The effect of stimulus content on volunteering for sexual interest research among college students.

Much of the literature on volunteer bias in sexual interest research has been limited to either identifying variables on which volunteers differ from nonvolunteers or examining how the intrusiveness to the measurement device affects volunteer rates. The present study was designed to examine whether stimulus content would affect volunteer rates and volunteer/nonvolunteer differences in a large sample of college students (206 men and 358 women). The study also sought to determine whether such finding could also apply to research that recruits from exclusively heterosexual samples. Students completed questionnaires in small groups and were asked whether they would be willing to volunteeer for studies that would involve viewing and rating five different types of sexually explicit images (nude men, nude women, heterosexual behavior, male homosexual behavior, and female homosexual behavior). Results indicate that men and women differed in the types of images that they would volunteer to view, based upon the content of the images. Furthermore, volunteer for each type of the image reported significantly greater self-monitoring as well as sexual and general sensation seeking than did nonvolunteers, while differences on other measures were less consistent. Discussion is given to specific ways in which the findings and generalizability of sexual interest study results can be affected.

It is commonly believed that sexuality research is more intrusive in nature than other types of research, leading to certain types of people being more or less likely to volunteer, which ultimately reduces the generalizability of findings from such research. Researchers over the past 3 decades have examined this potential for volunteer bias by studying differences in volunteer rates for different types of sexuality research as well as how volunteers differ from individuals who are given the same opportunity but decline to participate.

The utility of volunteer bias findings is twofold. First, these findings provide researchers with an estimate of the volunteer rates they might expect for different types of sexuality studies. For example, researchers have found that college students are more likely to volunteer for studies that involve merely completing sexuality-related questionnaires than for studies that involve being exposed to sexual images (hereafter referred to as sexual interest research; e.g., Wiederman, 1999b). Researchers have furthermore found that the intrusiveness of the measurement device to be used has a significant negative effect on volunteer rates (Gaither, 2000; Strassberg & Lowe, 1995; Wolchick, Braver, & Jensen, 1985).

The second way in which volunteer bias findings are helpful to sexuality researchers is that they inform us of the characteristics on which volunteers differ from nonvolunteers. This helps us to understand just how far we can generalize our result. In short, volunteer bias studies have consistently found that volunteers for sexual interest studies tend to report more positive affect related to sexuality (i.e., higher erotophilia and/or less sexual guilt; Gaither, 2000; Plaud, Gaither, Hegstad, Rowan, & Devitt, 1999; Strassberg & Lowe, 1995), a higher need for sexual novelty (i.e., greater sexual sensation seeking; Gaither, 2000; Wiederman, 1999b), and a wider range of sexual experiences (i.e., more sexual partners, a wider range of sexual behaviors experienced; Plaud et al., 1999; Wolchik et al., 1985) than nonvolunteers.

Studies that have examined personality characteristics have been less consistent in their findings. Older research studies failed to find significant differences on numerous personality variables (Barker & Perlman, 1975; Farkas, Sine, & Evans, 1978; Saunders, Fisher, Hewitt, & Clayton, 1985), while more recent studies have reported personality differences including delinquency and unconventionally (Bogaert, 1996), self-monitoring and interpersonal exploitiveness (Wiederman, 1999b), and sensation seeking (Bogaert, 1996; Gaither, 2000), with volunteers reporting higher levels of each than nonvolunteers. Thus, volunteers for sexual interest research may have a need for greater levels of sexual stimulation and be more willing to participate in "deviant" activities than their peers, or at least they may be more willing to present themselves as such. Do these findings mean that the findings of all sexual interest studies are inherently limited to the stone degree in their generalizability?

Bogaert (1996) suggested that the impact that volunteer/nonvolunteer differences have on the generalizability of a specific study depends upon the research question being addressed. Take, as an example, studies in which researchers have reported finding strong habituation effects in male sexual arousal that generally occur with the same stimulus but not with sexual stimuli in general (e.g., O'Donohue & Plaud, 1991; Plaud, Gaither, Henderson, & Devitt, 1997). Applying the results from the volunteer bias literature, one might suggest that although habituation does occur in some men (i.e., research study volunteers), it is not known whether it occurs for all men, as volunteers for this type of research tend to display a greater need for sexual novelty than their peers.

How do volunteer bias results affect sexual interest studies in which homosexual stimuli are presented? Take, for example, a study that gained a significant amount of attention from the popular press, in which Adams, Wright, and Lohr (1996) examined penile responses of a group of strongly heterosexual male college students (i.e., men who reported exclusive heterosexual arousal and experience on the Kinsey Heterosexual-Homosexual Rating Scale) to video clips depicting heterosexual, male homosexual, and female homosexual activity. These researchers divided the participants into two groups based upon their scores on a measure of homophobia (high and low) and found that those men in the high homophobia group displayed stronger penile responses to the male homosexual clip than did the low homophobia group. Adams and his colleagues concluded that their findings "indicate that individuals who score in the homophobic range and admit negative affect toward homosexuality demonstrate significant sexual arousal to male homosexual erotic stimuli" (p. 443). Again, volunteer bias findings would suggest that these results should only be generalized to men that have a high need for sexual novelty and not to all men who admit a negative affect toward homosexuals. However, two issues must be addressed before making this conclusion.

The first issue that limits the generalizability of volunteer bias findings to studies in which homosexual stimuli are presented is that none of the volunteer bias studies published to date have ever limited their participants to such a select group (i.e., strongly heterosexual men and/or women). Therefore, we cannot as yet rule out the possibility that the volunteer/nonvolunteer differences reported in the literature are attributed to participants who have a wider range of sexual interests or experiences, that is, individuals who would have been screened out of a study such as Adams et al.'s (1996). Plaud et al. (1999), in fact, found that male and female student volunteers for a physiological sexual interest study reported significantly more lifetime same-sex and opposite-sex partners than nonvolunteers.

The second issue that limits the generalizability of volunteer bias findings to studies in which homosexual stimuli are presented is how stimuli are described in volunteer bias studies compared to how they might be described during recruitment for actual sexual interest studies. Wiederman (1999a) pointed out that when research participants are to be drawn from university human subject pools, institutional review boards (IRBs) require researchers to inform them of the nature of the study, including the types of stimuli to be presented to participants either at the point of recruitment or the point of participation in the study. Volunteer bias researchers, however, have tended to use fairly ambiguous terms in their descriptions of the types of stimuli to which volunteers would be exposed, such as "sexually explicit videos" (Strassberg & Lowe, 1995, p. 373). Thus, participants in these studies may have assumed that the stimuli would be heterosexual in nature, and volunteered (or declined to volunteer) based upon their interest in viewing these types of stimuli. In support, Kenrick, Stringfield, Wagenhals, Dahl, and Ransdell (1980) conducted a study that demonstrated that the content of stimuli to he used (e.g. "soft-core" vs. "'hard-core" erotica) does in fact have an effect on volunteering among college students. Therefore, participants in volunteer bias studies may be very different from participants in sexual interest studies that present homosexual stimuli.

We designed the present study to examine whether the content of stimuli in a proposed study would affect volunteer rates as well as volunteer/nonvolunteer differences. Furthermore, we wanted to examine whether the rates and differences would be the same when we used a recruitment sample similar to those used in previous volunteer bias studies (i.e., not just exclusively heterosexual participants) as when we reduced the sample to only exclusively heterosexual participants. We believed this approach would provide sexual interest researchers with a stronger sense of whether volunteer bias results apply to their specific studies. We asked participants whether they would be willing to volunteer for a sexual interest study if different types of images (nude men, nude women, heterosexual activity, male homosexual activity, and female homosexual activity) would be presented. This study was grounded in two assumptions. First, we believed that students would be more likely to volunteer for studies that included stimuli consistent with their sexual interests than for those that included stimuli inconsistent with their sexual interests. Second, the volunteer bias findings reported in the literature are robust enough that we believed that the particular recruitment sample used would not affect the volunteer/nonvolunteer differences.

We developed two sets of hypotheses. The first set dealt with differences in volunteer rates across different image types as a function of participant sex and sexual interests (exclusively heterosexual experiences and fantasies vs. not exclusively heterosexual experiences and fantasies). The second set of hypotheses dealt with how volunteer/nonvolunteer differences would be affected by the different types of images and participants' sexual interests. We discuss the development of each set of hypotheses below.

First, we expected to find gender differences in volunteer rates as a function of stimulus type. More specifically, we hypothesized that consistent with previous volunteer bias studies in which gender differences in volunteer rates have been examined (Kenrick et al., 1980; Plaud et al., 1999; Saunders et al., 1985; Strassberg & Lowe, 1995; Wiederman, 1999b; Wolchik et al., 1985), more men than women would be willing to volunteer to view images containing heterosexual activity. We based this on the belief that participants in these previous studies, due to the ambiguous description of the stimuli, most often assumed that the stimuli would be heterosexual in nature. Since the previous studies were never limited to exclusively heterosexual participants, we had no reason to believe that this difference would be affected by excluding participants who were not exclusively heterosexual.

For the remaining images, we based our hypotheses on findings reported in the literature regarding gender differences in physiological response to (Steinman, Wincze, Sakheim, Barlow, & Mavissakalian, 1981), subjective reports of sexual arousal from (Hatfield, Sprecher, & Traupman, 1978; Steinman et al., 1981), and erotic value of (Louderback & Whitley, 1997) heterosexual and homosexual stimuli. Such studies have consistently found heterosexual men to display equivalent levels of arousal to heterosexual and female homosexual stimuli, both of which are significantly greater than their responses to male homosexual stimuli. These same studies, however, have found heterosexual women to display equivalent levels of sexual arousal to male and female homosexual stimuli, both of which were significantly less than their responses to heterosexual stimuli. Therefore, we hypothesized that male volunteer rates for studies containing images of nude women, heterosexual activity, and female homosexual activity would all be significantly higher than their rates for studies with images of nude men and male homosexual activity. In contrast, we hypothesized that female volunteer rates would be significantly higher for studies containing images of nude men and heterosexual activity than for those with images of nude women and male and female homosexual activity. We expected that when we excluded the participants who were not exclusively heterosexual, we would find even greater within-gender differences in volunteer rates.

To examine the effect of stimulus type on volunteer/nonvolunteer differences, we chose to include a number of different measures. First, we included measures of sexual and general sensation seeking, as researchers (e.g., Bogaert, 1996; Gaither, 2001; Wiederman, 1999b) have consistently reported volunteer/nonvolunteer differences on these variables. We also included a measure of self-monitoring, as Wiederman (1999b), in the only volunteer bias study to include this variable, found that volunteers reported a greater tendency to regulate their self-presentation than did nonvolunteers. Based upon this difference, Wiederman raised the issue of whether volunteers for sexuality studies may have a greater tendency to engage in socially desirable responding. Therefore, we included a measure of social desirability as well. To determine whether volunteers may be pathologically drawn to sexual stimuli, we included a measure of sexual compulsivity. Finally, we included a measure of homophobia to determine the extent in which volunteers and nonvolunteers would differ in their negative attitudes toward homosexuals.

We decided to examine differences between volunteers and nonvolunteers separately for heterosexual images, nude images (categorizing participants as volunteers if they volunteered for either or both of nude males and nude females), and homosexual activity (same categorization method). Our specific hypotheses were as follows: For heterosexual and nude images, results would conform with previous findings; that is, volunteers would report significantly greater levels of sexual and general sensation seeking, self-monitoring, and social desirability than nonvolunteers, though the groups would not differ on sexual compulsivity or homophobia. For homosexual images, we hypothesized that the same differences would be observed for the first four variables and that volunteers would report significantly less homophobia and significantly more sexual compulsivity than nonvolunteers. Finally, we hypothesized that we would find identical results when the nonexclusively heterosexual participants were included in the analyses and when they were excluded.

METHOD

Participants

Six hundred fifty-four students enrolled in introductory psychology courses at Ball State University participated in this study. However, 90 students were excluded due to missing data, leaving 206 (36.5%) male and 358 (63.5%) female students. The participants ranged in age from 18 to 50 years (M = 18.93, SD = 2.04). The sample was comprised mainly of individuals who were reportedly Caucasian (92.6%), of Catholic (n = 148, 26.2%) or Christian non-Catholic (n = 323, 57.3%) religious affiliation, and single or never married (n = 545, 96.6%).

When asked to indicate their sexual orientation, 551 (97.7%) of the participants reported that they were heterosexual, 7 (1.2%) reported being bisexual, and 6 (1.1%) reported being homosexual. Regarding past sexual experiences, 71 (12.6%) reported that they had not had a sexual experience, 463 (82.1%) reported having only opposite-sex experiences, 7 (1.2%) reported having only same-sex experiences, and the remaining 23 (4.1%) reported having had both same-sex and opposite-sex experiences. Regarding sexual fantasies, 457 (81.0%) reported having opposite-sex fantasies only, 7 (1.2%) reported having same-sex fantasies only, and the remaining 100 (17.8%) reported having both opposite-sex and same-sex fantasies. In sum, 173 (84.0%) of the male participants and 281 (78.5%) of the female participants were considered exclusively heterosexual as they indicated that they had had only opposite-sex experiences (or no sexual experiences) and fantasies and self-reported a heterosexual orientation. Participants received one hour of research credit toward their psychology courses in exchange for their participation.

Measures

The first measure participants completed was a demographics sheet, which contained items regarding the participant's age, gender, race, religious affiliation, education level, and marital status. This form also contained the Modified Kinsey Heterosexual-Homosexual Rating Scale (Adams et al., 1996). This scale is comprised of two 7-point subscales on which an individual separately rates his or her sexual experiences and fantasies from 1 (exclusively heterosexual) to 7 (exclusively homosexual). Participants were also asked to categorize their sexual orientations as either heterosexual, homosexual, bisexual, or asexual.

The Sexual Sensation Seeking Scale and the Nonsexual Experience Seeking Scale (Kalichman & Rompa, 1995) are both modified versions of Zuckerman's Sensation Seeking Scale (Zuckerman, Eysenck, & Eysenck, 1964). The 11-item Sexual Sensation Seeking Scale assesses sensation seeking related to sexual interests (i.e., seeking out novel and exciting sexual experiences), whereas the 11-item Nonsexual Experience Seeking Scale is a more global measure of sensation seeking using more contemporary language than does Zuckerman's scale. The 10-item Sexual Compulsivity Scale (Kalichman & Rompa, 1995) consists of items reflecting obsessive preoccupations with sexual acts and encounters. All three scales use a 4-point rating format, with responses ranging from 1 (not at all like me) to 4 (very much like me). Total scores for each scale are derived by summing responses. Cooper, Scherer, Boles, and Gordon (1999) included all three of these measures in their on-line study of over 9,000 adults who reported using the Interact for sexual purposes (e.g., viewing sexually explicit materials, participating in sex-related "chat rooms") at least one time. These researchers divided the participants into three groups based upon the amount of time they reported spending each week using the Internet for sexual purposes--low users (less than 1 hour per week), moderate users (1-10 hours per week), and high users (11 + hours per week)--and found significant between groups differences (low users < moderate users < high users) on all three measures. Internal consistency coefficients for these measures in the current sample were .85, .84, and .92, respectively.

Homophobic thoughts, feelings, and behaviors were assessed using the Homophobia Scale (HS; Wright, Adams, & Bernat, 1999). This measure contains 25 statements with which a respondent rates his or her level of agreement using a 5-point. Likert-type scale (1 = strongly agree, 5 = strongly disagree). The items were found to load on three separate factors, which Wright et al. labeled as negative cognitions regarding homosexuality (e.g., "Organizations which promote gay rights are not necessarily"), negative affect and avoidance of homosexual individuals (e.g., "I avoid gay individuals"), and negative affect and aggression toward homosexual individuals (e.g., "I would hit a homosexual for coming on to me"). Wright et al. reported good test-retest reliability in a sample of 122 college students and found that the HS was significantly correlated with the Index of Homophobia (Hudson & Ricketts, 1980). Lower scores on this measure indicate more negative attitudes toward homosexuals. The internal consistency coefficient for this measure was .83 in the current sample.

The Self-Monitoring Questionnaire (SMQ; Snyder & Gangestad, 1986) was used to assess the degree to which an individual regulates self-representation for the sake of desired public appearance. The SMQ contains 18 true/false statements, such as "Im not always the person I appear to be." Higher scores on the SMQ are thought to indicate greater responsiveness to social and interpersonal cues of situationally appropriate performance. The internal consistency coefficient of this measure was .65 in the current sample.

The extent to which responses to these measures are affected by socially desirable response bias was assessed using the Marlowe-Crowne Social Desirability Scale (MCSDS; Crowne & Marlowe, 1960). This measure contains 33 true/false statements, such as "Before voting, I thoroughly investigate the qualifications of all the candidates." Fifteen items, such as "On occasion I have had doubts about my ability to succeed in life," are reverse scored. The sum score of the scale is used in analyses, with higher scores indicating greater socially desirable responding. The internal consistency coefficient of this measure was .69 in the current sample.

Finally, we developed a Research Interests Survey specifically for this study to examine a participant's willingness to volunteer for sexual interest research studies that would involve viewing different types of images. Due to the low volunteer rates reported in the literature for physiological studies, we decided to pose the studies as self-report studies. Thus, this survey read as follows:

  This research team is currently in the process of designing several
  different research studies. Before we go further, however,
  we would like to find out about how many people we could
  expect to participate in each. These studies would require
  participants to rate sexually explicit images (pictures and/or
  videos) on several different dimensions. Assuming that you still need
  research credit when we act the studies ready, would you be willing
  to participate in a study that would involve rating images of:

This was followed by five items ("men posing in the nude," "women posing in the nude," "a man and a woman engaging in sexual activity together," "two men engaging in sexual activity together," and "two women engaging in sexual activity together"), each of which was followed by "yes" or "no" response options. We asked participants to indicate whether they were willing or not instead of how willing they would be in order to keep this study as similar to previous volunteer bias studies as possible.

Procedure

Students completed the packet of measures in groups of up to 30 students. A transcript of the oral instructions was read to the students, which included a statement that they were free to discontinue at any time without any prejudice from the researcher and that they could skip any question they did not wish to answer. Each student signed a consent form prior to receiving any materials. The material--the demographics sheet, a questionnaire packet, a response sheet, and the Research Interest Survey--were distributed in envelopes. Upon completing the measures, students placed their materials back in the envelopes and then placed their envelopes in a box at the front of the room. Students were then provided with a debriefing form which contained information regarding the area of research that this study addressed, as well as references to two sources from which they could obtain more information about human sexuality research (Allgeier & Allgeier, 1988) and volunteer bias more specifically (Plaud et al., 1999).

RESULTS

Gender Differences on Questionnaires

Because we hypothesized gender differences in volunteer rates as a function of image type and significant differences between volunteers and nonvolunteers on the questionnaires, we first examined gender differences on the Sexual Sensation Seeking Scale (SSSS), Nonsexual Experience Seeking Scale (NES), Sexual Compulsivity Scale (SCS), Self-Monitoring Questionnaire (SMQ), Marlowe-Crowne Social Desirability Scale (MCSDS), and Homophobia Scale (HS) using a MANOVA. Results revealed a significant multivariate effect for gender, Pillai's Trace F (6,557) = 30.71, p < .001. Follow-up univariate F tests are summarized in Table 1. A notable finding is that men scored significantly higher than women on all measures except for social desirability, which was not statistically significant.

Between-Gender Differences in Volunteering to View Heterosexual Images

To determine whether our volunteer rates would conform to the patterns previously established in volunteer bias studies, we compared male and female volunteer rates for heterosexual activity using a z test for comparing proportions. Results indicate that men were more likely to volunteer to view heterosexual activity than were women (81.1% vs. 46.4%, p < .01). We next ran the same analysis dropping the 110 participants who were not exclusively heterosexual and found very similar volunteer rates (77.5% vs. 39.1%, p < .01). Interestingly, when we compared male and female volunteer rates for the 110 participants who were not exclusively heterosexual, we found much higher volunteer rates, though still in the same direction (100% vs. 72.7% for men and women, respectively).

Within-Gender Differences in Volunteer Rates by Specific image Type

Table 2 presents the volunteer rates for each of the five different types of images broken down by participant sex. We hypothesized that among men, the volunteer rates for viewing heterosexual activity, nude women, and female homosexual activity would not differ significantly from one another but would all be significantly greater than the rates for viewing nude men and male homosexual activity. We tested this hypothesis using a tests for two proportions to compare volunteer rates for each image type. When we examined rates including all male participants and only exclusively heterosexual men, the results conformed to the hypothesis, with one exception: The volunteer rate for viewing nude women was significantly greater than the rates for all other image types. We found increased volunteer rates across all images types for the 33 men who were not exclusively heterosexual, although within this group the rates for viewing heterosexual activity, nude women, and female homosexual activity did not differ significantly.

For women, we hypothesized that volunteer rates for viewing heterosexual activity and nude men would not differ significantly from each other but would both be significantly greater than the rates for viewing nude women, male homosexual activity, and female homosexual activity, which would not differ from one another. Using similar z tests with all female participants, we found that the results generally conformed to our hypothesis, with two exceptions: The volunteer rate for viewing nude men was significantly higher than the rates for all other image types, and the volunteer rate for viewing nude women did not differ significantly from the rate for viewing heterosexual activity. When we separately examined the 281 exclusively heterosexual women, we found the same pattern (but lower levels) of volunteer rates. Finally, when we examined only the 77 women who were not exclusively heterosexual, we found much higher volunteer rates, but they again conformed to the hypothesized pattern.

Volunteer/Nonvolunteer Differences

We examined volunteer/nonvolunteer differences for three types of images: nudes, heterosexual activity, and homosexual activity. For nudes, we categorized participants as volunteers (n = 417, 73.9%) if they reported a willingness to volunteer to view images of nude men, nude women, or both. Similarly, for homosexual activity we categorized participants as volunteers (n = 256, 45.4%) if they reported a willingness to volunteer to view images of male and/or female homosexual activity. For heterosexual activity, 333 (59.0%) of the participants were categorized as volunteers.

We first conducted a series of univariate analyses of variance (ANOVAs) for each image type (nudes, heterosexual activity, and homosexual activity) with total scores on the six questionnaires as dependent variables and volunteer status and participant sex as independent variables, using data from all participants. Because of the large number of analyses, we used a Bonferroni correction to set our alpha level for statistical significance at .008 (.05 / 6) to control for family-wise error rate. Results of these analyses revealed no significant Participant Gender X Volunteer Status interactions. Table 3 presents the results of these analyses separately by image type only for those tests in which a significant main effect for volunteer status occurred. For all three types of images, volunteers scored significantly higher than nonvolunteers on the SMQ, SSSS, and NES. Volunteers for viewing heterosexual activity also scored significantly lower on the MCSDS and significantly higher on the SCS than did nonvolunteers. Finally, volunteers for homosexual images scored significantly higher than nonvolunteers on the SCS and the HS.

Next, we reran each series of ANOVAs without the 110 participants who were not exclusively heterosexual. Of the 454 participants included in these analyses, the following numbers of participants were categorized as volunteers for viewing each type of image: 320 (70.5%) for nudes, 244 (53.7%) for heterosexual activity, and 181 (39.9%) for homosexual activity. Results of the statistically significant tests are also included in Table 3. Once again, no interaction effects were significant. The same pattern of significant differences between volunteers and nonvolunteers occured again on the SMQ, MCSDS, SSSS, and NES. Differences on the SCS and HS were not statistically significant.

DISCUSSION

The current study was designed to determine the extent to which volunteer bias findings reported in the literature might apply to sexual interest studies in which college student participants are exposed to homosexual stimuli. In general, we found evidence that volunteer rates as well as differences between volunteers and nonvolunteers for self-report sexual interest studies vary as a function of the types of images to which participants are told they will be exposed. Our results are also consistent with findings from existing volunteer bias research (e.g., Gaither, 2000; Wiederman, 1999b). Below we describe the effects that different stimuli had on volunteer rates and we discuss how volunteer/nonvolunteer differences were affected, ways in which these results apply to specific studies, limitations of the study, and the unique contribution the present study makes.

In the current study, volunteer rates for viewing images of heterosexual activity were nearly identical to the rates reported in a previous study (Wiederman, 1999b). Though these rates may appear high, which some may attribute to the hypothetical nature of volunteering in both studies, it is important to note that they are also very close to the rates of students who actually did volunteer in other studies (Kenrick et al., 1980; Saunders et al., 1985), regardless of whether the stimuli were described to them beforehand. This leads to the conclusion that participants in other volunteer bias studies in which the stimuli were not described (e.g., Bogaert, 1996; Plaud et al., 1999; Strassberg & Lowe, 1995; Wolchik et al., 1985) may have assumed that the stimuli would be heterosexual in nature. Therefore, these rates do not provide any information about the types of volunteer rates to expect if other types of stimuli are to be used.

The results further suggest that men are more willing to view stimuli containing women than stimuli containing men. When we examined the results separately for exclusively heterosexual men and other men (see Table 2). We found that the pattern was the same for both. As might be expected, however, there was much less difference between the groups in volunteer rates for images containing women (13 23%) than for images not containing women (45-48%). Thus, it may be that men's willingness to volunteer to view images is related to their sexual experiences and/or fantasies. Another possibility that cannot be ruled out, however, is that the two groups do not really differ in what they would be willing to view, but that men who are willing to provide responses on one measure (e.g., sexual experience) that might be considered deviant are also more likely to do so on another (e.g., willingness to view).

Women, on the other hand, were most likely to volunteer to view images of nude men, followed by heterosexual activity and nude women, while they were equally less likely to report a willingness to view either type of homosexual activity. As was found among men, exclusively heterosexual women's volunteer rates were much lower than the other women's rates across all five image types. Interestingly, however, the differences between the two groups of women ranged from 22% to 40%, with a much less clear pattern in these differences than was the case for men. In essence, men appear to be interested in viewing nude women in any possible context, while women appear to be more interested in viewing images that are less sexually overt and less "deviant" or more socially acceptable. In other words, women do not appear to be seeking out images of nude men the way that men are seeking out images of nude women.

Our male and female students' patterns of responding in the current study are very consistent with the patterns of positive feelings and self-reports of sexual arousal after viewing video clips depicting individuals masturbating, heterosexual activity, and homosexual activity reported by Hatfield et al. (1978). They are also consistent with Louderback and Whitley's (1997) findings regarding the erotic value that exclusively heterosexual male and female college students ascribed to male and female homosexuality. Thus, our dependent measure of willingness to volunteer may actually be a measure of sexual attraction, or at least that may be how participants responded. Future research could examine this further by asking participants to rate their level of attraction to each type of stimulus on one measure and then asking them to rate their willingness to view each type of image on a separate measure at another time to reduce the chances that participants will base their responses to one measure on their responses to the other. Researchers could also examine what students say they would be willing to view and then assess what they actually do view in at lab or even an Internet-based free-choice situation.

Overall, our findings suggest that researchers can expect to find much higher volunteer rates among men than women for sexual interest studies. Furthermore, the results suggest that researchers can expect lower volunteer rates among exclusively heterosexual students than others and that the rates will decrease as the level of explicitness of the stimuli increase among all college students.

Results of analyses comparing volunteers for viewing nude images, heterosexual activity, and homosexual activity were nearly identical. For each type, volunteers scored significantly higher than nonvolunteers on measures of self-monitoring, sexual sensation seeking, and more general sensation seeking. These differences are consistent with those reported by other volunteer bias researchers (e.g., Bogaert, 1996; Gaither, 2000; Wiederman, 1999b). Thus, college students may view sexual interest studies as a potentially stimulating experience, which may factor into their decision to volunteer for (or possibly to avoid) them (e.g. Plaud et al., 1999).

As expected in the present study, volunteers for viewing homosexual images displayed less negative attitudes toward homosexuals than did nonvolunteers. They also displayed greater preoccupations with sex than nonvolunteers. These findings together with those reported above suggest that sexual interest studies that use homosexual stimuli may attract students who are more open to a wider variety of sexual experiences than their peers. This should be kept in mind when interpreting results of such studies, as participants may actually be more arousable than is true of the general population. For example, Adams et al.'s (1996) finding that highly homophobic men displayed greater physiological sexual arousal to a video of male homosexual behavior should not be generalized to all highly homophobic men. Adams and his colleagues did not report the number of students who were initially screened or what the students were told about the study prior to their participation.

Sexuality researchers (e.g., Meston, Heiman. Trapnell, & Paulhus, 1998) have found significant relationships between social desirability and responses on sexuality measures, with higher social desirability being related to more positive self-presentations. It is interesting that in the present study, volunteers for viewing each of the three image types scored higher on the Self-Monitoring Questionnaire (Snyder & Gangestad, 1986) than nonvolunteers, but volunteers only differed from nonvolunteers on the Marlowe-Crowne Social Desirability Scale (Crowne & Marlowe, 1960) for heterosexual images, with volunteers actually scoring lower than nonvolunteers. One possible reason for this discrepancy is that the measures are assessing different aspects of impression management. Items on the SMQ appear to relate more to impression management in social situations, whereas the MCSDS items are more related to overacknowledging one's postive qualities while completing questionnaires. Thus, volunteering, as it is a social act, should be more strongly affected by social impression management.

One limitation of the present study is that participants were a fairly homogeneous group, made up of primarily young, single, Caucasian college students. Although this may be representative of college student participants in sexual interest research, future research should include more diverse samples to determine whether similar patterns of findings would hold. For example, is the bias as strong in community volunteers for sexual interest research, or in homosexual samples? Having an answer to such a question would help researchers be more confident in generalizing their results beyond the samples that participate.

In conclusion, the present study was the first to assess volunteer rates and volunteer/nonvolunteer differences for studies that would involve viewing different types of images. Volunteer rates tended to be consistent with the participants' sexual interests. This was also the first volunteer bias study to examine how volunteer/nonvolunteer differences reported in the literature would apply to specific samples of exclusively heterosexual college students. Results indicate that the differences probably do apply to such exclusive groups. The findings from this study suggest that researchers should be diligent in reporting their recruitment strategies as well as characteristics of their samples, beyond demographics, to determine the extent for which their findings may be generalized.

Table 1.Mean (Standard Deviation) and Gender Differences in Mean Scale Scores

                                               Men            Women
Scale                                        (n=206)         (n=358)

Self-Monitoring Questionnaire              10.71 (3.81)     9.80 (3.37)
Marlowe-Crowne Social Desirability Scale   13.81 (5.55)    14.53 (5.38)
Sexual Sensation Seeking Scale             28.03 (7.12)    22.54 (7.35)
Nonsexual Experience Seeking Scale         30.81 (7.23)    27.33 (7.50)
Sexual Compulsivity Scale                  19.75 (7.19)    15.77 (7.60)
Homophobia Scale                           42.48 (14.06)   32.1  (11.06)

Scale                                     (1,563)                   d

Self-Monitoring Questionnaire               8.70*                  0.26
Marlowe-Crowne Social Desirability          2.30                  -0.13
Sexual Sensation Seeking Scale             74.63**                 0.71
Nonsexual Experience Seeking Scale         28.87**                 0.46
Sexual Compulsivity Scale                  37.36**                 0.52
Homophobia Scale                           91.97**                 0.78
* p < .005. ** p < .001.

Table 2. Percentages of Participants (by Sex) Who Reported a Willingnessto Volunteer to View Each Image Type

                                                Image type

                                     Heterosexual  Nude men  Nude women

All men (n = 206)                         81.1        22.8       89.3
  Exclusively heterosexual (n= 173)       77.5        15.6       87.3
  Other (n = 33)                         100.0        60.6      100.0

All women (n = 358)                       46.4        64.0       41.9
  Exclusively heterosexual (n = 281)      39.1        59.1       33.5
  Other (n = 77)                          72.7        81.8       72.7

                                               Image type

                                     Male homosexual  Female homosexual

All men (n = 206)                           14.1             76.7
  Exclusively heterosexual (n = 173)         6.4             74.0
  Other (n = 33)                            54.5             90.9

All women (n = 358)                         25.4             26.0
  Exclusive heterosexual                    18.1             17.4
  Other (n = 77)                            51.9             57.1

Note. Exclusively heterosexual participants are those who reported only opposite-sex experiences and fantasies and self-reported heterosexual orientation. Others refers to all other participants.

Table 3. Comparisons of Volunteers and Nonvolunteers by Image Type When All Participants Are Included and When Only Exclusively Heterosexual(a) Participants Are Included

                                         All participants (n = 564)

Image type scale                      Volunteers         Nonvolunteers
                                        M (SD)               M (SD)

Nudes
  SMQ                                10.50 (3.61)          9.10 (3.20)
  SSSS                               26.00 (7.02)         20.43 (8.19)
  NES                                29.60 (7.44)         25.78 (7.30)
Heterosexual activity
  SMQ                                10.94 (3.57)          8.97 (3.21)
  SSSS                               27.07 (6.66)         20.92 (7.74)
  NES                                30.16 (7.44)         26.36 (7.24)
  MCSDS                              13.51 (5.46)         15.35 (5.27)
  SCS                                18.46 (7.09)         15.44 (8.18)
Homosexual activity
  SMQ                                10.97 (3.75)          9.46 (3.25)
  SSSS                               27.94 (6.70)         21.83 (7.43)
  NES                                30.76 (6.99)         26.87 (7.61)
  SCS                                18.98 (7.41)         15.76 (7.62)
  HS                                 37.00 (14.79)        35.10 (11.66)

                                        All participants (n = 564)

Images type scale                              F
                                            (1,563)              d

Nudes
  SMQ                                       12.50****           0.41
  SSSS                                      17.44****           0.73
  NES                                        7.23***            0.51
Heterosexual activity
  SMQ                                       29.65****           0.55
  SSSS                                      41.97****           0.80
  NES                                       11.87***            0.50
  MCSDS                                      7.55**            -0.34
  SCS                                        3.93*              0.40
Homosexual activity
  SMQ                                       18.34****           0.42
  SSSS                                      43.41****           0.79
  NES                                       15.86****           0.51
  SCS                                        4.88*              0.42
  HS                                         6.08*              0.14

                                Exclusively heterosexual only (n = 454)

Image type scale                    Volunteers         Nonvolunteers
                                     M (SD)                M (SD)

Nudes
  SMQ                              10.27 (3.47)          9.04 (3.22)
  SSSS                             25.42 (6.90)         19.99 (7.94)
  NES                              29.29 (7.47)         25.38 (6.95)
Heterosexual activity
  SMQ                              10.67 (3.44)          9.02 (3.22)
  SSSS                             26.47 (6.59)         20.74 (7.61)
  NES                              29.92 (7.45)         26.07 (7.10)
  MCSDS                            13.65 (5.35)         15.40 (5.25)
  SCS
Homosexual activity
  SMQ                              10.70 (3.68)          9.39 (3.17)
  SSSS                             27.37 (6.66)         21.47 (7.32)
  NES                              30.43 (7.10)         26.62 (7.43)
  SCS
  HS
                               Exclusively heterosexual only (n = 454)

Image type scale                           F
                                        (1,453)               d

Nudes
  SMQ                                    9.42***             0.35
  SSSS                                  14.00****            0.70
  NES                                    6.44*               0.52
Heterosexual activity
  SMQ                                   19.34****            0.48
  SSSS                                  28.68****            0.75
  NES                                    9.88***             0.51
  MCSDS                                  6.08*              -0.32
  SCS
Homosexual activity
  SMQ                                    9.17***             0.38
  SSSS                                  24.46****            0.76
  NES                                    8.44***             0.50
  SCS
  HS

Note. SMQ = Self- Monitoring Questionnaire (Snyder & Gangestad, 1986); SSSS = Sexual Sensation Seeking Scale (Kalichman & Rompa, 1995); NES = Nonsexual Experience Seeking Scale (Kalichman & Rompa, 1995); SCS = Sexual Compulsivity Scale (Kalichman & Rompa, 1995); MCSDS = Marlowe-Crowne Social Desirability Scale (Crowne & Marlowe, 1960); HS= Homophobia Scale (Wright, Adams,& Bernat, 1999). (a) Exclusively heterosexual participants are those who reported only opposite-sex experiences and fantasies ans reported their sexual orientation to be heterosexual. * p < .05. ** p < .01. *** p < .005. **** p < .001.

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Manuscript accepted January 22, 2003

Address correspondence to George A. Gaither, Ph.D., Department of Psychological Science, Ball State University, IN 47306-0520; e-mail: [e-mail].