A September 2019 study by the University of Birmingham found,
"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.
"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."
"A solution to this tension may be to eat at least as much as 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."
"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."