The Cook and Medley HO scale: a heritability analysis in adult male twins

Thirty-seven pairs of monozygotic (MZ) and 60 pairs of dizygotic (DZ) middle-aged, male, American twins were studied to determine the heritability of the Cook and Medley Hostility (HO) scale and its two subscales, Cynicism and Paranoid Alienation. No clear evidence for a significant genetic component was indicated for the full HO scale and the Paranoid Alienation subscale. Scores on these scales were found to be associated with age, socioeconomic status, the twinning condition, and the Minnesota Multiphasic Personality Inventory (MMPI) K scale. DZ twins scored higher than MZ on HO and Cynicism, and lower on the MMPI K scale. Statistical adjustments for age and socioeconomic status removed twin mean differences and weakened MZ intrapair correlations on the HO scale but did not change overall conclusions regarding heritability. After the association with K was partialled out from these scales and retested for heritability, both HO and Paranoid Alienation showed a weaker twin pair similarity than that observed in the unadjusted scales. However, Cynicism, the scale with the greatest item overlap with the K scale, was not affected by the adjustment. Since no genetic component was evident for the K scale in this sample it was concluded that if a genetic influence in these MMPI scales is present, it is mostly in the Cynicism subscale of HO.