A comparison of volume and circumference phallometry: response magnitude and method agreement

Penile circumference and penile volume phallometry are laboratory methods of assessing sexual arousal. Volume phallometry is reportedly more sensitive to responses, but comparative studies have been inconclusive and beset with methodological problems. In this study, 42 self-professed heterosexual volunteers were assessed with both methods simultaneously, employing a standard test for erotic partner preference. Pearson correlations between test outcome profiles were very high (r > .80) for subjects whose circumferential increase was > 2.5 mm [10% of a full erection (FE)]. However, among lower responders the agreement dropped precipitously (mean r = -.15). Moreover, as a group higher responders differentiated adult and pubescent age female stimuli from each other and all other categories with either method, but lower responders made this differentiation only with the volume method. We conclude that (l) at high levels of response both methods are equally good, (2) at low levels of response volumetric phallometry is a more accurate measure of arousal, and (3) 10% FE, or a 2.5-mm circumference increase, should be the minimum response criterion for the circumferential measure.