The equal environments assumption is important in behavior-genetic research on monozygotic (MZ) and dizygotic (DZ) twins, and was tested in 525 MZ, 200 same-sex DZ, and 68 opposite-sex DZ pairs. Similarity of twin environments was assessed by a self-report questionnaire on childhood experiences, and personality resemblance was measured using the self-report and peer-report versions of the NEO-Five-Factor Inventory. MZ twins reported more similar experiences than DZ twins and were also more alike in personality. Across twin pairs, however, treatment similarity was unrelated to personality resemblance, except in the combined group of MZ and DZ twins. These results are consistent with the equal-environments assumption.
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