Changes in Vaginal pH and Lubrication in Response to Sexually Coercive Stimuli

Women’s genital response to sexual stimuli is cue non-specific, such that women display similar levels of genital arousal in response to preferred and non-preferred sexual stimuli, which is discordant with their subjective reports of sexual arousal. One explanation for this phenomenon is the preparation hypothesis, which proposes that women’s genital response to sexual stimuli evolved to be cue non-specific to reduce injury to the vaginal walls during both consensual and non-consensual intercourse (i.e., sexual coercion, forced copulation, or rape). Consistent with the preparation hypothesis, women display genital arousal (including vasocongestion and vaginal lubrication) in response to audio stimuli depicting rape. However, over human evolutionary history, rape imposed special and severe adaptive costs on women, notably loss of control over mate choice and, potentially, rearing without co-parental support a child whose sire is of unknown genetic quality. The spermicidal hypothesis of vaginal acidity proposes that women’s cue non-specific genital responsivity evolved to prevent fertilization by sexually coercive men of unknown genetic quality. Women’s vaginal pH, which changes over the ovulatory cycle, and in response to sexual stimulation, may afford a mechanism by which women prevent fertilization by rape. The current research employed a within-subjects design to compare changes in vaginal lubrication and pH in response to different audio stimuli to investigate whether women have evolved a physiological mechanism (i.e., production of more acidic vaginal pH) to avoid fertilization by rape. The main goal of the present research was to determine the effect of coercive versus consensual sexual stimuli on women’s genital arousal. Specifically, I predicted that women would produce more acidic (i.e. lower pH) vaginal lubricant in response to sexually coercive audio stimuli than to neutral, sexually consensual, and violent non-sexual audio stimuli. An additional goal of this research was to replicate and extend previous research by testing the effect of different audio stimuli on vaginal lubrication. Therefore, I also predicted that women would produce more vaginal lubrication in response to a sexually coercive audio stimulus than to neutral and violent non-sexual audio stimuli, but less lubrication in response to a sexually consensual audio stimulus. Results largely failed to support the predictions of the present study; however, several significant interactions emerged from the repeated-measures ANCOVA. Namely, the effect of experimental condition on amount of vaginal lubrication, and changes in vaginal pH depended on women’s fertility status. Discussion offers potential explanations for these interactions, addresses the many limitations faced by the present study (e.g., potentially inaccurate assessments of pH and fertility), and outlines a path forward for future research regarding changes in vaginal pH.