People tend to eat more when they’re having dinner with friends and family. Scientists refer to this effect as the “social facilitation” of eating. Now, a new study has come up with several evolutionary and psychological hypotheses that explain our tendency to eat more grub when having company.

Researchers at the University of Birmingham, UK, performed a meta-analysis of 42 studies that focus on the effects of social facilitation of eating. Some of these observed participants eating either alone or with company, while others relied on the participants’ self-reported social eating habits, such as those recorded in a food diary.

The results suggest that when we’re eating out with friends, meal sizes were between 29% and 48% larger compared to when eating alone. This was especially the case for fatty and protein-rich foods such as hamburgers and other meat-based meals.

Interestingly, sometimes people would eat less than they would normally do alone in certain social situations. For instance, women tended to eat smaller portions in front of men — whether they were strangers or friends — and overweight people ate smaller portions in public, perhaps out of fear of being judged. According to one study, these groups would eat 18% less food when with others compared to when they were alone.

“People want to convey positive impressions to strangers. Selecting small portions may provide a means of doing so and this may be why the social facilitation of eating is less pronounced amongst groups of strangers,” Dr. Helen Ruddock, from the School of Psychology at the University of Birmingham, said in a statement.

“Findings from previous research suggest that we often choose what (and how much) to eat based on the type of impression that we want to convey about ourselves. Evidence suggests that this may be particularly pronounced for women eating with men they wish to impress and for people with obesity who wish to avoid being judged for overeating.”

The researchers claim that social facilitation may be a remnant of our hunter-gatherer days when food was scarce and communities used to share a common food source. In such tight-knit groups, equal distribution of food resources must have been important — a behavior that has been observed elsewhere, in other species like chickens, rats, and gerbils.

Eating more than other tribe members would have been seen as unfair, leading to ostracization, which could jeopardize food security in the future for a greedy individual. But, if that’s the case, why eat more?

Dr. Ruddock says that the eat more when we’re trying to match the consumption of others in the group. “Individual members match their behaviour to others, promoting a larger meal than might otherwise be eaten in the absence of this social competition,” she commented.

“What we describe as ‘social facilitation’ can be seen as a natural by-product of social food sharing – a strategy that would have served a critical function in our ancestral environments. This also explains why it is more likely to occur in groups with individuals who are familiar with each other,” Ruddock added.

Eating in the presence of others is also generally thought of as more pleasant, so there may also be a psychological reward for social eating, which may trigger over-consumption.

The problem is that we’re now living in a food-abundant society. Although we have no issue accessing food, we may still be acting on psychological mechanisms that are ingrained in us from our hunter-gather days. Just another thing to look out for when trying to meet your dietary goals.

Findings appeared in The American Journal of Clinical Nutrition.