a table full of people, Wansink recommends you go into your meal with a plan.

“Look up the menu beforehand and select a meal that suits your dietary goals,” he says. “Or, if you’re going to a buffet, pre-commit to selecting modest portions of healthy foods and with that goal in mind, those around you will have less of a negative influence over what you eat.”

MORE: Why You Need a Food Buddy