The Eyes Have It: Sex and Sexual Orientation Differences
in Pupil Dilation Patterns
Gerulf Rieger*, Ritch C. Savin-Williams
Department of Human Development, Cornell University, Ithaca, New York, United States of America
Recent research suggests profound sex and sexual orientation differences in sexual response. These results, however, are
based on measures of genital arousal, which have potential limitations such as volunteer bias and differential measures for
the sexes. The present study introduces a measure less affected by these limitations. We assessed the‘ pupil dilation of 325
men and women of various sexual orientations to male and female erotic stimuli. Results supported hypotheses. In general,
self-reported sexual orientation corresponded with pupil dilation to men and women. Among men, substantial dilation to
both sexes was most common in bisexual-identified men. In contrast, among women, substantial dilation to both sexes was
most common in heterosexual-identified women. Possible reasons for these differences are discussed. Because the measure
of pupil dilation is less invasive than previous measures of sexual response, it allows for studying diverse age and cultural
populations, usually not included in sexuality research.
Citation: Rieger G, Savin-Williams RC (2012) The Eyes Have It: Sex and Sexual Orientation Differences in Pupil Dilation Patterns. PLoS ONE 7(8): e40256.
Editor: Andrew H. Kemp, University of Sydney, Australia
Received January 16, 2012; Accepted June 4, 2012; Published August 3, 2012
Copyright: ? 2012 Rieger, Savin-Williams. This is an open-access article distributed under the terms of the Creative Commons Attribution License, which permits
unrestricted use, distribution, and reproduction in any medium, provided the original author and source are credited.
Funding: This research was funded by the American Institutes of Bisexuality (http://www.bisexual.org/home.html) and the United States Department of
Agruculture’s Federal Formula Funds (http://cuaes.cornell.edu/support/index.cfm) given to Cornell University for project NYC-321421. The funders had no role in
study design, data collection and analysis, decision to publish, or preparation of the manuscript.
Competing Interests: The authors have declared that no competing interests exist.
* E-mail: Gerulf@cornell.edu
Recent research suggests that self-reported sexual orientation
more strongly corresponds with sexual arousal to male or female
sexual stimuli in men than in women . In addition, the
expression of bisexual arousal differs between the sexes. There is
conflicting evidence regarding whether bisexual-identified men
have a bisexual arousal pattern, but among women, a bisexual
arousal pattern is most common among those who self-identify as
heterosexual [2–4]. These findings, however, are based on
measures of genital responses, which have potential limitations
such as volunteer bias and different measurement devices for men
and women. The present study introduces a measure that is less
affected by these limitations: pupil dilation. We use this measure to
examine previously suggested sex and sexual orientation differ-
ences in sexual response.
Sex Differences in Sexual Response
Research has established that men and women react differen-
tially to sexual stimuli, with men’s responses more influenced by
the erotic information of stimuli and women’s responses more
dependent on other, nonsexual aspects of the stimuli [5–7]. These
findings concur with the proposal that in contrast to men, the
sexual attraction patterns of women are less affected by a partner’s
sex and more by cultural, social, and situational variables [8–10].
These variables include pair bonds, attachment history, educa-
tional experiences, religious attitudes and beliefs, and accultura-
tion [5,6]. Through socialization, these experiences alter women’s
capacity for sexual response, leading to greater variability in sexual
arousal . The consequence, according to Peplau , is that
both sexual and nonsexual experiences are crucial for shaping
women’s sexual orientation and attraction, including the potential
for some women to change their sexual attraction to men and
women over time and across contexts.
Baumeister  proposed that the underlying mechanism for this
sex difference in the variability of sexual attraction has an
evolutionary basis. Baumeister writes on page 347 that sexuality
evolved to ‘‘suit the reproductive contingencies of males and
females so as to maximize the passing on of each person’s genes.’’
Men evolved to be strongly driven by innate motivational patterns
that are relatively constant and unchanging across time and
situations. Women evolved to be responsive to the male sex drive
and to be flexible in whether and what situations they respond to
the male drive. From this perspective, female sexuality, more than
male sexuality, adapts to changing circumstances. This difference
influences women’s sexuality to be more responsive to environ-
As a reflection of this general sex difference, the relationship
of genital arousal to either male or female sexual stimuli with
self-reported sexual orientation is considerably stronger in men
than in women [2,12]. Most men are exclusively aroused to the
sex consistent with their reported sexual orientation; for
example, most heterosexual men are almost exclusively aroused
to women and most homosexual men are almost exclusively
aroused to men. In contrast, women’s sexual orientation is
poorly reflected in their genital response because they respond
with substantial arousal to both sexes [2,12]. These results
suggest a substantial difference between the sexes in the
organization of sexual orientation.
If sex differences in sexual orientation and arousal are robust
and not restricted to measures of genital response, then other
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measures of sexual arousal should also indicate them. The present
research used pupillary response as a measure of sexual arousal.
Thus, our first hypothesis was that the correspondence of pupil
dilation to male or female sexual stimuli with self-reported sexual
orientation would be stronger in men than women.
Sexual Orientation Differences in Sexual Response
In addition to the aforementioned sex differences, genital
arousal measures illuminate differences in sexual response,
depending on a person’s sexual orientation. Although most men
show nearly exclusive sexual arousal to one sex, a substantial
minority of men identifies as bisexual and therefore might be
expected to display strong arousal to both sexes. Some research
suggested, however, that bisexual men exhibit almost exclusive
sexual arousal to either men or women, but not to both [3,13].
According to this research, bisexual men are similar to most other
men in that they show substantial arousal to only one sex. More
recent research by Rosenthal et al. , however, indicated that
more stringent recruitment methods produce self-reported bisex-
ual men who show a bisexual genital arousal pattern. For example,
bisexual-identified participants were recruited through websites
that cater towards men who seek sexual relations with both men
and women. According to another study, bisexual men have a
genital arousal pattern that can be described as bisexual ,
although other interpetation of these data was given . In total,
there is some but inconclusive support for the hypothesis that
bisexual men’s sexual response is a reflection of their self-reported
The present study used recruitment methods similar to those
employed by Rosenthal et al. , but used pupillary response
instead of genital response to measure sexual arousal. Thus, our
second hypothesis was that bisexual men would show greater pupil
dilation to both male and female sexual stimuli compared to
heterosexual and homosexual men.
We previously noted that men and women differ in their
sexual attraction and behavioral patterns because environmental
and innate factors influence the sexes differently [8–10]. Other
authors have pointed out that innate factors account for sexual
orientation differences in sexual attraction [16,17], although
some effective socialization processes can be theorized. For
example, cross-cultural research compared the same-sex sexual
behaviors of heterosexual men in Turkey, Thailand, and Brazil
[18,19]. The incidence of same-sex sexual behavior was greater
in the first two groups, but especially among working class
Turkish heterosexual men. According to Cardoso, because most
Turkish women of this social class are sexually unavailable, men
are more likely to engage in same-sex behavior. Hence, the
finding that some men show sexual arousal to both sexes, or
engage in sexual behavior with both sexes, may be due to
cultural and social class influences.
Unlike most men, many women show substantial sexual arousal
to both sexes. This general pattern, however, is moderated by
women’s sexual orientation and is most common among
heterosexual women, who show similarly strong sexual arousal
to male and female sexual stimuli. In contrast, this pattern is less
common among homosexual women, who show more sexual
arousal to female stimuli and somewhat less sexual arousal to male
stimuli [2,12]. In this sense, homosexual women show more male-
typical sexual arousal patterns compared to other women. This
observation led to the third hypothesis of this study: Homosexual
women would show greater pupil dilation to female stimuli, and
less to male stimuli, and heterosexual women would show more
equal dilation to both sexes.
Pupil Dilation as a Measure of Sexual Response
Potential limitations in assessing sexual orientation by genital
arousal have been pointed out . First, a substantial number of
people are reluctant to participate in a study that assesses genital
response [12,21] and those who do might represent an unusual
population in unknown respects, thus creating results that may not
apply broadly. Second, genital arousal is measured differently in
the two sexes. In men, a common instrument measures penile
circumference; in women, a common instrument assesses vaginal
pulse amplitude . Thus, it cannot be concluded with
confidence whether distinctions between men and women are
due to actual sex differences in sexual arousal or to dissimilarities
of measurement. Third, some people can willingly suppress genital
arousal to sexual stimuli, which affects the accuracy of the measure
One measure that is less affected by these limitations is pupil
dilation. Participants are arguably less likely to opt out of an
experiment that assesses their eye gaze rather than their genital
response. Across both sexes, response of the same organ (the eye)
can be measured with identical instruments. Furthermore, pupil
dilation to stimuli indicates activation of the autonomic nervous
system [25,26]. This system is associated with many automatic
processes such as perspiration, digestion, blood pressure, and heart
rate . For this reason, pupil dilation has been used as indicator
of automatic response, for example, in studies of implicit reaction
and cognitive load [28,29]. Other research suggests that pupil
dilation can reflect automatic attention, or attention that is likely
not in the conscious control of participants . It is therefore
unlikely that participants suppress pupil dilation to stimuli they are
sexually attracted to. Pupil dilation patterns could therefore reflect,
with high sensitivity, automatic attention related to sexual
attraction and sexual orientation.
One study previously employed pupil dilation to measure sexual
orientation . This study investigated the dilation patterns of
five heterosexual men and five homosexual men to images of nude
men and women. Pupil dilation patterns corresponded highly with
sexual orientation. However, the majority of participants was
associated with the investigators, which could have affected results.
To date, there has been no known attempt to confirm these
findings with a larger and more representative sample.
Other than for basic research, pupillary responses as a measure
of sexual orientation have been employed by the Canadian
Government between the 1950s and 1970s. The goal of this
program was to detect homosexual individuals, who were at the
time considered a national risk. These experiments were noted to
be fallacious [32,33] because of several methodological limitations
. The authorities had great difficulty recruiting both hetero-
sexual and homosexual volunteers, which likely meant that they
did not have sufficient statistical power for comparisons. Only
heterosexual and homosexual identities were taken into account;
thus, if bisexual individuals participated, they were not noted as
such, which could have distorted results. The experimenters had
problems adjusting for variation in pupil size. Without this
adjustment, individual responses, including those of individuals
with different sexual orientations, cannot be accurately compared.
It is unknown how consistent the distance was between the pupil
and the camera that captured its image. Without this information
it is problematic to compare responses across individuals. Many
pupillary changes that occurred were in a range that was less than
one millimeter. Because these changes were measured by hand, it
is likely that a large amount of error was added to the data. Sexual
stimuli varied in degree of luminance, which could have caused
pupillary responses unrelated to the individual’s sexual attraction
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to stimuli. These experiments never efficiently produced results,
despite possessing some unique technological qualities.
The present research is free of the vast majority of the
aforementioned limitations. Over 320 participants agreed to take
part in this study and they had multiple options to indicate their
sexual identity, ranging from exclusively heterosexual, to bisexual,
to exclusively homosexual. An infrared gaze tracker automatically
recorded for each participant both pupil size and degree of
dilation. Data were standardized within participants to allow
accurate comparisons across participants. The distance of partic-
ipants to the camera was kept constant. The measurement of pupil
size was in terms of camera pixel occluded by the pupil; thus, the
measure of pupillary response did not rely on an assessment by
hand. In addition, the present research had some, albeit crude,
control over the luminance of stimuli. Overall, compared to the
studies sponsored by the Canadian Government [33,34], the
present research employed methodologies that were considerably
more advanced and precise.
Correspondence of Measures
Because pupil dilation has rarely been used in systematic
research as an indicator of sexual orientation, it is important to
assess its validity. Other research has measured time spent viewing
male or female stimuli to examine sex and sexual orientation
differences in sexual response [35–39]. There may be correspon-
dences among pupil dilation to stimuli, viewing time of stimuli,
self-reported sexual attraction to stimuli, and self-reported
orientation. These correspondences would point to the general
validity of all measures. Thus, the fourth hypothesis was that pupil
dilation would be positively related to other measures of sexual
attraction, and all measures would be positively related to self-
reported sexual orientation.
Summary of Hypotheses
Based on previous research using other measures of sexual
response, the following hypotheses were tested:
(1) The correspondence of pupil dilation to male or female sexual
stimuli with self-reported sexual orientation will be stronger in
men than women.
(2) Bisexual men will show greater pupil dilation to both male
and female sexual stimuli compared to heterosexual and
(3) Homosexual women will show greater pupil dilation to female
stimuli and less to male stimuli, and heterosexual women will
show more equal dilation to both sexes.
(4) Pupil dilation to sexual stimuli will correspond with time spent
viewing these stimuli, self-reported sexual attraction to these
stimuli, and self-reported sexual orientation.
Advertisements were placed on several websites for members of
a Northeast university. We also recruited from a web forum where
men sought both men and women for sexual reasons. The latter
method was previously successful in recruiting bisexual-identified
men , a group less prevalent than other men . A total of 165
men and 160 women indicated their sexual orientation identity on
a 7-point scale, ranging from ‘‘exclusively straight’’ to ‘‘bisexual’’
to ‘‘exclusively gay/lesbian.’’ Table 1 shows the number of
participants for each sexual orientation identity by age and
The average age (SD) was 23.36 (6.62) years in men and 27.70
(6.78) years in women. The most common ethnicity was Caucasian,
indicated mixed ethnicities, and the remainder identified as Asian,
African-American, Hispanic, and Native-American. Across sexual
orientation identities there were differences in age and proportions
of being Caucasian (Table 1).These differences did not significantly
affect patterns of results reported below.
Self-reported sexual orientation.
their sexual orientation identities, attractions, fantasies, and
infatuations on four Kinsey-type scales . These measures were
highly correlated in men (all p’s ,.0001, all r’s $.95) and women
(all p’s ,.0001, all r’s $.92), and were averaged within
participants. For this composite, a score of 0 indicated an
exclusively heterosexual orientation and a score of 6 an exclusively
Each stimulus was a 30-second video showing either
a naked male or female model masturbating. In total, 12 male
stimuli and 12 female stimuli were selected from a large pool of
videos drawn from sites on the Internet. In a pilot study,
heterosexual and homosexual men and women rated these videos
on the models’ sexual appeal, and the most appealing stimuli were
used in the present study. Two 1-minute videos, showing
landscapes, were taken from a nature documentary for creation
of neutral stimuli. Luminance of stimuli was set to equal thresholds
using FinalCut Pro.
An SR Research Remote infrared gaze tracker
recorded participants’ eyes. The program EyeLink computed
pupil area as the number of the tracker’s camera pixels occluded
by the pupil. Pupil dilation and constriction was based on changes of
pupil area when viewing sexual stimuli as compared to neutral
stimuli. Given that results are presented with respect to dilation,
we refer to this measure as pupil dilation.
Viewing time was measured as percentage of time participants
looked at a male or female sexual stimulus.
Cornell University’s Institutional Review Board for Human
Participants (at the Office of Research Integrity and Assurance)
specifically approved this study. Participants provided written
informed consent once they arrived at the university lab. They
were seated in a dimly lit room facing a monitor with a screen
resolution of 1024 by 768 pixels. The gaze tracker was placed
underneath the screen and collected data every two milliseconds
with a 16 mm lens focused on participants’ preferred eye.
Participants’ heads rested on a mount 500 mm from the lens,
and the head’s exact position was automatically recorded by
measuring the distance from the lens to the forehead.
Stimuli were presented in two modules. The first module was
used for collection of pupil dilation data. Participants watched a
neutral stimulus, followed, in random order, by the sexual stimuli.
After each stimulus, participants answered three questions
regarding, in random order, how sexually attractive they found
the person, how sexually appealing they found the person, and
how much they would like to date this person. Participants
answered each question with a 7-point scale ranging from ‘‘not at
all’’ to ‘‘average,’’ to ‘‘very much.’’ These questions were written
on the screen, set against a white background. Participants used a
mouse to answer questions, by clicking at the number of their
choice. This visual display was the same before each subsequent
stimulus. Participants were instructed to give answers without
thinking too hard, but they had as much time as needed to answer
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questions. The vast majority of participants answered these
questions within three seconds. After they answered the last
question, the next stimulus was presented.
with a neutral stimulus. Then, two stimuli were presented
simultaneously. Half of these paired stimuli showed the male to
the right of the female; the other half had the opposite presentation.
Paired stimuli were shown in random order. This module was
women. After each stimulus pair, participants responded to three
questions, in random order, about which person they found more
sexually attractive, more sexually appealing,and would morelike to
date. Participants answered each question with a 7-point scale
ranging from ‘‘very much the left,’’ to ‘‘equal,’’ to ‘‘very much the
Finally, participants completed a questionnaire with demo-
graphic information and sexual orientation and received payment.
The total procedure took, on average, 45 minutes.
Because each stimulus was presented immediately after the
questions for the previous stimulus, there was a chance that
undesired factors influenced pupillary response to stimuli. Specif-
ically, degree of attention to the previous stimulus and its questions
could result in cognitive load and, thus, affect pupillary response to
the subsequent stimulus. To avoid such influences, data analyses
were restricted to the last 10 seconds of each stimulus. The
subsequent results were virtually identical, regardless of whether
the full stimuli length or the last 10 seconds were used for analyses.
However, restricting analyses to the last 10 seconds yielded, in
general, marginally stronger effect sizes.
For each participant, pupil size data were averaged in two steps,
within stimulus and across stimuli of the same type. Specifically,
for each participant, pupil size data were first averaged across the
last 10 seconds of each stimulus. Averaged pupil size was
multiplied with averaged head distance for each stimulus, to
account for heads moving somewhat between stimuli. There is no
consensus as to the most appropriate technique of measuring and
analyzing pupil size data . We computed within each
participant z-scores of pupillary response because pupils vary in
size and in degree of dilation. For men and women, pupil size data
were highly reliable for all male sexual stimuli, female sexual
stimuli, and neutral stimuli (all Cronbach’s a’s $.96, respectively).
This means, for example, if participants showed increased
pupillary response to a male sexual stimulus (as compared to
both a neutral stimulus and other participants), they were likely to
respond in this way to all other male sexual stimuli. Because of
these consistent responses within stimuli types, we computed, for
each participant, three mean values reflecting average pupil
dilation to male sexual stimuli, female sexual stimuli, and neutral
stimuli. Whenever pupillary response to sexual stimuli was used in
analyses, we first subtracted response to neutral stimuli. Thus,
positive scores of these variables indicated increased pupil dilation
to sexual stimuli as compared to neutral stimuli; a negative score
would indicate constriction compared to neutral.
Viewing time was computed such that higher numbers
indicated higher percentage of time viewing the same sex. These
percentages were highly reliable across paired stimuli (Cronbach’s a
$.96 in both sexes) and averaged within participants.
The three ratings of stimuli were reliable within each stimulus,
across all male stimuli, across all female stimuli, and across all male-
female stimuli (all Cronbach’s a’s $.95). Thus, for each participant
and each stimulus type, an average was computed across ratings.
These averages represented, respectively, participants’ general self-
reported sexual attraction to stimuli of the same sex, sexual
attraction to stimuli of the other sex, and, from the second module,
a contrast reflecting sexual attraction to the same or other sex.
Sex Differences in Sexual Orientation and Pupil Dilation
Our first hypothesis stated that the relation of pupil dilation to
male and female stimuli with self-reported sexual orientation
would be stronger in men than women. For both men and women,
Table 1. Distribution of Sexual Orientation Identities, Ages, and Ethnicities.
Sexual Orientation Identity
Straight Bisexual Leaning Straight BisexualBisexual Leaning Gay Mostly Gay
Number3124 15 1021 3331
Average Age (SD) 20.83 (2.79)23.58 (7.00)25.67 (7.51) 28.67 (12.09)25.50 (6.96) 22.76 (5.65) 24.00 (7.16)
Percentage Caucasian65 63676759 6477
Variable Sexual Orientation Identity
Bisexual Leaning Straight BisexualBisexual Leaning
Number34 27 111716 3223
Average Age (SD) 21.88 (6.73)21.67 (4.29) 22.36 (4.25)24.41 (6.92)20.81 (2.07) 26.31 (9.46)24.87 (5.17)
47 63557669 84 91
Note.1Caucasian was the most common ethnicity in men (65%) and women (69%). The second most common group indicated mixed ethnicities in men (11%) and
women (9%), and the remainder identified as Asian, African-American, Hispanic, and Native-American.
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there was a strong and inverse correlation of pupil dilation to
same-sex stimuli with pupil dilation to other sex stimuli, p,.0001,
r=2.77 and p,.0001, r=2.77, respectively. We therefore
created a contrast by subtracting participants’ dilation to the
other sex from dilation to the same sex. For this pupil dilation
contrast a positive score represented more dilation to the same sex
and less dilation to the other sex, a negative score represented
more dilation to the other sex and less dilation to the same sex, and
a score of zero represents equal dilation to both sexes. For each
sex, this contrast was regressed against self-reported sexual
orientation by conducting a regression analysis that included both
the linear and curvilinear effect of sexual orientation. The
curvilinear effect was included because, for example, the
relationship of sexual orientation with pupil dilation to the
preferred sex may not be equally strong in heterosexual and
In men, the linear relationship of the pupil dilation contrast with
sexual orientation was significant, p,.0001, b=.55. (The effect
size b is the standardized regression coefficient and can be
interpreted similarly to a correlation coefficient.) Heterosexual
men dilated most to the other sex, homosexual men dilated most
to the same sex, and bisexual men dilated more equally than other
men to both sexes (Figure 1A). For heterosexual, bisexual, and
homosexual women, pupil dilation to the same or other sex was
similarly related to their sexual orientation (Figure 1B), p,.0001,
The curvilinear effects of sexual orientation on pupil dilation
was not significant in men, p=.49, b=-.05. Figure 1A shows that
exclusive heterosexual men (Kinsey Score of 0) and homosexual
men (Kinsey Score of 6) dilated almost equally strongly to their
preferred sex, yielding the aforementioned linear effect. In
contrast, in women, there was indication of a curvilinear effect.
Figure 1B shows that exclusive heterosexual women had
significantly greater pupil dilation to the other sex than to the
same sex; however, in magnitude, exclusive homosexual women
dilated more to the same sex than the other sex. This difference
yielded, within women, a significant curvilinear effect, p=.03,
b=.15. The finding that homosexual women had more orienta-
tion-specific dilation patterns than other women is in line with the
third hypothesis investigated below.
A subsequent multiple regression analysis tested for sex
differences in these effects. Pupil dilation the same sex or other
sex was predicted by the linear effect of sexual orientation, the
curvilinear effect of sexual orientation, and by participant’s sex
(converted into a numerical variable with values 0 for males and 1
for females). Other predictor variables included the interactions of
the linear and curvilinear effects of sexual orientation with sex.
These interactions tested whether the sexes differed in these
effects. Results of this regression analysis indicated no significant
sex difference for the linear relation of pupil dilation with sexual
orientation, p=.82, b=.01. There was, however, a significant sex
difference for the curvilinear relationship, p=.04, b=.18. This
result suggested that the curvilinear relationship is more common
in women than men; this sex difference is reflected in the
comparisons of Figures 1A and 1B.
Figure 1 also shows that within exclusive heterosexual men and
women (Kinsey Score of 0) there was an additional sex difference.
Exclusive heterosexual men showed greater pupil dilation to the
other sex than same sex compared to exclusive heterosexual
women. We computed an independent-sample t-test, comparing
exclusive heterosexual men and women on their pupil dilation to
the other sex over the same sex. This sex difference was marginally
significant, p=.08, d=0.66.
Pupil Dilation Patterns within Men
Our second hypothesis stated that bisexual men would show a
bisexual dilation pattern. We computed two new variables, one for
participants’ pupil dilation toward the more arousing (dilation-
triggering) sex and one for the less arousing sex . We assumed
the level of pupil dilation toward the more arousing sex to be
similar across men of different sexual orientations. The crucial
analysis concerned the less arousing sex; if bisexual men have
bisexual pupil dilation patterns, then they should show signifi-
cantly greater pupil dilation toward the less arousing sex
(whichever that sex happens to be) than do either heterosexual
or homosexual men.
We conducted a multiple linear regression analysis predicting
men’s pupil dilation to the less arousing sex by the linear and
curvilinear effect of sexual orientation. Results confirmed this
hypothesis. Figure 2A indicates that, compared to a neutral
stimulus, men with bisexual orientations (Kinsey Scores of 2, 3, or
4) displayed significantly greater pupil dilation to the less arousing
sex than either heterosexual or homosexual men. This curvilinear
effect was significant, p,.0001, b=2.32. Thus, with respect to
pupil dilation, bisexual men had bisexual responses. The linear
effect of men’s sexual orientation on their pupil dilation to the less
arousing sex was not significant, p=.26, b=.09.
Figure 1. Pupil dilation to the same sex and the other sex. Panel A shows men’s responses and Panel B shows women’s’ responses. Y Axes
reflect z-scores within participants: positive numbers indicate dilation to the same sex, and negative numbers indicate dilation to the other sex. X
Axes reflect self-reported sexual orientation: 0 represents an exclusive heterosexual orientation, 3 an even bisexual orientation, and 6 an exclusive
homosexual orientation. Triple lines represent regression coefficients with 95% confidence intervals. Dots represent participants’ average scores. b’s
are standardized coefficients for linear effects.
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To obtain a substantial sample of bisexual men, we recruited
from a web forum where men sought sex with men and women.
Thus, our bisexual men may have had bisexual pupil dilations
because they presented as highly sexual men, in general, and not
because they had responses specific for their identity. We re-
analyzed data after excluding the 24 bisexual men recruited from
the forum and kept the 22 bisexual men recruited similarly to
other participants. Results remained similar; for example, bisexual
men showed bisexual pupil dilation patterns before and after
exclusion, p,.0001, b=2.32, and p=.02, b=2.25, respectively.
Differences between bisexual men and other men were therefore
not strongly based on recruitment venue.
Notably, Figure 2A also shows that bisexual men dilated
significantly less to their more arousing sex, compared to both
heterosexual and homosexual men, p=.02, b=.18. This effect was
weaker, in magnitude, than bisexual men’s greater pupil dilation
to their less arousing sex, compared to other men, p,.0001,
Pupil Dilation Patterns within Women
Our third hypothesis indicated that homosexual women would
have greater pupil dilation to their more arousing sex and less
dilation to the less arousing sex compared to other women. In
contrast, heterosexual women would show more equal pupil
dilation to both sexes.
We conducted two multiple linear regression analyses predicting
either women’s pupil dilation to the more arousing sex or their
pupil dilation to the less arousing sex by sexual orientation.
Figure 2B shows that pupil dilation to the more arousing sex,
compared to a neutral stimulus, was stronger in homosexual
women than in other women, p=.001, b=.25. By contrast,
dilation to the less arousing sex was weaker in homosexual women
than in other women, p=.01, b=2.19. Thus, among women,
homosexual women showed the strongest contrast in pupil dilation
to the more versus less arousing sex.
Figure 2B also shows that for bisexual women (Kinsey Scores of
2, 3, or 4) the contrast in pupil dilation to the more versus less
arousing sex was stronger than the contrast found in heterosexual
women, but less strong than the contrast found in homosexual
women. When compared to the heterosexual women only, their
pupil dilation contrast was significantly stronger, p=.003, b=.24;
when compared to the homosexual women only, their pupil
dilation contrast was significantly weaker p=.009, b=2.23,
respectively. Thus, in their pupil dilation to the more and less
arousing sex, bisexual women were in an intermediate position
between heterosexual and homosexual women.
Finally, Figure 2 shows that for exclusive heterosexual men and
women, pupil dilations to the more and less arousing sex were
different. Heterosexual men dilated strongly to the more arousing
sex and little to the less arousing sex, compared to neutral stimuli.
By contrast, heterosexual women dilated less to the more arousing
sex, compared to both neutral stimuli and men, and they dilated
more to the less arousing sex, compared to both neutral stimuli
and men. Hence, heterosexual women dilated more strongly to the
less arousing sex than did heterosexual men, p=.01, d=1.00.
Consistency across Measures
Our fourth hypothesis regarded the correspondence of mea-
sures. Table 2 shows the correlations across pupil dilation, viewing
time of same-sex or other-sex stimuli, self-reported sexual
attraction toward stimuli, and self-reported sexual orientation. In
general, these correlations were modest and significant, suggesting
that all measures, including pupil dilation, are indicators of sexual
attraction and orientation.
One of the strongest correlations was between self-reported
sexual orientation and time spent viewing same-sex or other-sex
stimuli (Table 2). The weakest correlations were found between
self-reported sexual attraction to same-sex stimuli and sexual
attraction to other-sex stimuli.
The average absolute correlation was.64 in men and.55 in
women. A repeated measures t-test, with calculated sex differences
in correlations within pairs of variables, suggested that the
magnitude of absolute correlations was significantly stronger in
men than women, t(20)=7.83, p,.0001, d=1.43.
Results suggested that pupil dilation is a significant indicator of
sexual orientation. Within heterosexual men and women, findings
confirmed hypothesized sex differences in sexual response.
Furthermore, results indicated that bisexual men have bisexual
dilation patterns, and homosexual women have male-typical
The overall relationships of sexual orientation with pupil
dilation and other measures were stronger in men than in women
(Table 2). Other research has reported corresponding sex
Figure 2. Pupil dilation to the more arousing sex and less arousing sex. Panel A shows men’s responses and Panel B shows women’s’
responses to the more arousing sex (i.e., more dilation-eliciting sex; upper lines) and the less arousing sex (lower lines). Y Axes reflect, within
participants, z-scores compared to a neutral stimulus. X Axes reflect self-reported sexual orientation: 0 represents an exclusive heterosexual
orientation, 3 an even bisexual orientation, and 6 an exclusive homosexual orientation. Triple lines represent regression coefficients with 95%
confidence intervals. b’s are, for men, standardized coefficients for curvilinear effects, and, for women, standardized coefficients for linear effects.
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differences [7,43]. The simple relationship of sexual orientation
with pupil dilation patterns to sexual stimuli was not significantly
stronger in men than women, largely because among homosexual
women the relationship of their sexual orientation with dilation
patterns was similar to the relationship seen in men.
Among heterosexual participants, substantial sex differences in
pupil response were found (Figures 1 & 2). Compared to
heterosexual men, heterosexual women showed a more equal
sexual response to male and female stimuli. This sex difference is
consistent with previously reviewed theoretical writings [5,6,8,9]
and documented sex differences in genital response . Baume-
ister  argued that the sexes evolved to differ in their sexual
responsiveness, and this is an adaptation to the sexual behavior of
the other sex. One hypothesis related to Baumeister’s proposal is
that sexual response has different biological functions for men and
women . For men, an important function is to facilitate erection
and penetration; for women, to facilitate lubrication and prevent
genital injury in case of penetration. Support for this hypothesis is
derived from both cross-species and cross-cultural comparisons.
Forced copulation in several species [44–46] and in most human
societies [47,48] indicate that it may have occurred throughout
human evolution . Because forced copulation can lead to
genital trauma , the female response to any sexual stimulus
may have evolved in part to mitigate this risk. Related reasons
have been discussed regarding how and why mate choice  and
sexual arousal  differ between the sexes.
When proposing these evolutionary hypotheses for sex differ-
ences in sexual arousal, we focus on heterosexual men and women.
The vast majority of people is heterosexual [40,52], and a sexual
orientation towards the other sex is likely promoted by evolution-
ary mechanisms; thus, a focus on heterosexual individuals is
justified. From an evolutionary perspective, exclusive homosexu-
ality as found in humans is a conundrum . Some research has
suggested that, at least in men, the decreased fecundity of
homosexual males is counter-balanced by the increased fecundity
of their relatives . Why such a balancing mechanism might
exist and how it would relate to general sex differences in
attraction and arousal is still unknown.
Sexual Orientation Differences In Men
Bisexual men displayed bisexual pupil dilation patterns consis-
tent with the finding that bisexual men show bisexual genital
arousal . The previous conclusion that bisexual men do not
show such arousal  may have been based on a sample that
identified as bisexual for reasons other than having strong sexual
responses to both sexes. We also point to other findings that only a
few men report attraction to both sexes [40,52]. Men who have
both bisexual identities and bisexual responses may therefore
constitute an uncommon group that differs in some aspects from
bisexually-identified men who have sexual responses to only one
sex. It is possible, for example, that some men identify as bisexual
not because they show bisexual arousal but because they have
distinct personalities that open them to a variety of sexual
experiences, including sexual experiences with the less preferred
Notably, bisexual men in the present study dilated somewhat less
to their more arousing sex, compared to both heterosexual and
homosexual men (Figure 2A). Blanchard, a leading figure in
psychophysiological research on sexuality [57,58], noted in
unpublished data a similar pattern with respect to genital arousal.
Perhaps, some bisexual men need further input other than visual
stimulation to achieve maximum sexual arousal to their preferred
sex. Possibilities include a need for higher levels of tactile
stimulation or proprioceptive feedback from their own sexual
Table 2. Correlations between Pupil Dilation, Viewing Time, Self-Reported Attraction to Sexual Stimuli, and Self-Reported Sexual Orientation across Male (N=165, above
Diagonal) and Female (N=160, below Diagonal) Participants.
Pupil Dilation To
Pupil Dilation To
Attraction to Same
Attraction to Other Sex
Pupil Dilation To Same Sex
Pupil Dilation To Other Sex
Sexual Attraction to Same Sex Stimuli
Sexual Attraction to Other Sex Stimuli
Sexual Attraction Contrast1
Note.1Higher scores indicate stronger response or attraction to the same sex and less to the other sex.
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behavior in order to show maximum sexual response (Blanchard,
Sexual Orientation Differences In Women
The present study confirmed the hypothesis that homosexual
women have more male-typical sexual responses when compared
to other women . Homosexual women are, on average, more
masculine than other women not only in motor behavior, voice
pattern, facial features, and appearance (both self-reported and
perceived by others) but also in their self-concepts and interests
[59–62]. Prospective studies suggest that differences in masculin-
ity-femininity appear in early childhood and prior to the
development of an adult sexual orientation identity [63,64].
Cross-culturally, sexual orientation differences in masculinity-
femininity are not restricted to Western cultures but are found in
other societies [65,66]. These observations lead to the hypothesis
that non-social and non-cultural factors are important for the co-
development of sexual orientation with masculinity-femininity.
Social factors are certainly important for the development of
some gender-typed behavior , but there is little if any evidence
that they affect the co-development of sexual orientation with
masculinity-femininity . To date, prominent candidates for
this co-development include prenatal gonadal influences [68,69]
and genetic influences [70,71]. If these factors account for a
general association of homosexuality with masculinity in women,
they may also explain the present finding that, compared to other
women, homosexual women had male-typical sexual responses.
Utility of Pupillary Response
Pupil dilation was, in general, a robust indicator of sexual
orientation. Pupillary response, viewing time, and self-reported
sexual attraction to stimuli correlated with each other and with
sexual orientation (Table 2). We note that these measures have
their limitations. Pupillary response can be influenced by factors in
addition to sexual orientation, including luminance and cognitive
load [28,29]. Aspects of viewing time can be under the conscious
control of participants [72,73], and the same is the case for self-
reported attraction. Yet, despite their different methodological
limitations, the present measures corresponded with each other,
which supports the general validity of all measures and points to a
core factor of sexual attraction and orientation.
In the present study the simple relationship of male self-reported
sexual orientation with pupil dilation to the same or other sex
(r=.57) was weaker in effect than corresponding effects of other
research based on genital arousal measures, with r’s ranging
from.77 to.83 [2,3]. In this sense, the assessment of genital arousal
measures appears to be the more precise measure of the two. Yet,
because the assessment of genital response is more invasive than
the assessment of pupillary response, the latter is more appealing
for a wide range of participants. Moreover, in combination with
stimuli that are not sexually explicit (e.g., images of dressed men
and women), pupil dilation could be used with populations for
which it would be problematic to use genital arousal measures,
such as the study of sexual orientation among minors or in
In the present study, the relationship of female self-reported
sexual orientation with pupil dilation to the same sex or other sex
(r=.47) was larger in effect than corresponding effects in other
studies based on genital arousal, with r’s ranging from.21 to.24
[2,12]. Perhaps, pupil dilation is a more sensitive indicator of
female sexual orientation than vaginal pulse amplitude. Future
research should compare these measures systematically in order to
investigate which is the more robust correlate of female sexual
Viewing time of male or female stimuli was more strongly
related to self-reported sexual orientation than was pupil dilation
(Table 2). As aforementioned, because aspects of viewing stimuli
are controllable by participants [72,73], similar to self-reports,
their strong correspondence would be expected. In contrast, pupil
dilation is likely a measure of autonomic or unconscious response
[25,30]. Thus, pupil dilation may be the more desirable measure
in future research, despite its weaker correlations with self-reported
We emphasize that in the present study, sexual orientation
differences in pupillary response were significant, on average, and
with effects that were moderate to strong in magnitude, but not
perfect, using the guidelines suggested by Cohen . Conse-
quently, not every participant’s sexual orientation was correctly
classified, based on his or her pupil dilation to the same or other
sex. Figure 1 illustrates that an observable amount of variability in
pupil dilation was unrelated to the participant’s sexual orientation.
The present study had several methodological limitations. Pupils
dilate to factors other than to how arousing stimuli are, such as the
amount of cognitive load they produce and the stimuli’s level of
luminance and contrast [28,29]. In the present study, average
luminance of stimuli and contrast was not set equal across stimuli.
This was attempted but the resulting stimuli appeared extremely
distorted and could not be used, which is not an uncommon
problem when adjusting these factors, especially in videos.
We therefore selected videos that had, subjectively, similar
luminance and then applied the filters on luminance thresholds.
Although this procedure has clear limitations, it is unlikely that
they resulted in confounded findings. Figure 1A shows that
heterosexual and homosexual men dilated almost equally strong to
their preferred sex, and bisexual men showed almost equal dilation
to both sexes. Such patterns were consistent with our expectations
about differences in sexual orientations. These patterns were, in
general, similar in women, although we point to hypothesized sex
differences in this effect. Given these systematic differences, results
are less likely due to a lack of control of luminance or other factors.
In fact, given the lack of control, it is likely that there was a certain
level of noise in our data and that actual sexual orientation
differences in pupillary response would have been stronger in
effect if we had full control over all visual factors.
Another limitation was that neutral stimuli were presented
twice, one time each before a block of stimuli was presented. A
refined methodology would include a neutral stimulus that is
presented before each sexual stimulus and would be used as
specific comparison for this stimulus.
The present study suggests that measures of pupil dilation and
genital arousal show similar patterns related to sex and sexual
orientation. This proposal is indirect, however, because only pupil
dilation was assessed in the present study. An important future
study is to compare participants whose pupillary response and
genital response are simultaneously assessed with participants who
were recruited for a study on only pupil dilation. A comparison of
these groups could systematically assess whether type of measure
leads to ascertainment biases and affects patterns of results.
Personal and interpersonal factors other than sexual attraction
may affect pupil dilation to stimuli of the same sex or other sex.
For example, participants may dilate in response to comparing
themselves, on a physical or social level, more with stimuli of their
own sex than with the other sex (e.g., by asking whether they are
better looking than the stimulus). Similarly, perceived personality
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PLoS ONE | www.plosone.org8 August 2012 | Volume 7 | Issue 8 | e40256
characteristics of men and women used for stimuli could affect
pupillary responses. Future research should assess how such effects
compare, in magnitude, to the effects of sexual orientation on
Findings from the present study suggest, for the first time across
a large sample, that pupil dilation patterns are significant
indicators of sexual orientation. As opposed to previous measures
of sexuality, the possible benefits of pupil dilation include a
similarity of assessment across the sexes, its potential to reach a
wide range of participants, and its ability to capture automatic
responses. The assessment of pupil dilation can therefore
complement other physiological and self-report measures of sexual
We thank Sam B. Hutton and David A. Puts for scientific advice, and
David Post, James Jones-Rounds and Sanjay Muralidharan Dharmavaram
for technical assistance.
Conceived and designed the experiments: GR RSW. Performed the
experiments: GR RSW. Analyzed the data: GR RSW. Wrote the paper:
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