Genetics and Personality Inventories: The Limits of Replication with Twin Data

The consistency o f 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 M Z twins are fairly consistent across different samples while the correlations for DZ twins fail to show as much consistency. Differences, moreover, between M Z 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 CP1 scales loading on the extraversion-introversion factor are the most heritable. The implications o f sampling error and sampling bias for estimating genetic parameters from correlational twin data, for uncovering differential heritability o f personality traits, and for designing future research are discussed.