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Personality trait similarity among ordinary relatives is surprisingly low, with parent–offspring and sibling–sibling correlations usually r ≤ .15. We explain why these correlations are biased in typical single-method studies and argue that this problem can only be addressed with multimethod designs. We also explain why ordinary relative comparisons can provide a more generalizable way of estimating (additive, narrow-sense) heritability than the better known twin comparisons. In a sample of parent–offspring (Npairs = 522), sibling–sibling (Npairs = 388), and second-degree relative pairs (Npairs = 476), who rated their Big Five personality traits and life satisfaction and were each rated by an independent informant (Nparticipants = 2,258 + informants), we found that parent–offspring and sibling correlations were about one third higher than typically shown (r ≈ .20). Based on the ordinary relative comparisons, the heritability of personality traits and life satisfaction was about 40%, compared with about 26% typical to self-report studies. Life satisfaction was as heritable as personality traits, sharing about 80% of its genetic variance with neuroticism, extraversion, and conscientiousness. About half of life satisfaction’s phenotypic correlations with neuroticism and extraversion and its entire correlation with conscientiousness were explained by shared genetic factors. Using data from a larger sample of relatives with only self-reports (Nparticipants = 32,004; Npairs = 24,118), we provide further evidence that growing up together does not make people more similar. The results were consistent for both aggregate traits and individual items. Only multimethod designs can accurately reveal traits’ similarity among relatives and their genetic and environmental transmission. (PsycInfo Database Record (c) 2025 APA, all rights reserved)
