The many faces of assumed similarity in perceptions of personality

•Assumed and actual similarity on 74 variables were assessed in roommate dyads.•Findings suggest that participants do not assume others are similar to them.•Rather, they make use of actual similarity to derive accurate ratings of roommates.•A model of the judgment process was proposed to distinguish 3 forms of similarity.

We propose that most coefficients of assumed similarity in acquaintance ratings are based on actual similarity between observer and target rather than any bias to project self onto other. We assessed assumed similarity, actual similarity, and rater accuracy in 62 university roommate dyads who rated each other on 74 personality-related variables. Our findings indicate that our roommates (a) were actually alike on many variables, (b) knew this, rather than assumed it, and (c) applied this knowledge to enhance the accuracy of their ratings. We outline a model of judgment that urges three distinct concepts: true assumed similarity, known similarity, and spurious similarity. We conclude that personality ratings among acquaintances reflecting true assumed similarity are probably rare.