When the actress served herself, while wearing the 'fat suit', those taking part in the study opted for the more unhealthy option - pasta.
When she was slimmer, they were more inclined to choose the salad.
Furthermore when the 'fat' actress chose to eat a large portion of salad, the participants ate less salad.
The scientists, from Southern Illinois and Cornell universities, concluded the effect is the result of people being less reminded of their health goals when they are around overweight people.
The study authors wrote: "The purpose of this study was to examine whether or not the presence of an overweight eating companion influences healthy and unhealthy eating behavior, and to determine if the effect is moderated by how the companion serves himself or herself.
"Results demonstrated that regardless of how the confederate served, participants served and ate a larger amount of pasta when she was wearing the prosthesis than when she was not.
"In addition, when the confederate served herself healthily, participants served and ate a smaller amount of salad when she was wearing the prosthesis than when she was not.
"Consistent with the 'lower health commitment' hypothesis, these results demonstrated that people may eat larger portions of unhealthy food and smaller portions of healthy food when eating with an overweight person, probably because the health commitment goal is less activated.
"More generally, this study provides evidence that the body type of an eating companion, as well as whether she serves herself healthily or unhealthily, influences the quantity of food intake."
It comes as another study, by researchers at Liverpool University found almost nine in 10 people can't tell when someone is dangerously overweight.
And the more fat friends someone has - the worse they are at judging when someone needs to lose weight.
The Liverpool University researchers blamed the rising tide of obesity for changing our perception of what is a normal weight.
They said the phenomenon of fat blindness is important because acknowledging we have a problem is the first step in embarking on a diet.
Researcher Melissa Oldham showed 1,000 people photos of men of various sizes and asked them to say whether the males were of a healthy weight, overweight or obese.
The volunteers correctly judged 75 per cent of the photos of the slim men.
However, they were less good at recognising when someone was overweight, the UK Congress on Obesity in Birmingham heard.
They judged the pictures of overweight men correctly only 30 per cent of the time. And worryingly, recognised obesity in just 13 per cent of cases.
Analysis showed that they weren't just bored or bad at the task - instead they were getting it wrong because they were systematically under-estimating weight.
Miss Oldham, a PhD student, also showed that more overweight and obese friends someone had, the more likely they were to be blind to fatness.
She said she was surprised at just how bad the volunteers were at judging weight and added: "This could be of public health relevance as recognition of obesity is necessary to motivate behaviour change and weight loss."
Previous research has shown that parents often underestimate their children's weight. Even doctors are not immune to the trick of the mind, with studies showing they "quite regularly" think patients are thinner than they are.
- Daily Mail
