NCBI ROFL: For some reason, women don't volunteer for vaginal photoplethysmographs.

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.