Psychological characteristics in twins discordant for smoking behavior: a matched-twin-pair analysis.

The matched-twin-pair methodology was used to investigate the pattern of interactions between twinning and personality in 127 monozygotic and 140 dizygotic adult male twin pairs. Eighteen standard and three research scales from the California Psychological Inventory were used to identify differences in personality between twins discordant for smoking and in nonsmoking and ever-smoking twins treated as individuals. Although no evidence was obtained for a significant heritability component in the smoking habit, numerous interactions between the twinning condition (e.g., monozygotic or dizygotic) and personality were identified. As expected, monozygotic twins discordant for smoking show fewer personality differences than do individuals discordant for smoking. Discordant dizygotic twins show more disparity than unrelated individuals. The only personality scale showing a consistent discriminating effect regardless of the twinning condition was that measuring socialization. Results are discussed in terms of the environmental conditions that may contribute differentially to the smoking behavior in related and unrelated individuals.