Adult monozygotic (MZ) twins (63 pairs), same-sex dizygotic (DZ) twins (35 pairs), and same-sex non-twin siblings (40 pairs) completed the Personality Research Form-E (PRF-E) and the Sibling Evaluation of Personality Characteristics (SEPC). The PRF-E yields scores and profiles on 20 personality traits. The SEPC presents subjects with brief descriptions of the same 20 traits and asks them to rate how well they think each description fits themselves and fits their sibling. The results indicated that MZs, DZs, and sibs all rate themselves on the SEPC as being more similar to one another than the PRF-E indicates they ''actually'' are. However, the discrepancies between rated (SEPC) and ''actual'' (PRF) degrees of similarity are quite small and essentially the same for the 3 groups. This notwithstanding, across all 20 personality traits them is a substantially higher correlation between rated (SEPC) and actual (PRF-E) degrees of similarity for MZs (r = 0.53) than for DZs (r = 0.15) or sibs (r = 0.24). Thus, MZs appear to be better judges than either DZs or sibs of how similar/dissimilar they are across the 20 personality traits assessed by the PRF-E.
