It is difficult to know whether assumed similarity reflects a psychological process or reflects a method effect: Ratings by the same observer of self and others share the same measuring device. However, based on extensive experimental evidence, it appears that assumed similarity is real. Assumed similarity effects are strongest for the Big Five factor of agreeableness. Assumed similarity seems to increase with acquaintance. Although one might thing that there would be assumed dissimilarity for ratings of outgroup members, no such effect is found.
Chapters 9 and 10 of Interpersonal Perception: A Social Relations Analysis
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