A sex difference in features that elicit genital response
Meredith L. Chiversa,*, J. Michael Baileyb
aCentre for Addiction and Mental Health, 250 College St., Toronto, Ont., Canada M5T 1R8
Received 10 September 2004; accepted 29 December 2004
Available online 9 March 2005
Previous research suggests that women’s genital arousal is an automatic response to sexual stimuli, whereas men’s genital arousal is
dependent upon stimulus features specific to their sexual interests. In this study, we tested the hypothesis that a nonhuman sexual stimulus
would elicit a genital response in women but not in men. Eighteen heterosexual women and 18 heterosexual men viewed seven sexual film
stimuli,sixhumanfilmsandonenonhumanprimatefilm,while measurementsofgenitalandsubjectivesexualarousalwererecorded. Women
showed small increases in genital arousal to the nonhuman stimulus and large increases in genital arousal to both human male and female
stimuli. Men did not show anygenital arousal to the nonhuman stimulus and demonstrated a category-specific pattern of arousal to the human
stimuli that corresponded to their stated sexual orientation. These results suggest that stimulus features necessary to evoke genital arousal are
much less specific in women than in men.
# 2005 Elsevier B.V. All rights reserved.
Keywords: Female sexual arousal; Male sexual arousal; Sex difference; Gender difference; Sexual features; Vaginal photoplethysmography; Penile
Men’s sexual interests vary by the preferred category of
target and by the preferred activity (Freund et al., 1997).
Sexual interests are usually directed toward other sexually
mature humans or, much less frequently, in the case of
paraphilias, toward animals, body parts, or nonliving
objects; sexual interests also involve some type of sexual
activity with that target. A relatively objective indicator of
men’s sexual interests is genital arousal to stimuli
reflecting their interest. In laboratory settings, men
typically show patterns of genital responding that
correspond to their preferred gender (e.g., opposite- or
same-sex adults; Freund, 1963, 1974; Freund et al., 1973;
Mavissakalian et al., 1975; Sakheim et al., 1985), their
preferred age (e.g., adult versus prepubescent/pubescent
in the case of pedophilia; Blanchard et al., 2001; Freund
and Watson, 1991; Freund and Blanchard, 1989; Seto
et al., 2000), and preferred sexual object (e.g., Blanchard
et al., 1986). Similarly, incarcerated rapists and sexually
coercive men from the community demonstrate relatively
greater genital arousal to stimuli depicting sexual
aggression than noncoercive men (Bernat et al., 1999;
Lalumie `re and Quinsey, 1994; Lalumie `re et al., 2003;
Lohr et al., 1997). These data indicate a strong relation-
ship between genital sexual arousal and sexual interests in
Women’s sexuality seems to be organized differently.
Genital arousal does not correspond to a woman’s stated
sexual interests in the way that it does for men. Both lesbian
and heterosexual women show substantial genital arousal to
films of both preferred and nonpreferred genders (Chivers
et al., 2004; Laan et al., 1996). Women also show genital
arousal to stimuli depicting non-preferred sexual activities:
Stock (1983) reported that women experienced similar
levels of genital response to an audio narrative of sexual
assault as to a narrative describing consensual sexual
intercourse. Laan et al. (1995) and Both et al. (2003)
observed significant increases in genital arousal to depic-
tions of a sexual threat when compared to responses to a
Biological Psychology 70 (2005) 115–120
* Corresponding author. Tel.: +1 416 535 8501x6960;
fax: +1 416 979 6965.
E-mail address: firstname.lastname@example.org (M.L. Chivers).
0301-0511/$ – see front matter # 2005 Elsevier B.V. All rights reserved.
responses are not as informative about women’s sexual
interests as they are for men.
If women’s genital arousal is nonspecific, as the above
data suggest, then what aspect of a sexual stimulus causes a
genital response? Laan and Everaerd (1995) proposed that
women’s genital vasocongestion is a reflexive response to
automatic processing of the sexual features of a stimulus.
Exactly which features make a stimulus ‘‘sexual,’’ however,
is unclear. The literature on men’s sexual arousal responses
suggests that relevant features indicate preferred categories
of sexual target and preferred sexual activity. For example,
features that indicate a preferred gender would include
primary and secondary sexual characteristics (e.g., penis,
full breasts). Features that indicate a preferred sexual
activity, such as sexual intercourse, would include depic-
tions of genital configurations and characteristic movements
(e.g., thrusting). For women, the stimulus features necessary
to evoke genital arousal do not seem to reflect preferred
gender or activity (Both et al., 2003; Chivers et al., 2004;
Laan et al., 1995; Laan et al., 1996; Stock, 1983).
We propose that women’s genital vasocongestion is
provoked by exposure to nonspecific sexual features, that
is, any sexual content, whether or not the categories of
sexual targets or sexual activities presented are preferred.
To test this hypothesis, it was necessary to create a
stimulus that excluded plausible sexual targets (human
actors) but included sexual content. Our solution was to
use a film stimulus depicting nonhumans engaged in
copulation. In the present study, we examined the genital
and the subjective sexual arousal of women and of men to
stimuli depicting human male–male, female–male, and
female–female sexual interactions as well as to a stimulus
depicting female and male bonobos (Pan paniscus)
engaging in sexual activity (the nonhuman stimulus).
We expected that the sexual content presented in the
nonhuman stimulus would be sufficient to elicit genital
response in women, but not in men. We did expect
women’s subjective arousal patterns to reflect their sexual
interests, specifically, and that women would not report
subjective arousal to the nonhuman stimulus. We expected
that men would show category-specific responses to the
human stimuli, as consistently demonstrated in previous
studies, and would not show any increase in genital
responding or report subjective sexual arousal to the
These results suggest that genital
We recruited heterosexual women and men via adver-
tisements in an ‘‘alternative’’ urban newspaper (Chicago
Reader). Fifty women responded to the advertisement, 28
were eligible to participate and scheduled an appointment,
and 20 attended the experimental session. Thirty-nine men
responded to the advertisement, 35 were eligible to
participate and scheduled an appointment, and 20 attended
the experimental session. Women were much more likely to
be excluded than men because women who used oral
contraceptives or were pregnant were not eligible to
participate in the study. Individuals were also excluded if
they endorsed any of the following: consistent problems
with sexual arousal, using medications known or believed to
influence sexual arousal and response, having a sexually
transmitted disease, and having a nonheterosexual orienta-
tion. All participants were offered financial compensation.
Heterosexuality was operationally defined as self-report of
exclusive or nearly exclusive opposite-gender sexual
feelings (i.e., scores of 0 or 1) as assessed using the Kinsey
Sexual Fantasy Scale (Kinsey et al., 1953).
Of these participants, 18 women and 18 men produced
genital responses that met the minimum response inclusion
criterion (a minimum difference of 0.5 standard deviations
between maximum arousal to either a human male or human
female stimulus and arousal to the neutral stimulus; see
Chivers et al., 2004). Mean ages were 28.7 (S.D. = 4.8) and
29.4 (S.D. = 5.3) years for the female and male samples,
respectively. Women and men did not differ significantly on
any demographic variables but did differ on some sexual
experience variables, reflecting commonly observed sex
differences in sexuality (Oliver and Hyde, 1993). Men
reported more frequent masturbation, F(2, 36) = 5.9,
p < 0.05, a greater desire for more frequent sexual contact
with a partner, F(2, 36) = 9.7, p < 0.001, and a greater
36) = 10.5, p < 0.001, than women did. Women reported
more nonheterosexual sexual fantasies, F(2, 36) = 12.6,
p < 0.05 and less frequent orgasm during sexual inter-
course, F(2, 36) = 5.0, p < 0.05, than men did.
1.2. Audiovisual stimuli
The human sexual stimuli were identical to those used by
Chivers et al. (2004): Six, 2 min films depicting female–
female, female–male, and male–male oral and penetrative
sexual interactions were presented with sound. The nonhu-
man sexual stimulus consisted of a 2 min film, presented
with sound, of female and male bonobos engaging in
repeated penile–vaginal. Neutral nonsexual stimuli, depict-
ing landscapes or primates engaging in nonsexual beha-
viours (e.g., relaxing in a hot spring), were included to
compare genital and subjective arousal to sexual versus
M.L. Chivers, J.M. Bailey/Biological Psychology 70 (2005) 115–120116
1Although women in these studies were not directly asked whether they
preferred coercive sex, it is very unlikely that thesewomen had this interest.
A sexual preference manifests as persistent sexual thoughts, feelings, and
behaviour involving the object or activity of interest that occur more
frequently than sexual thoughts, etcetera for nonpreferred objects or activ-
ities. Studies of sexual fantasy content, for example, suggest between 20
and 51% of women report fantasizing about being coerced sexually, but the
frequency of these fantasies does not exceed the frequency of fantasies
involving consensual sexual contact (Leitenberg and Henning, 1995).
nonsexual stimuli. All stimuli were presented in random
order. An 11 min adaptation film (depicting landscape
scenes accompanied by relaxing music) was used to
establish baseline arousal.
and digitized during baseline and stimulus conditions using
Acqknowledge III, Version 3.2 (BIOPAC Systems Inc.,
Santa Barbara, CA), a MP100 data acquisition unit
(BIOPAC Systems Inc.), and a PowerMac 6500 computer.
Women’s genital responses were assessed via change in
vaginal pulse amplitude (VPA), a measure of vaginal
vasocongestion specific to sexualarousal (Laan et al., 1995),
using a vaginal photoplethysmograph (Sintchak and Geer,
s throughout all stimuli, band-pass filtered (0.5–10 Hz), and
digitized (40 Hz). VPA was measured as peak-to-trough
amplitude for each vaginal pulse.
Men’s genital responses were assessed with penile
plethysmography (Janssen, 2002), using a mercury-in-
rubber strain gauge to measure changes in the circumference
of the penis as erection developed. The signal was sampled
at a rate of 100 samples/s, low-pass filtered (to 0.5 Hz),
digitized (40 Hz), and transformed into millimeters of
circumference change from baseline. The gauge was
calibrated over six, 5 mm steps between sessions (Janssen,
1.6. Subjective arousal
Participants indicated their subjective sexual arousal
while viewing audiovisual stimuli by using a lever that
moved through a 1808 arc; 08 represented no subjective
sexual arousal and 1808 the subjective sexual arousal
associated with orgasm. The signal was low-pass filtered (to
0.5 Hz), digitized (40 Hz), and transformed into percent
Participants were assessed individually in a dimly lit,
private room, seated in a comfortable recliner with a
television monitor five feet away. Participants received
instruction on the genital gauge and fitted the gauge
themselves. Participants watched the adaptation film and
then the experimental stimuli, separated by return-to-
baseline intervals. Participants completed distraction tasks
during inter-stimulus intervals and, after the sexual arousal
assessment, completed questionnaires assessing sexual
orientation, sexual experience, masturbation frequency,
and orgasmic capacity.
1.8. Data reduction
Both genital and subjective arousal measures were
standardized within-subjects to control effects of idiosyn-
cratic variation in responsiveness (Harris et al., 1992), then
averaged, separately and within stimulus category, yielding
mean genital and subjective arousal values for responses to
human (female–female, male–female, and male–male),
nonhuman, and neutral stimuli.
Genital and subjective sexual arousal to neutral, nonhu-
man, and human stimuli were examined separately for
women and for men. We submitted subjective and genital
responses to each stimulus category to planned repeated
contrasts, comparing the mean response to each stimulus
category to the mean of adjacent categories. The human
sexual stimuli were entered in a separate order for each
group, reflecting the expected arousal pattern for hetero-
sexualwomen and men. For women, we enteredbothgenital
and subjective arousal to the stimuli in the order of
increasing arousal predicted for subjective sexual arousal,
that is, neutral, nonhuman, female–female, female–male,
and male–male. We expected no significant differences in
women’s genital arousal to the human stimuli. For men, the
male–male, female–male, and female–female. We include
the omnibus ANOVA results for sake of completeness.
68) = 23.6, p < 0.001, indicating that the stimuli elicited
significantly different levels of genital arousal (Fig. 1).
Planned, repeated contrasts revealed that women’s genital
arousal to the nonhuman sexual stimulus was significantly
greaterthantotheneutralstimulus, F(1,17) = 8.9, p < 0.01,
and significantly smaller than to the male–male stimulus,
F(1, 17) = 12.5, p < 0.01. Women’s responses to the human
replicating the nonspecificity effect reported by Chivers et al.
female–male, F(1, 17) = 0.1, p > 0.05; female–male versus
male–male, F(1, 17) = 0.75, p > 0.05.
A main effect of stimulus category was also found using
subjective sexual arousal as the dependent variable, F(4,
68) = 17.9, p < 0.001 (Fig. 2). Women’s subjective arousal
showed a different pattern than their genital arousal: Women
did not report being sexually aroused by the nonhuman
stimulus. Planned contrasts revealed that women’s sub-
M.L. Chivers, J.M. Bailey/Biological Psychology 70 (2005) 115–120117
jective arousal to the nonhuman stimulus was not
significantly greater than to the neutral stimulus, F(1,
17) = 1.85, p > 0.05. Women’s subjective arousal was
greatest to the female–male sexual stimulus: Women
reported significantly less arousal to the female–female
than female–male stimulus, F(1, 17) = 11.9, p < 0.05, and
significantly greater arousal to the female–male versus
male–male stimulus, F(1, 17) = 14.8, p < 0.01.
The omnibus test of stimulus category was significant,
F(4, 68) = 14.2, p < 0.001 (Fig. 3). Planned contrasts
showed that men, unlike women, did not show genital
arousal to the nonhuman stimulus when compared to the
neutral stimulus, F(1, 17) = 0.04, p > 0.05. Men’s genital
arousal to human sexual stimuli showed a category-specific
pattern: Arousal tothe male–male stimulus was significantly
lower than to the female–male stimulus, F(1, 17) = 4.93,
p < 0.05;therewasnosignificantdifferencebetweengenital
responses to the female–male and female–female sexual
stimulus, F(1, 17) = 1.2, p > 0.05.
Men’s subjective arousal responses mirrored their genital
responses. A significant main effect of stimulus category
wasfound, F(4,68) = 30.1, p < 0.001(Fig.4).Theplanned
contrasts showed that men did not report greater subjective
arousal to the nonhuman stimulus, relative to the neutral
stimulus, F(1, 17) = 0.09, p > 0.05. Men’s subjective
arousal to the human sexual stimuli was also category-
specific. Men reported significantly greater arousal to the
M.L. Chivers, J.M. Bailey/Biological Psychology 70 (2005) 115–120 118
Fig. 1. Women’s mean genital sexual arousal responses to neutral, nonhu-
man, female–female (FF), female–male (FM), male–male (MM) stimuli.
Error bars show standard error of the mean. Units are within-subject
Fig. 2. Women’s mean subjective sexual arousal responses to neutral,
nonhuman, female–female (FF), female–male (FM), male–male (MM)
stimuli. Error bars show standard error of the mean. Units are within-
subject standard deviations.
Fig. 3. Men’s mean genital sexual arousal responses to neutral, nonhuman,
male–male (MM), female–male (FM), female–female (FF) stimuli. Error
bars show standard error of the mean. Units are within-subject standard
Fig. 4. Men’s mean subjective sexual arousal responses to neutral, nonhu-
man, male–male (MM), female–male (FM), female–female (FF) stimuli.
Error bars show standard error of the mean. Units are within-subject
female–male than male–male stimuli, F(1, 17) = 23.4,
p < 0.001, and relatively equivalent arousal to the female–
male and female–female stimuli, F(1, 17) = 0.25, p > 0.05.
Our results suggest that nonspecific sexual features are
sufficient to induce intermediate levels of genital arousal in
women, even in the absence of a subjective appraisal of the
stimulus as sexually arousing. In other words, women
showed a significant increase in genital arousal to a
nonhuman sexual stimulus, but did not report being sexually
aroused by this stimulus. In contrast, men showed neither
genital nor subjective arousal to the nonhuman sexual
stimulus; nonspecific sexual features were not sufficient to
elicit genital arousal in the male sample. Women also
showed a nonspecific pattern of genital arousal to human
sexual stimuli, replicating the effects reported by Laan et al.
(1996) and Chivers et al. (2004) in a different sample of
women. Conversely, men’s genital arousal was category-
specific, with the highest level of response to the female–
female and female–male stimuli. These results suggest there
Although women showed significantly greater arousal to
magnitude of this response was lower than that to human
sexual stimuli. This difference might be attributable to the
amount of sexual activity depicted in the nonhuman sexual
stimulus materials compared with the human stimulus
materials. Sexual activity in the nonhuman stimulus was
presented in short episodes (each intromission lasted for
approximately 10 s) compiled to create a 120 s stimulus,
whereas the human stimuli depicted sustained sexual activity
for the entire 120 s. If the nonhuman stimulus depicted
sustained and intense sexual activity, which is uncommonfor
nonhuman primates, it is possible that women’s genital
responses to the nonhuman stimulus would have been
comparable to their responses to human stimuli. It is also
possible that gender-nonspecific human features are neces-
sary for women to achieve higher levels of genital arousal.
These data provide support for Laan and Everaerd’s
(1995) idea that women’s genital vasocongestion is an
automatic response to any ‘‘sexual’’ features: Women
experienced a rapid genital response to a stimulus that
displayed frank sexual activity but neither corresponded
with their interests nor evoked concomitant subjective
sexual arousal. Another example of this automatic response
is the lack of habituation in women’s genital arousal to
repeated presentations of a sexual stimulus (Laan and
Everaerd, 1995b). Men, in contrast, do show habituation of
genital arousal (O’Donahue and Geer, 1985).
There is other evidence that genital arousal is partially
independent of psychological processes in women. The
correlation between genital and subjective arousal is
consistently smaller in women than in men (Chivers et al.,
in preparation; Laan and Everaerd, 1995). Research examin-
ing the temporal relationship between genital and subjective
arousal in women shows that maximum genital arousal
precedes maximum subjective arousal by about one minute
(Polan et al., 2003). Studies examining the effects of
pharmaceutical agents on women’s sexual arousal have
in genital vasocongestion do not translate into significant
increases insubjective arousal(Laanetal.,2002;Mestonand
studies indicate that a fundamentally different process
underlies the sexual arousal systems of women and men.
Why would women’s genital arousal system be auto-
matically responsive to depictions of nonhuman sexual
activity? Visual sexual features common to nonhuman and
human sexual activity may be among a class of biologically
prepared stimuli (cf., Seligman, 1971) that automatically
initiate neural sexual responding (Geer et al., 1992). Genital
vasocongestion prepares the vagina for sexual activity via
lubrication, facilitating penile penetration and reducing the
likelihood of genital injury or infection of the reproductive
tract. Reflexive genital vasocongestion in response to
nonspecific visual sexual features may be an evolved
protective mechanism (Laan, 1994). Having reflexive and
low-cost vasocongestion to nonspecific sexual features may
have improved fitness in ancestral environments by reducing
intercourse. The evolutionary psychology perspective from
which we derive these speculations may provide a helpful
framework from which to develop and test these and other
hypotheses about sex differences in sexual psychology (e.g.,
That women demonstrated a genital response to a
nonhuman sexual stimulus does not suggest women have a
latent preference for sex with animals. Although genital
response to preferred sexual stimuli has long been equated
with a sexual preference in sex research, this assumption is
most likely valid for males only. There is, to date, no
convincing evidence that women’s genital response to
preferred and nonpreferred sexual stimuli corresponds to
their stated sexual preferences (Chivers et al., 2004). In fact,
the current study and ourprevious researchsuggestthat little
can be inferred about a woman’s sexual preference on the
basis of her genital responses alone.
Our results further our understanding of women’s sexual
arousal processes in three ways. First, our data suggest that
women’s genital arousal is elicited by nonspecific sexual
features. That is, sexual activity is a sufficient sexual feature
for females. Second, they suggest that women’s genital
arousal is not dependent on preferred sexual partner features.
Third, they indicate that significant increases in genital
as other researchers have also noted (Laan and Everaerd,
1995). Our study, however, cannot specify what comprises a
nonspecific sexual feature, other than to rule out features
M.L. Chivers, J.M. Bailey/Biological Psychology 70 (2005) 115–120 119
related to preferred sexual targets. By comparing the Download full-text
nonhuman to human stimuli, we can speculate that
nonspecific features may concern movements (penile thrust-
ing), postures (face-to-face intercourse) or cross-species
physical features (an erect penis) associated with sexual
and other stimulus features would address this question.
This research was based, in part, on the first author’s
doctoral dissertation, presented to the Department of
Psychology, Northwestern University. A National Institute
of Child and Human Development Grant R03-HD41770-01
and a grant from the American Psychological Foundation
funded this research. We thank R. Blanchard, J. M. Cantor,
M. Lalumie `re, and M. Seto for helpful comments, G. Reiger
for his assistance with video editing and data collection, and
we thank the study participants.
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