A number of studies have shown that the probability that a man is homosexual increases with the number of older brothers (but not sisters) born to his mother. This older-brother effect suggests that a progressive maternal immunosensitization process is involved in producing homosexual preferences. A recent demonstration of the older-brother effect in homosexual pedophilia raises the question about whether it is involved in other anomalous sexual preferences as well. In the current test of the older-brother effect, phallometric data were obtained from 29 child molesters, 38 rapists, and 11 individuals who had offended against both children and adults. In all groups, a phallometric index of sexual deviance (a relative preference for children or for coercive sex) was positively correlated with the offenders' number of older brothers (but not sisters). These findings suggest that the maternal immunosensitization hypothesis may explain some variations in male sexual preferences. 1 figure and 33 references