The heritability of adult attachment styles as measured by the Relationship Scales Questionnaire (RSQ) was estimated on data from a volunteer general population sample of 220 adult twin pairs (116 monozygotic pairs. 104 dizygotic pairs). Additive genetic effects accounted for 37%, 43%, and 25% of the variance in the secure, fearful, and preoccupied adult attachment styles, respectively, but none of the variance in the dismissing style. Nonshared environmental influences accounted for the majority of the variance in all styles: 63% secure, 57% fearful, 75% preoccupied, and 71% dismissing. Shared environmental effects were negligible for all styles except dismissing (29%).