Before Freud, homosexuality was usually thought to have constitutional causes. Indeed, some recent studies have again proposed that genetic factors play an important role in its aetiology. Kallmann (1952), for instance, studying forty male homosexuals with identical twins, found that 39 of the twins were also homosexual. Slater (1962) suggests that homosexual patients tend to be the younger children in their families and to have elderly mothers, so that they might have been born with chromosomal abnormalities. There is, however, a growing tendency to assume that one of the causes of homosexuality is the lack of normal childhood relations with the parents. Fenichel (1946) suggests that homosexuals tend to be mother-fixated and to have identified with their mothers because, while their fathers were weak, their mothers were domineering, and psychoanalysts, in general, agree with his views. West (1959), who compared homosexual and heterosexual in-patients in a mental hospital, found that the combination of an over-intense relationship with the mother and an unsatisfactory relationship with the father is typical for the homosexual. Bieber et al. (1962) compared homosexual and non-homosexual patients in psychoanalysis. They claim that in the majority of cases the behaviour of the mothers of homosexuals toward their sons was seductive and over-indulgent, domineering and inhibiting, while the behaviour of their fathers was hostile, ambivalent or indifferent. O'Connor (1964) compared homosexual patients with heterosexual neurotic ones, all of whom were serving in the Air Force. He found that a high proportion of the homosexuals had, in their childhood, poor relations with their fathers and had been more attached to their mothers, though some had not got on well with their mothers either. He also reports that a number of the homosexual patients were raised in foster-homes, while none of the heterosexual ones were. The main difference between his two groups was that a much greater proportion of the homosexuals did not have normal relationships in their childhood with their fathers. West (1959) quotes Liddicoat (1956), whose subjects were non-patients and who found that unsatisfactory relations with the father were more common in her sample than undue attachments to the mother. So did Westwood (1960), whose subjects were also recruited from among non-patients, and who states that many homosexuals from his sample had over-possessive or over-protective mothers, but even more had inadequate or absent fathers.