Genetic and environmental influences for type A-like measures and related traits: a study of twins reared apart and twins reared together.

The relative influences of genetic and environmental factors for Type A-like behaviors and related traits were examined in the Swedish Adoption/Twin Study of Aging. The sample consisted of 99 pairs of monozygotic twins separated at an early age and reared apart, 229 pairs of dizygotic twins reared apart, 160 monozygotic pairs reared together, and 212 dizygotic pairs reared together. The average age of the Swedish Adoption/Twin Study of Aging twins at the time of data collection in 1984 was 58.6 (SD 13.6); 72% of the pairs were over 50 years of age and 60% were female. The Framingham Type A Scale, three descriptors of the Type A behavior pattern (pressure, hard-driving, and ambitious), and measures of hostility and lack of assertiveness were assessed in a mailout questionnaire. Heritability (the proportion of total variance due to genetic effects) was 27%, 28%, 43%, 37%, 20%, and 12%, respectively, for the six measures. The most conservative test of significance indicated significant genetic influence for all but the hostility and assertiveness scales. Sharing the same rearing environment was generally unimportant for twin similarity in the Type A behaviors later in life; however, 20% of the variation in the hostility and assertiveness measures could be attributed to shared family environment. Evidence for the effects of correlated post-rearing environments was found for hostility. Approximately 60% of the variation in each of the measures can be attributed to non-shared environmental experiences unique to the individual.