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. (C) 2013 Elsevier Inc. All rights reserved.