I must not go to enough dinner parties, because I haven’t seen this behaviour. Sure, people behave differently in some social situations than they might in othe

I must not go to enough dinner parties, because I haven’t seen this behaviour. Sure, people behave differently in some social situations than they might in others, but I would judge it’s for all different reasons — an urge to impress, shyness, unfamiliarity etc. Characterising this particular behaviour as pretending to be stupid is a bit of a stretch in my mind. I think the answer is more simple; people often get anxious or uncomfortable in front of others they don’t know, or who they are attracted to. Consequently they can come off as a bit dumb or inarticulate. No need to "pretend to be stupid".