Genetics and personality inventories: The limits of replication with twin data

The consistency of twin data with personality questionnaires is examined using all reported twin samples which have been administered the California Psychological Inventory. The scale correlations for the MZ twins are fairly consistent across different samples while the correlations for DZ twins fail to show as much consistency. Differences, moreover, between MZ and DZ correlations fail to replicate across samples. Sampling error and sampling bias are proposed as the major reasons for the inconsistency, and when these factors are taken into account the resulting heritabilities suggest that the CPI scales loading on the extraversion-introversion factor are the most heritable. The implications of sampling error and sampling bias for estimating genetic parameters from correlational twin data, for uncovering differential heritability of personality traits, and for designing future research are discussed.