Abstract The sample consisted of 419 twins, born in 1965–1973, from Northern Finland who had been followed at ten-year intervals, at 2–10 years, 12–20 years and at 22–30 years of age. Data on psychosomatic symptoms, parent-twin and inter-twin relationships were elicited, and twins completed the Children's Depression Inventory modified for age. Middle adolescence appeared to be the most difficult phase of puberty for twins as far as depressive and psychosomatic/somatic symptoms were concerned. When different twin pairs were evaluated separately, the males of opposite-sex twin pairs seemed to be in the most favourable position, particularly in late adolescence, as they reported least depressive symptoms. Depressive and psychosomatic symptoms were evaluated in relation to parental preference among young adult twins. Parental preference was evaluated in two directions: which one of the parents was reported to feel as being closer to the twin – experienced parental preference evaluated by the twin, and which one of the parents the twin felt closer to – twin's own preference. Those males who were equally close to both parents (experienced parental preference) had least total depressiveness, while females in intermediate situation had the highest self-confidence and least anhedonia and nervousness. According to twins' own preference, twins who felt equally close to both parents had least total depressiveness and anhedonia. The intermediate position seems to be the best alternative, as these twins had the least symptoms. Psychosomatic and depressive symptoms were evaluated in relation to co-twin dependence in young adult twins. MZ twins, especially MZ females, reported most often co-twin dependence at all ages. There were no significant differences in depressive symptoms between dependent and independent twins. Twin's subjective experience about co-twin dependence appeared to be important for the twin's mental well-being, as dependence-independence imbalance within twin pair was associated with elevated levels of depressive symptom reporting, especially in twins who perceived themselves as dependent and the co-twin as independent. Dominance-submissiveness between co-twins and its relationship to mental health was assessed in young adulthood. Dominance-submissiveness in the twin relationships was assessed separately in three domains of life: physical and psychological dominance-submissiveness and the role of a spokesperson. Submissiveness in the psychological domain seemed to be associated with increased depressiveness, nervous complaints and psychosomatic symptoms in males of male-female twin pairs. Among females of same-sex twin pairs, submissiveness in the psychological domain was most clearly associated with depressive symptoms. 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. This was in contrast to co-twin dependency, which was experienced positively when occurring towards a twin sister.