234 pairs of twins were studied from pregnancy up to 21 years of age on the basis of records from maternity hospitals, neonatal wards and children's health centres and questionnaires filled in by the parents when the twins were aged 2-10 and 12-21 years, and by the twins themselves at age 12-21. 74 twins were personally interviewed about human relationships in their families and with the Present State Examination (PSE) at age 15-21. When the evaluation of parental preference was made by the parents, the mother's favourites had learned to speak earlier and were more often the psychic leader of the pair, but they more often had sleeping difficulties and other psychosomatic symptoms in adolescence. They were most often scored in class 2-3, non-specific neurotic symptoms in the PSE, but none of them was placed in the higher classes of possible or probable psychiatric disorder. Mothers seem to develop a tighter affectionate bond towards their favourites than do fathers, thus inducing a good basic trust and faster language acquisition in childhood, but probably also transient non-specific neurotic symptoms in adolescence in face of the developmental task of entering autonomous adulthood. The father's favorites were more often the physical leaders of the pair, showed less accident proneness and most often reported tendencies towards autonomy from their co-twins, thus indicating that the fathers' attitudes may be more encouraging towards independence. As the least psychosomatic symptoms were seen in twins in the intermediate position regarding parental preference, it seems reasonable that the division of twins between parents on the grounds of favouritism should not be strict.