Measurement of Sexual Interests with Pupillary Responses: A Meta-Analysis. Janice Attard-Johnson, Martin R. Vasilev, Caoilte Ó Ciardha, Markus Bindemann & Kelly M. Babchishin. Archives of Sexual Behavior, Sep 23 2021. https://rd.springer.com/article/10.1007/s10508-021-02137-y
Abstract: Objective measures of sexual interest are important for research on human sexuality. There has been a resurgence in research examining pupil dilation as a potential index of sexual orientation. We carried out a meta-analytic review of studies published between 1965 and 2020 (Mdn year = 2016) measuring pupil responses to visual stimuli of adult men and women to assess sexual interest. Separate meta-analyses were performed for six sexual orientation categories. In the final analysis, 15 studies were included for heterosexual men (N = 550), 5 studies for gay men (N = 65), 4 studies for bisexual men (N = 124), 13 studies for heterosexual women (N = 403), and 3 studies for lesbian women (N = 132). Only heterosexual and gay men demonstrated discrimination in pupillary responses that was clearly in line with their sexual orientation, with greater pupil dilation to female and male stimuli, respectively. Bisexual men showed greater pupil dilation to male stimuli. Although heterosexual women exhibited larger pupils to male stimuli compared to female stimuli, the magnitude of the effect was small and non-significant. Finally, lesbian women displayed greater pupil dilation to male stimuli. Three methodological moderators were identified—the sexual explicitness of stimulus materials, the measurement technique of pupillary response, and inclusion of self-report measures of sexual interest. These meta-analyses are based on a limited number of studies and are therefore preliminary. However, the results suggest that pupillary measurement of sexual interest is promising for men and that standardization is essential to gain a better understanding of the validity of this measurement technique for sexual interest.
This meta-analysis examined whether changes in pupil size in response to viewing images of adult males and females can provide a consistent measure of sexual orientation in men and women. Overall, this was found to be the case for gay and heterosexual men, whereby heterosexual men showed greater pupil dilation to other-sex (i.e., female) stimuli, and gay men showed greater pupil dilation to same-sex (i.e., male) stimuli. The overall effect sizes for these observer groups ranged from small to moderate across studies, similarly to other measures of sexual interest using cognitive tasks (e.g., Viewing Time, Ó Ciardha et al., 2018), but smaller effect sizes relative to those obtained with genital arousal measurements (e.g., Jabbour et al., 2020; Semon et al., 2017). Results for bisexual men demonstrated the same direction of effect as gay men, of greater pupil dilation to male than to female stimuli. In fact, this effect was larger for bisexual than for gay men. Previous studies recording pupil dilation and genital arousal have found varied response patterns such that men who identify as bisexual inconsistently demonstrate arousal to both men and women (Rieger & Savin-Williams, 2012; Rosenthal et al., 2011). However, in some studies this group has also exhibited a stronger arousal response to men (Rieger et al., 2005; Slettevold et al., 2019; Tollison et al., 1979). These diverse findings suggest that although sexual arousal is one facet in an individual’s identification with a sexual orientation, other aspects are likely to be involved, too. One such factor is sexual curiosity. This has been found to interact with arousal responses in bisexual men, whereby those with lower levels of sexual curiosity are more aroused by same-sex men compared to those with higher level of sexual curiosity, who record bisexual arousal patterns (Rieger et al., 2013).
A different pattern was observed with female observers. Heterosexual women showed a trend of larger pupil sizes during the viewing of other-sex (i.e., male) stimuli, but the magnitude of this effect was small and not significant. The gender differences in the strength of responses observed here are consistent with those found in the wider literature on the measurement of sexual interest in men and women using other physiological measurement techniques, such as direct measures of genital arousal (Chivers et al., 2004). Similarly to the inconsistent pattern of responding in bisexual men, it is possible that sexual arousal is only one element in women’s sexual identification and may form a smaller part in the makeup of their sexual orientation than in heterosexual men (Peplau & Garnets, 2000). We return to this issue in the “Moderators of Pupil Response Measures” section.
The responses of bisexual women were not analyzed due to an insufficient number of studies (Rieger et al., 2016; Snowden et al., 2019). Lesbian women showed greater dilation to other-sex stimuli. While this last result was unexpected, the analysis included only 3 studies (total N = 132) and was affected by one outlier study with a large negative effect size. As the analyses of gay and bisexual groups were also based on a limited number of either 3 or 4 studies, the meta-analysis of these should be considered as preliminary until more evidence becomes available.
Finally, we note that in this study the difference scores in pupil responses for images of men and women were calculated separately for male and female observers. This provides separate response patterns and effect size estimates for men and women of different self-reported sexual orientation. This approach is applied most frequently in this literature and was taken here to examine the potential applicability of pupillary response in assessing sexual interest within each group. However, some studies also include comparisons between male and female observers, or between sexual orientation groups, for each image category (Rieger et al., 2012; Savin-Williams et al., 2016). These between-subject comparisons would yield different effect sizes and may also produce a different pattern of results to this meta-analysis.
Moderators of Pupil Response Measures
We examined three moderators associated with methodological variations across studies: the sexual explicitness of stimuli, the methodological technique applied to measure pupillary response, and the inclusion of a subjective measure of sexual orientation or preference. Moderator analysis was only performed for heterosexual men and heterosexual women due to the insufficient number of studies for this analysis for the other participant groups. In heterosexual men, visual stimuli which comprised of partially nude images of people provided greater discrimination compared to studies using highly sexually explicit stimuli (i.e., complete nudity). Stimulus sets that comprised a mixture of explicitness level, such as dressed, partially nude, or completely nude, produced negligible effects. However, it is important to note that stimulus type might have been confounded with other possible moderators, such as the approach used to measure pupillary responses. Pupillary responses measured using an EyeLink eye trackers, for example, provided greater discrimination of pupil size change for male and female images in heterosexual men compared to other equipment.
These findings indicate that pupil response measurement and stimulus type can provide a source of variability. The specific effects of stimulus type and approach to measuring pupillary response are difficult to untangle because the three categories of explicitness (fully nude, partial or no nudity, and mixed nudity) do not span across the different types of pupillary response measures. For heterosexual men, for example, all studies with partially nude stimuli in the meta-analysis also applied EyeLink eye trackers to measure pupil response (Attard-Johnson et al., 2016; De Winter et al., 2021), while none of the studies using other techniques (i.e., SMI/Tobii eye trackers, manual measurement) included comparable stimuli of in low explicitness. In contrast, fully nude stimuli were employed by studies using EyeLink (Rieger et al., 2013; Watts et al., 2018) and SMI eye trackers (Finke et al., 2018), but not by any of the studies with Tobii eye trackers or manual pupil measurement. This could provide some explanation for a larger effect being observed for studies using partially nude compared to studies using more sexually explicit stimuli (i.e., complete nudity).
In heterosexual women, stimuli with low sexual explicitness demonstrated a moderate positive effect size (i.e., larger pupils when viewing persons of the same-sex), but a negative effect size (i.e., larger pupils when viewing other-sex stimuli) when mixed stimulus sets were used. The effect size for high sexual explicitness was negligible. These divergent findings could reflect the general pattern of inconsistency in female responding across studies and sexual arousal assessments more widely (Chivers, 2005; Chivers et al., 2010). Several theories have been proposed to explain inconsistencies in female sexual responding and comprehensive reviews have been published (see, for example, Chivers, 2005, 2017; Chivers et al., 2010). These theories include a range of methodological, biological, psychological and sociological factors. For example, potential differences could result from hormonal variations associated with fertility and menstrual cycle (Diamond, 2007; Shirazi et al., 2018), identification with the sexual pleasure that is perceived in other women leading to an arousal response to same-sex stimuli (Chivers, 2017), and greater malleability of sexuality by external contextual influences such as relationships (Baumeister, 2000). These possibilities have not been explored here (for a complete review of theories, see Chivers, 2017).
The comparatively small effect for stimuli of high sexual explicitness that was found in this meta-analysis could provide some support for the preparation hypothesis (Dawson et al., 2013; Lalumière et al., 2020), which suggests that any strong sexual cues provoke an indiscriminate arousal response to prepare a woman for the possibility of a sexual encounter (Chivers, 2017; Laan & Everaerd, 1995; Suschinsky & Lalumière, 2011). However, there is no clear explanation for the opposing effects of the mixed and low explicitness stimuli that were revealed in the current meta-analysis. Furthermore, the analysis considers difference scores of pupillary responses between male and female images, and from this it is therefore not possible to conclude whether the small effect is a result of no dilation or pupil dilation to a similar degree for both images. We also cannot exclude the possibility that other moderating factors could have contributed to this finding. For example, studies in both these subcategories included participants who were not asked to report their sexual orientation and were therefore assumed to be of heterosexual orientation (for example, De Winter et al., 2021; Hamel, 1974; Scott et al., 1967). Yet, the prevalence of same-sex behavior is reported to be higher in women compared to men (Diamond, 2016) with a difference of around 9% (7% of men versus 16% of women; Mercer et al., 2013). Thus, it is conceivable that the samples in these studies also contained a proportion of women who would have identified as bisexual or gay more so than samples of men. We note, however, that the moderator analysis also included measurement of sexual interest (i.e., whether researchers recorded self-reported sexual interest or assumed participants were heterosexual) and this was not found to moderate effect sizes.
Taken together, the findings from the present meta-analysis suggest that differences in type of stimulus and measurement technique across studies might not only influence the degree of pupillary response effects of sexual interest, but also the direction of these effects. Additional primary studies are required to draw firm conclusions on the individual influence of any of the moderators examined in the current meta-analysis.
Limitations and Future Research Directions
Due to the low number of studies that were available, we were selective in the choices of variables that were used in the moderator analysis. Consequently, other potential variables associated with methodological techniques were not included in the meta-analysis, such as low-level stimulus characteristics. Image luminance, for example, has the potential to evoke bottom-up pupillary responses that might interfere with responses to the task-specific processes under investigation. One mechanism through which such interference could occur is the pupil light reflex, which refers to the constriction or dilation of the pupil in response to changes in the intensity of light levels that enter the eye (Ellis, 1981). The pupils begin to constrict within 200 ms after an increase in light, reaching a minimum diameter of approximately 2 mm at around 1500 ms (Mathôt, 2018). However, precise durations and pupil sizes vary depending on the intensity of the light change and individual differences (Ellis, 1981). In addition to luminance, color of the perceived light can also influence the duration of a constriction. For example, while blue light leads to sustained constriction of the pupil beyond 1500 ms, red light leads to a degree of re-dilation in the absence of light change, termed pupil escape (Mathôt, 2018). Variations in image luminance and color might, therefore, be a factor to consider when measuring pupillary responses to visual stimuli.
One method for controlling this factor is to equalize brightness across stimuli to match the mean luminance of the stimulus set (Attard-Johnson et al., 2016; Snowden et al., 2019). However, while this approach eliminates mean luminance differences across images, it could also create additional artifacts by introducing distortion of natural luminance levels within images, such as light–dark contrasts. Such image distortions might decrease the realism of images and could unwittingly interact with the attractiveness of a depicted person or draw the observer’s attention to information in images that is irrelevant for the task at hand. In the study of pupillary responses as an index of sexual interest, only a few studies have so far adopted such luminance manipulations, with results that are difficult to assimilate, demonstrating a need for further research (cf. Attard-Johnson et al., 2016; Snowden et al., 2019). However, most studies in this meta-analysis also demonstrate divergent pupillary responses to the same stimuli, based on differences in observers’ sex and sexual orientation, indicating that low-level image characteristics are unlikely to explain the study findings.
Another factor that was not included in the meta-analysis here is the approach that is taken to analyze pupillary response data, which also varies across research studies. A recent paper compared four widely used methods for analyzing pupil responses (Attard-Johnson et al., 2019). Specifically, Attard-Johnson et al. analyzed (unadjusted) raw pupil scores obtained from eye-tracking, z-scores of this data, a conversion of the pupil data that captures differences between conditions in terms of the percentage change in eye pupil size, and pupillary responses that have been adjusted on the basis of a baseline measure. They reported that fundamental pupillary response patterns were consistent across all methods.
Finally, although it was possible to obtain information on all three methodological moderators from published sources or from direct contact with the authors, we could not acquire the actual correlations for pupil responses between male and female stimuli for some of the earlier studies included in the meta-analysis. Therefore, estimated correlations were used as a proxy when coding these studies. Actual correlation data produced a larger effect size compared to estimated correlations for heterosexual men. For heterosexual women, actual and estimated correlations produced effect sizes of similar magnitude but in different directions. In order to facilitate accumulation of knowledge, studies on pupil responses should provide correlations between sets of stimuli.
Implications for the Measurement of Sexual Interest
The current meta-analysis offers some tentative guidance for the measurement of pupillary responses of sexual interest. Across both heterosexual men and women, the low sexual explicitness stimuli (comprising partial or no nudity) appeared to provide clearer discrimination in pupillary response to sexual interest compared to high explicitness stimuli (complete nudity). We note, however, that for heterosexual women this effect was not in the direction consistent with their sexual orientation. Current evidence is too limited to determine whether some eye-tracking apparatus are better suited to this research. The desk-mounted EyeLink provided the strongest effect for heterosexual male observers, but this did not extend to heterosexual women for whom the largest effect was obtained for manual recording, and there were insufficient eligible studies for inclusion of remote Tobii eye trackers. Thus, further investigation comparing these methods is warranted, especially in women.
Furthermore, it is also possible that the affective state underlying these arousal patterns could be attributed to states evoked from other interpretations of the stimulus being perceived by the observer as pleasant or unpleasant. For instance, an arousal response would also be evoked if the stimulus is perceived as threatening or unpleasant when viewing, for example, stimuli depicting illness or violence (Bradley et al., 2008). It may not be possible to establish for certain whether sexual arousal underpins pupil dilation responses to sexual stimuli. Equally, we cannot exclude the possibility that other ‘pleasant’ emotions, for instance affection or aesthetic appeal, could be underlying the pupil dilation responses to sexual stimuli. We would argue that it is implausible that, for example, heterosexual male arousal to female images is better explained by perceived threat or unpleasantness, or pleasant emotions given that correlations of pupillary responses to sexual stimuli with genital arousal patterns, subjective sexual arousal (Rieger et al., 2015) and sexual appeal ratings (Attard-Johnson et al., 2016). These correlations provide some evidence to support the notion that pupil dilation responses to sexual stimuli observed are related to sexual arousal—at least in men. However, we acknowledge that further research is needed to untangle the affective states underlying the arousal response when viewing these images.
This highlights the importance of obtaining confirmatory measure of sexual interest to validate against pupillary responses. In many studies this was obtained through subjective self-report of sexual orientation which, at least in men, demonstrates convergence with pupillary responses (Rieger et al., 2015), cognitive tasks (Ó Ciardha et al., 2018), and other physiological measures of sexual arousal (Chivers et al., 2010; Rieger et al., 2015). Few direct attempts have been made to examine the convergent validity of pupillary responses with self-report sexual orientation and cognitive-based measures of sexual interest (Attard-Johnson et al., 2016; Ó Ciardha et al., 2018; Rieger et al., 2015) and only one study with genital responses (Rieger et al., 2015). From these, men’s pupillary responses demonstrated modest correlations with genital response (average r = 0.59; Rieger et al., 2015), viewing time (average r = 0.51; Ó Ciardha et al., 2018; Rieger & Savin-Williams, 2012), self-report sexual orientation (average r = 0.56; Ó Ciardha et al., 2018; Rieger et al., 2015; Rieger & Savin-Williams, 2012), and subjective sexual appeal and arousal ratings (average r = 0.58; Attard-Johnson et al., 2016; Rieger et al., 2015). In contrast, women’s pupillary responses correlated weakly with genital response (average r = 0.19; Rieger et al., 2015), subjective sexual appeal, and arousal ratings (average r = 0.15; Attard-Johnson et al., 2016; Rieger et al., 2015), but demonstrated a moderate correlation with viewing time tasks (average r = 0.47; Ó Ciardha et al., 2018; Rieger & Savin-Williams, 2012) and self-report sexual orientation (average r = 0.31; Ó Ciardha et al., 2018; Rieger et al., 2015; Rieger & Savin-Williams, 2012). We recommend that future primary studies seeking to use pupillary responses as a measure of sexual interest include also one or more alternative measures of sexual orientation to strengthen our understanding of the validity of pupillary responses for the assessment of sexual interests in men and women.
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