Not nearly as well as we think.
This study shows that our confidence about how well we know someone comes from the vividness of our impression of them — but that’s not the same as accuracy.
Rich impressions make us more confident that we know our long term partner but the research shows our beliefs might not be any more accurate than those of people who have dated briefly:
…Our findings suggest that the confidence people have in their impressions of others is, at best, sporadically related to the accuracy of those impressions. Such confidence-accuracy dissociations seem to occur because people base the confidence of their impressions on cues that are largely unrelated to accuracy. Specifically, people base their confidence on the richness of their impressions. What they seem not to recognise is that the richness of impressions is not equivalent to the veracity of impressions. As a result, people in long-term, committed relationships may be quite bullish about the accuracy of beliefs that are no more accurate than the beliefs of people who have just initiated their relationships.
Source: “Confidence and Accuracy in Person Perception: Do We Know What We Think We Know About Our Relationship Partners?” from Journal of Personality and Social Psychology, Vol. 73, No. 4, 747-757
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