Pupillary response as a measure of individuals' sexual interest in specific other individuals

Increases in pupil size seem to indicate observers' sexual interest in other people. This pupillary response was first discovered in the 1960s, by filming observers' eyes whilst viewing sexually explicit material before manually measuring their pupil size in still video frames. Until recently, however, this effect remained largely unnoticed in the psychological literature, when the original findings were replicated with modern eye-tracking equipment. In the last few years, the study of this phenomenon has gathered pace, revealing that pupillary responses to sexually interesting images reflect observers' sexual orientation similarly to genital arousal, can discriminate the sex and age of a viewed person, are consistent across analysis protocols, and provide concurrent validity with latency-based (i.e. reaction time) measures of sexual interest.

Pupillary responses appear to hold a key advantage over these latency-based methods, which are open to manipulation by participants. In contrast, pupillary response is linked to activation of the autonomic nervous system and is potentially impervious to conscious control. This makes pupillary response a valuable measure for unbiased assessment in the study of sexual cognition. In light of evidence that pupillary responses also predict observers' self-reported interest in child molestation, this indicates strong potential for applied settings (e.g., for treatment monitoring of paedophilic sexual offenders in forensic psychology).

However, the conditions under which this response reliably measures sexual interest have been heavily debated and much evidence has been contradictory. For example, there is some evidence that this response is unaffected by the sexually explicitness of stimuli, while others have suggested that the relationship between pupillary responses and sexually non-explicit stimuli is complicated and at best modest. And whereas this effect is consistently observed in male observers (e.g., with pupil dilation in straight men during the viewing of women), it is typically absent in female observers. There is, however, limited evidence that it can be elicited in women with photographs of their romantic partners during ovulation, but not men generally, hinting at links to affect rather than sexual arousal.

Resolving these issues is of clear importance to advance theory of pupillary responses and the study of sexual interest. We need to know whether these measures truly reflect sexual interest, or whether they are driven by affect for other people. Similarly, we need to establish whether these measures are sensitive to individuals' sexual interest in a specific other. Resolving these issues will speak to the differences that have been observed between men and women in pupillary response (i.e. with men's but not women's sexual interest consistently apparent from this measure), which are also evident in other measures of sexual interest that have been studied more widely, and will directly inform theory. In turn, this research has relevance for forensic practice, commercial where latency-based measures are already in use. If autonomic pupillary responses indeed reflect sexual interest, are robust at the level of the individual, and its differences across men and women are more fully understood, then this is likely to provide a much more appropriate measure for applied settings than those currently available. Moreover, since sexual offences against children are most frequently committed by family members, research that seeks to isolate the contributions of sexual and affective components in assessment is important.

The aims of this research are therefore to determine whether pupillary response can robustly indicate sexual interest for individual observers; whether differences between men and women reflect general sexual interest in other people or interest in specific people; and whether pupillary responses reflect sexual interest or affect for other people.