The most recent version of the phallometric test of erotic gender and age preferences was administered to obtain a differential diagnosis between pedophilia and gynephilia for the sample of 187 male, non-psychotic, and almost exclusively white 18- to 55-year-old sex offenders against females: 95 were incest offenders, 54 were non-incest offenders, and 38 were offenders against adult females. There were significantly more gynephiles among incestuous sex offenders against children than among non-incestuous offenders, which validates the father role as a predictor of gynephilia. Victimization of only one child emerged as another such predictor but appeared to be weaker than an incestuous situation. This suggests that an incestuous situation and victimization of only one child are each predictors in their own right but most likely do not interact with each other, and, in an incestuous situation, victimization of only one child loses its potential as a predictor of gynephilia. Finally, the study demonstrated a difference between the proportions of single-case victimizers in incestuous and non-incestuous offenders. More single- case victimizers emerged among incestuous offenders. 2 tables and 33 references