Volunteer bias in erotica research: effects of intrusiveness of measure and sexual background.

Volunteer characteristics and volunteer rates across several laboratory experiments of sexual arousal were compared. Conditions were created to assess which component of the experimental setting was responsible for low volunteer rates in experiments using genital measurement. Subjects were 324 male and 424 female undergraduate students who had volunteered for an experiment on sexuality and personality. After completing several measures of sexual experience and attitude, subjects received a written description of one of the following conditions and were asked if they wished to volunteer: sexual film, sexual film and subjective rating of arousal, sexual film and assessment through forehead temperature, sexual film and assessment with a device that was placed over the clothes and measured genital heat flow, sexual film and assessment with the heat flow device while partially undressed, or sexual film and assessment with the vaginal photoplethysmograph or penile strain gauge while partially undressed. Men were significantly more likely to volunteer than women, and volunteer rates for both men and women decreased significantly when and only when subjects were required to undress. Multivariate analyses of variance revealed that both male and female volunteers were more sexually experienced, reported more exposure to erotic materials, and worried less about their sexual performance than nonvolunteers. No differences in volunteer characteristics occurred across the increasingly intrusive conditions for women while a few differences occurred for men. The present findings suggest that researchers should be cautious about discussing the generality of findings of studies involving exposure to a sexually explicit film alone as well as of experiments that involve self-report or physiological measures of sexual arousal.