Twin study is extended to include intrafamilial dynamics apparently leading to the choice of homosexuality or heterosexuality in twins. Four sets of twins are studied. Of these, two cases, not of the same pair, were in psychoanalysis; the remainder were in psychotherapy. Factors such as the prenatal fantasies of the parents regarding the sex of the twin, difficulties in childbirth, naming, anatomical differences, and the father's position in the family lead to a preference of one of the pair by one parent and the other child by the other parent. Effects on subsequent Oedipal outcome and object choice are discussed.