I am not sure about this. I think the research is skewed some. I may be different as I like to see people happy and it does not matter if they are a stranger or not if they are happy and their faces reflect that I see them as happy. I do not look at people that I know as being happier.
KN,
I agree with you. I tend to look for the expression rather than if I know the person. Maybe it's because I have a couple of friends who are spooks and they have taught the rudiments of reading facial expressions… I'm by no means an expert here, barely a beginner. But I don't think knowing someone would skew my perception of their feelings.