ABSTRACT Preliminary findings from an on‐going study of monozygotic twins reared apart (MZA) and data from a larger sample of twins reared together (MZT and DZT), indicate a surprisingly strong influence of genetic variation on aptitudes, psychophysiological characteristics, personality traits and even dimensions of attitude and interest. For some of these variables, MZT and MZA twins show high intra‐class correlations while DZT twins are no more similar than pairs of unrelated persons. It is suggested that such traits are “emergenic,” i.e., that they are determined by the interaction—rather than the sum—of genetic influences. Emergenic traits, although perhaps strongly genetic, will not tend to run in families and for this reason have been neglected by students of behavior genetics. For this and several other listed reasons, wider use of twins in psychological research is strongly recommended.
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