Classically, incest has been considered from both a psychological and sociological point of view to have harmful consequences. Genetic research, though by no means lacking controversy of its own, generally supports the notion that inbreeding has untoward genetic consequences. The psychodynamics of all three parties to father-daughter incest seem to indicate that people who become involved in incestuous behaviour are often psychologically damaged before the fact, so that if they show subsequent evidence of psychological impairment the incestuous behaviour can be as plausibly viewed as a dysfunctional attempt at solving problems as it can a cause of subsequent psychopathology. Girls involved in the father-daughter incest present in one of half a dozen frequent clinical syndromes. The presentation is influenced by the degree to which the girl may have participated in ongoing incestuous behaviour as opposed to being the presumed victim of an older adult's coercive actions or her own temporary suspension of a behavioural taboo. Research is inconclusive as to the psychological harmfulness of incestuous behaviour, and evidence is reviewed on both sides of this complicated and controversial question. Quite apart from the general issue of the harmful-ness of incest, a number of indicators can be derived from the nature of the incestuous episode and the early response to therapeutic assessment which aid in the clinical forecasting of probable outcome.