We evaluated dominance-submissiveness between co-twins and its relationship to mental health in a cohort study of 419 twins followed from pregnancy to 22–30 years of age. Dominance-submissiveness between co-twins was assessed from three separate perspectives: physical dominance, psychological dominance, and verbal dominance. Depressive, nervous, and psychosomatic symptoms were analyzed in different twin groups. In the physical domain, males were more commonly dominant than females at school age and in adulthood. Before and at school age, girls were more dominant than boys in the psychological and verbal domains, as well as in total dominance. These differences disappeared in adulthood, and 81% of adult twins felt themselves equal to their co-twin in total dominance. Submissiveness in the psychological domain seemed to be associated with increased depressiveness, nervous complaints and psychosomatic symptoms in males of male-female twin pairs. Verbally submissive males in same-sex twin pairs had more depression and psychosomatic symptoms. Among females of same-sex twin pairs, submissiveness in the psychological domain was most clearly associated with depressive symptoms, whereas psychological or verbal dominance-submissiveness among females from male-female twin pairs was not associated with symptoms. Psychologically dominant males and females of same-sex twin pairs expressed greater nervousness than did their co-twins. We conclude that being submissive, especially in the psychological domain, to a female twin partner seems to be stressful, whereas it is easier, especially for females, to be submissive to a male twin partner.

From 1964 to 1971, twins and singleton controls were followed from 10 to 16 years of age in Swedish schools. It was a nationally representative sample of 323 twin pairs, and 1193 classmate controls. In 1991 a follow-up was made of this sample with the purpose of investigating genetic influences on experienced family and school environments, as well as coping ability at the age of 35. The follow-up also evaluated the relation between coping ability at mid-life and family environment, as well as coping ability and school environment at adolescence for males and females. Genetic influences on perceived family and school environments, as well as on self-reported coping ability, have been investigated by means of comparisons of MZ and DZ twin pairs. A stepwise linear regression analysis was performed to assess environmental influences on coping ability at mid-life. The findings indicated that coping ability at mid-life is influenced by both genetic and environmental factors. Different environmental factors were operating for males and females, however. The results demonstrate that early environmental factors and genetic disposition have a long-term effect on coping ability.