Differentiating rapists and non-offenders using the rape index

Some studies have suggested that phallometrically derived rape indices can differentiate groups of rapists and non-offenders. There are other studies however which cast doubt on this assertion, at least in so far as it applies to all but those few rapists who are sadistic. These studies have used rape indices which are derived from rapists' sexual responses to audiotaped descriptions of mutually consenting sex and brutal sexual assaults. It was hypothesized however, that stimuli which put more emphasis on the degrading and humiliating elements of rape would improve discriminability. Stimuli were compiled to test this hypothesis. The results indicate that neither rape indices derived from the physically brutal elements nor the degrading elements of rape were able to discriminate between rapists and non-offenders. Furthermore, these indices were not related to offence histories. The results are discussed in terms of their implications for the assessment of rapists and theoretical considerations of their behaviour.