Volunteer Bias In Human Psychophysiological Sexual Arousal Research: To Whom Do Our Research Results Apply?

Journal of Sex Research, May, 1999 by Joseph J. Plaud, George A. Gaither, Holly J. Hegstad, Leslie Rowan, Mary K. Devitt

Psychophysiological sexual arousal research has added considerably to our understanding of normal sexual functioning. Several researchers (Morokoff, 1986; Strassberg & Lowe, 1995; Wolchik, Braver, & Jensen, 1985), however, have begun to question just how far the results of this type of research can be generalized, as they have found a volunteer bias to exist. That is, when given the opportunity to participate in this type of research, only a small minority of the population does so. How does this impact our knowledge base? It is hard to tell, as most researchers do not explore this issue when reporting on their samples. Furthermore, although researchers have found volunteers for sexual arousal studies to differ from nonvolunteers on some personality and sexuality measures, the particular measures or constructs on which the differences have been found has varied across studies (Morokoff, 1986). Finally, no one has examined the reasons that individuals have given for choosing not to volunteer. Knowing why people choose not to volunteer would be beneficial as it would allow us to make inferences regarding the role of the personality and sexuality differences that exist. For example, if nonvolunteers gave pragmatic reasons for not participating (e.g., a lack of time or not finding the reimbursement for participating rewarding), the role of the personality and sexuality differences would probably have far less of an impact than if they gave more emotional, attitudinal, or value-laden reasons (i.e., not feeling comfortable having their sexual arousal measured or not wanting to be exposed to sexually explicit material). The purpose of the present study, then, was to explore the volunteer bias in more detail by examining not only differences between volunteers and nonvolunteers for a psychophysiological sexual arousal study on a number of different sexuality and personality measures, but also the reasons that people give for choosing not to volunteer. First, a brief review of the most prominent psychophysiological sexual arousal measures currently used will be presented, followed by a review of the literature on volunteer bias in this area.

Psychophysiological Measures of Arousal

The assessment of male sexual arousal patterns was first conducted by Freund (1963), using an instrument which recorded changes in penile volume (phallometry). In this seminal study, Freund was able to correctly classify participants according to their stated sexual preferences by measuring changes in penile volume in response to stimuli depicting male and female adults and children. Different types of penile circumference gauges (plethysmography) have been developed by other researchers (Bancroft, Jones, & Pullan, 1966; Barlow, Becker, Leitenberg, & Agras, 1970) since and are, in fact, more commonly used due to practical concerns (i.e., they are easier to apply and use, and they are more sturdy and reliable; Howes, 1995). Finally, Proulx (1989) reviewed all of the physiological measures in use at the time (e.g., skin conductance, heart rate, blood pressure, pupillary responses, and temperature) and concluded that penile responses are the only physiological response which is specific to sexual arousal in men, and can differentiate between sexual arousal and other arousal states such as anger and fear.

Among females, measures of physiological sexual response have generally included devices such as the isothermal relative blood flow transducer, the vaginal photoplethysmograph, and the labial thermistor-clip. The isothermal relative blood flow transducer consists of a diaphragm to which two thermistors are attached which measure relative vaginal temperature (Henson & Rubin, 1978). Similarly, the labial thermistor clip measures temperature change in the vaginal area by attaching a clip to the labia minora (Henson, Rubin, & Henson, 1979). The vaginal photoplethysmograph, perhaps currently the most widely accepted psychophysiological measure of female sexual arousal, was first used by Sintchak and Geer (1975). This device consists of a light source and a light sensor within a small cylinder that is inserted into the vagina in a manner similar to a tampon. This device provides both AC and DC output, measuring pressure pulse volume and pooled blood changes of the vagina, respectively (Henson et al., 1979).

To date, there has been no exhaustive review of the efficacy or reliability of the various measurement devices of sexual response in women. However, researchers have concluded that the measures are generally reliable (e.g., Henson & Rubin, 1978; Henson et al., 1979) although they are more physically intrusive than the male devices.

Volunteer Bias and External Validity of Research Findings

Some researchers (e.g., Rosenthal & Rosnow, 1975) believe that volunteer bias may be greater in sex research than in any other area of psychology. Researchers who have examined this issue have mainly been concerned with personality variables which may discriminate volunteers from nonvolunteers (e.g., Farkas, Sine, & Evans, 1978; Morokoff, 1986; Strassberg & Lowe, 1995; Wolchik, Spencer, & Lisi, 1983; Wolchik et al., 1985). For example, in one study (Farkas et al., 1978) male participants completed sexuality and personality questionnaires. They were also given a description of a further study that would involve having their sexual arousal measured via a penile plethysmograph and were asked to indicate whether they would like to volunteer. Farkas et al. found that while volunteers reported significantly more sexual experience, less sexual guilt, less sexual fear, and more past sexual difficulties (e.g., erectile dysfunction) than nonvolunteers, they did not differ on any of the other personality variables measured.