Ovulatory cycle effects on tip earnings by lap dancers:
economic evidence for human estrus?
Geoffrey Miller, Joshua M. Tybur, Brent D. Jordan
Department of Psychology, University of New Mexico, Albuquerque, NM 87131, USA
Initial receipt 16 April 2007; final revision received 26 June 2007
Abstract
To see whether estrus was really "lost" during human evolution (as researchers often claim), we examined ovulatory cycle effects on
tip earnings by professional lap dancers working in gentlemen's clubs. Eighteen dancers recorded their menstrual periods, work shifts, and
tip earnings for 60 days on a study web site. A mixed-model analysis of 296 work shifts (representing about 5300 lap dances) showed an
interaction between cycle phase and hormonal contraception use. Normally cycling participants earned about US$335 per 5-h shift during
estrus, US$260 per shift during the luteal phase, and US$185 per shift during menstruation. By contrast, participants using contraceptive
pills showed no estrous earnings peak. These results constitute the first direct economic evidence for the existence and importance of
estrus in contemporary human females, in a real-world work setting. These results have clear implications for human evolution, sexuality,
and economics.
 2007 Elsevier Inc. All rights reserved.
Keywords: Estrus; Female sexuality; Behavioral economics; Sexual service industries; Hormonal contraception
1. Introduction
Estrus is a phase of increased female sexual receptivity,
proceptivity, selectivity, and attractiveness. It is common
across mammalian species (Lange, Hartel, & Meyer, 2002;
Lombardi, 1998), including primates (Dixson, 1998; Nelson,
2000), and seems functionally designed to obtain sires of
superior genetic quality (Gangestad, Thornhill, & GarverApgar,
2005; Thornhill, 2006). However, the conventional
wisdom holds that human female estrus became uniquely
"lost" or "hidden" over evolutionary time (e.g., Burt, 1992),
perhaps to promote male provisioning and paternal care in
long-term pair-bonded relationships (Strassmann, 1981;
Turke, 1984). Contrary to this "hidden-estrus" view, recent
laboratory-based studies show that women near the most
fertile point of their cycle (just before ovulation) are more
attractive to males, as manifest through more attractive
body scent (Havlíček, Dvořáková, Bartoš, & Flegr, 2006;
Kuukasiarvi et al., 2004; Singh & Bronstad, 2001), greater
facial attractiveness (Roberts et al., 2004), increased softtissue
body symmetry (Manning, Scutt, Whitehouse, Leinster,
& Walton, 1996), decreased waist-to-hip ratio (Kirchengast &
Gartner, 2002), and higher verbal creativity and fluency (Krug,
Moelle, & Fehm, 1999; Symonds, Gallagher, Thompson, &
Young, 2004).
While such laboratory-based findings are theoretically
important, only four studies have, to our knowledge,
investigated the real-world attractiveness effects of human
estrus outside the laboratory.
Haselton, Mortezaie, Pillsworth, Bleske-Recheck, and
Frederick (2007) photographed 30 young women--all in
steady relationships and not using the pill--twice each,
wearing their self-chosen clothing, once during estrus (as
confirmed by hormonal assay) and once during a lowerfertility
(luteal) cycle phase. Then, 42 mixed-sex raters made
a forced-choice judgment ("In which photo is the person
trying to look more attractive?") between the two photos of
each woman (with faces obscured, leaving only body and
clothing cues). They chose the woman when she was in
estrus about 60% of the time--modestly but significantly
above chance. This result confirmed that both male and
Evolution and Human Behavior 28 (2007) 375­381

Brent D. Jordan's contribution to this project was supported by a
McNair/ROP Scholars Program Fellowship.
 Corresponding author.
E-mail address: gfmiller@unm.edu (G. Miller).
1090-5138/$ ­ see front matter  2007 Elsevier Inc. All rights reserved.
doi:10.1016/j.evolhumbehav.2007.06.002
female observers are perceptually sensitive to women's
choice of more conspicuous and fashionable clothes during
estrus. (A related real-world study by Grammer, Renninger,
& Fisher, 2004, found that mated women attending Vienna
discotheques without their partners tended to dress more
provocatively if they had higher estradiol levels; however,
that study did not directly assess cycle phase.)
Gangestad, Thornhill, and Garver (2002) found convergent
evidence that men's real-world behavior is sensitive
to their female partners' estrous cues. (Note that estrous is
the adjectival form of the noun estrus.) Among 31 mated
women not using the pill, their sexual partners were reported
as using more mate-guarding behaviors, including higher
proprietariness, attentiveness, and vigilance (e.g., calling the
women's cell phones at random times to see what they were
doing) when the women were in estrus. This effect was
especially strong when the relationship was not yet steady or
exclusive--consistent with the theory that increased mate
guarding of estrous women by male partners is functionally
designed to deter extra-pair copulation.
Haselton and Gangestad (2006) replicated this mateguarding
result among 25 normally cycling women, using a
stronger repeated-measures design based on daily reports.
They further found that estrous mate guarding by male
partners was mediated by both men's sexual attractiveness
(less-attractive men mate guarded especially more during
estrus) and women's own attractiveness (less-attractive
women were especially mate guarded during estrus, whereas
more-attractive women were mate guarded all the time). In a
subsequent study, Pillsworth and Haselton (2006) found
similar results on measures of male attentiveness and
expressed love.
These four studies provide some evidence that men are
sensitive to estrous cues in real-world situations. However,
the photo-choice results were rather modest in strength
(Haselton et al., 2007), and the mate-guarding results were
rather indirect measures of estrous female attractiveness
(Gangestad et al., 2002; Haselton & Gangestad, 2006;
Pillsworth & Haselton, 2006). In this article, we build upon
such research by presenting the first real-world economic
evidence of male sensitivity to cyclic changes in female
attractiveness. Specifically, we measured the tips earned by
professional lap dancers in gentlemen's clubs over a 2-month
span. These dancers are highly motivated to maximize tip
earnings during every shift they work, which they do by
appearing more sexually attractive than the other 5 to 30 rival
dancers working the same shift and by doing the "emotional
labor" of "counterfeiting intimacy" with male club patrons
(Barton, 2006; Beasley, 2003; Deshotels & Forsyth, 2006;
Pasko, 2002). Hence, if the hidden-estrus view is right, there
should be no reliable cues of fertility available or expressed
to patrons and no ovulatory cycle effects on dancer tip
earnings. On the other hand, if women retain perceivable
cues of estrus--if they become more attractive to male club
patrons at midcycle, just before ovulation--then they might
be in higher demand and earn higher tips.
2. Background
Because academics may be unfamiliar with the gentlemen's
club subculture, some background may be helpful
to understand why this is an ideal setting for investigating
real-world attractiveness effects of human female estrus. The
following information was gathered from interviews with
local club managers and from the sociological and feminist
literature on erotic dancing (Barton, 2006; Beasley, 2003;
Brewster, 2003; Deshotels & Forsyth, 2006; Enck & Preston,
1988; Forsyth & Deshotels, 1997; Hall, 1993; Hochschild,
1983; Lewis, 2006; Linz et al., 2000; Pasko, 2002; Ronai &
Ellis, 1989; Thompson, Harred, & Burks, 2003).
All participants in this study worked as lap dancers in
Albuquerque "gentlemen's clubs" circa November 2006
through January 2007. The clubs serve alcohol; they are
fairly dark, smoky, and loud (with a DJ playing rock, rap, or
pop music). Most club patrons are Anglo or Hispanic men
aged 20 to 60, ranging from semiskilled laborers to
professionals; they typically start the evening by getting a
stack of US$20 bills from the club's on-site ATM and having
a couple of drinks.
Dancers in these clubs perform topless but not bottomless;
law requires them to wear underwear, bikinis, or similar
garments to cover the pubis. Thus, menstruating dancers can
wear tampons (with strings clipped short or tucked up) and
change them often during heavy-flow days, without revealing
any visual signs of menstruation. Dancers typically wear
very little perfume, but they often have breast implants, dye
their head hair, trim their pubic hair, shave their legs and
underarms, and adopt a "stage name" different from their real
first name. They typically do regular aerobic and resistance
exercise to maintain a fit, lean body shape.
During work, each dancer performs one to three "stage
dances" on an elevated central stage about every 90 min to
advertise her presence, attractiveness, and availability for lap
dances. These result in only modest tip earnings (typically
US$1­5 tips from the men seated closest to the stage,
totaling only about 10% of her earnings). The rest of the
dancer's time is spent walking around the club asking men if
they want a "lap dance." A lap dance typically costs US$10
per 3-min song in the main club area or US$20 in the more
private VIP lounge. Dancers typically make about two thirds
of their income from the main club area and one third from
the VIP area; thus, average income from each lap dance is
about US$14. Lap dances require informal "tips" rather than
having explicit "prices" (to avoid police charges of illegal
"solicitation"), but the economic norms of tipping are
vigorously enforced by bouncers. Dancers thus maximize
their earnings by providing as many lap dances as possible
per shift.
In each lap dance, the male patron sits on a chair or couch,
fully clothed, with his hands at his sides; he is typically not
allowed to touch the dancer. The topless female dancer sits
on the man's lap, either facing away from him (to display her
buttocks, back, and hair) or facing him (either leaning back
376 G. Miller et al. / Evolution and Human Behavior 28 (2007) 375­381
to display her breasts, and to make conversation and eye
contact, or learning forward to whisper in his ear). Lap
dances typically entail intense rhythmic contact between the
female pelvis and the clothed male penis (Barton, 2006;
Beasley, 2003).
Thus, lap dances are the most intimate form of sex work
that is legal in most American cities--much more intimate
than the stage dances and table dances that characterized
such clubs until the late 1990s. However, most lap dance
clubs strongly discourage more intimate patron­dancer
contact, even in private VIP rooms, since clubs can be
closed swiftly if undercover police discover that prostitution
is occurring. Rather, the dancer's earnings are maximized by
inducing the man to agree to further lap dances after the first
3-min song is over (Deshotels & Forsyth, 2006; Pasko,
2002). Thus, a dancer can make up to US$400 per hour
performing for a rich, attentive patron in a VIP lounge. Club
patrons will often "sample" several different dancers with
one lap dance each before picking one for a more expensive
multisong bout of dancing. Thus, patrons can assess the
relative attractiveness of different women through intimate
verbal, visual, tactile, and olfactory interaction, and those
attractiveness judgments can directly influence women's tip
earnings, through the number of 3-min dances that patrons
request from each dancer. In these ways, estrous attractiveness
effects on lap-dancer earnings in gentlemen's clubs may
be stronger than in other kinds of psychology research that
use photo ratings (e.g., Haselton et al., 2007) or other kinds
of sex work (e.g., visual pornography, phone sex) that give
fewer fertility cues across fewer modalities.
3. Methods
Participants were recruited through indirect e-mails
(forwarded through local industry contacts), newspaper
advertisements, and flyers posted near clubs. To minimize
possible response biases through demand characteristics, we
mentioned the ovulatory cycle only in recruitment and
consent and we did not suggest that tip earnings would be
examined specifically as a function of days since menstrual
onset. We believe that this brief mention of possible cycle
effects on tip earnings in the consent form did not bias
participant responses in any particular direction.
To enter the study, each participant collected an experiment
packet (containing a consent form, 14-page questionnaire,
and instructions for using the online web site) from
a public location on the University of New Mexico (UNM)
campus. The questionnaire (with an attached subject ID
number) asked about age, ethnicity, work experience, sexual
experience and attitudes, menstrual cycle characteristics,
contraception use, physical characteristics, education, intelligence,
and personality. Signed consent forms and completed
questionnaires were returned to separate boxes at
UNM to maintain anonymity. Participants were also asked to
use their anonymous subject ID number to log in to the web
site every day for 60 days. Each day, they were to report their
mood, work hours, work location, and tip earnings in US
dollars and whether they had begun or ceased menstruation.
Participants were offered a payment of US$30 upon
completion of the study.
A total of 18 women gave analyzable data (signed consent
forms, completed questionnaires, and reported menstrual
cycle data over the 60-day period); 1 additional woman
completed the study but reported highly irregular menstrual
activity (e.g., three different menstrual periods beginning in
the same 10-day interval), rendering her unsuitable for
relevant analyses. Because recruitment was through forwarded
e-mails, advertisements, and flyers, it is hard to
estimate the proportion of women who responded.
4. Results
4.1. Participant demographics and traits
The 14-page questionnaire completed by each participant
gave extensive background information, only some of which
is relevant to this study. All participants were exclusively or
primarily heterosexual. All women reported regular cycles of
28­30 days; 7 were using the hormonal contraception pill,
and 11 were not (and had not within the previous 3 months);
none used any other form of hormonal contraception (e.g.,
the patch or the implant). On average, participants were 26.9
years old (S.D.=5) and had 6.4 years of experience as exotic
dancers (S.D.=2.1). Their demographics seem representative
of exotic dancers studied in previous sociological research
(e.g., Barton, 2006; Deshotels & Forsyth, 2006; Forsyth &
Deshotels, 1996; Pasko, 2002).
4.2. Daily online reports and fertility estimates
We asked participants to log in to our study web site daily
to report whether they had started or stopped menstruating
on that day and to report their tip earnings and other details of
any shift they worked that day. Each participant's menstruation
data were plotted on a calendar, and we recorded how
many days into the cycle each participant was for each shift
reported. The online data revealed that all 18 participants
showed quite regular cycles ranging from 28 to 29 days in
cycle length--a good match to their self-reported cycle
lengths of 28­30 days.
Actuarial data (e.g., Wilcox, Dunson, Weinberg, Trussell,
& Baird, 2001) suggest that fertility is high around Days 9­
15 of the cycle and is low around Days 1­8 (early follicular
days including menstruation) and Days 16­28 (days in the
luteal phase). We divided nonestrous parts of the cycle into
menstrual and luteal phases because we expected that
menstrual side effects (e.g., fatigue, bloating, muscle pains,
irritability) might reduce women's subjective well-being and
tip earnings and we wanted to be able to distinguish an
estrous increase in tips from a menstrual decrease, relative to
the luteal phase. Also, because fertility estimation is
imperfect and fertility may be high a few days before or
377G. Miller et al. / Evolution and Human Behavior 28 (2007) 375­381
after the typical fertile window, we used methods similar to
those of Haselton and Gangestad (2006) by conservatively
estimating cycle phase. Thus, the cycle was broken up into
three phases: menstrual [Days 1­5 of the cycle (Days 6­8
were dropped because participants could have been fertile
and were likely not menstruating)], fertile (Days 9­15 of the
cycle), and luteal [Days 18­28 of the cycle (Days 16 and 17
were dropped because participants could have been fertile)].
We then calculated an average tip-earning level for each
participant in her menstrual, fertile, and luteal phases, based
on all available online data in each category.
Apart from the ambiguous-fertility days dropped from our
analysis, participants reported tip earnings in a total of 296
online entries, averaging 16.4 entries each (S.D.=5.2,
range=9­29), out of the 60 days requested. Participants
only logged on to the web site on about half of the days when
they worked (27% of the 60 requested days).
The average work shift lasted 5.2 h (S.D.=1.7, range=0­
12), usually starting between 5 and 10 p.m. and ending
between midnight and 4 a.m. Participants reported mean
earnings of US$248.73 per shift (S.D.=US$125.30). With
lap dances yielding an average of about US$14, this mean
earnings level of about US$250 reflects about 18 dances per
shift. Shift length was unrelated to shift earnings (r=-.03,
p=.63); thus, "earnings per shift," rather than "earnings per
hour," was used in subsequent tip analyses. Additionally,
number of hours worked per shift did not differ across cycle
phase [F(2, 249)=1.19, p=.306; menstrual mean=5.44, fertile
mean=5.26, luteal mean=5.04]. Fig. 1 shows average tip
earnings for Days 1­28 in the cycle for normally cycling
women versus pill-using women.
4.3. Effects of ovulatory cycle and contraception on
tip earnings
Our design involved multiple observations (i.e., tips per
shift) for dancers, who were nested within contraception use
and crossed with cycle phase; hence, we analyzed effects of
cycle phase and contraception use on tip earnings using
multilevel modeling (hierarchical linear modeling). Multilevel
modeling is most appropriate in this context because it
allows interpretable tests of cycle phase and contraception
use despite multiple observations and the nonindependence
of tip earnings and despite the differential number of
observations between participants. Analyses were thus
performed using SPSS 14.0 Mixed Models. Restricted
maximum likelihood criteria were employed. Shifts (nested
within individuals) were the Level 1 units of analysis, and
participants were Level 2 units of analysis. Accordingly,
cycle phase was a Level 1 factor and contraception use was a
Level 2 factor. We report significance levels using both
traditional p values for null-hypothesis testing and prep
values (Killeen, 2005).
Main effects of cycle phase [F(2, 236)=27.46, pb.001,
prep=1.00] and contraception use [F(1, 17)=6.76, pb.05,
prep=.929] were moderated by an interaction between
cycle phase and pill use [F(2, 236)=5.32, pb.01, prep=.964].
Figs. 1 and 2 show these key results by plotting average
tip earnings within each running 3-day period (Fig. 1) or
each cycle phase (menstrual, fertile, or luteal; Fig. 2) for
normally cycling participants versus pill-using participants.
We conducted two planned contrasts of the interaction to
investigate how cycle phase moderated the difference in tip
earnings between normally cycling participants and pill
users. The first contrast investigated how pill use moderated
differences in tip earnings between the menstrual and luteal
phases. We predicted that pill-using and normally cycling
participants would demonstrate a similar difference
between these two phases. The second contrast investigated
how pill use moderated differences in tip earnings between
the fertile phase and the other two phases. We predicted
that normally cycling participants would demonstrate a
larger increase in the fertile phase relative to the other
phases than pill-using participants.
Consistent with our predictions, the first contrast
indicated that pill use does not moderate the difference
between tips earned in the menstrual and luteal phases [F(1,
Fig. 1. Effects of ovulatory cycle (Days 1­28) on average tip earnings per
shift, for normally cycling women versus women using hormonal contraception
(pill users); each data point represents a 3-day average of the
indicated day, the previous day, and the following day.
Fig. 2. Effects of ovulatory cycle phase (menstrual phase, fertile estrous
phase, or luteal phase) on tip earnings per shift, for normally cycling women
versus women using hormonal contraception (pill users). Error bars
represent 95% confidence intervals.
378 G. Miller et al. / Evolution and Human Behavior 28 (2007) 375­381
234)=0.012, p=.911], with normally cycling participants
demonstrating a similar increase in tip earnings from
menstrual to luteal phases (+70.45; 95% confidence limits
from +25.68 to +115.16) as compared to pill-using
participants (+66.60; 95% confidence limits from +20.72
to +112.26). Also consistent with our predictions, the second
contrast indicated that pill use does moderate the difference
between tips earned in the fertile phase and the other two
phases [F(1, 238)=10.52, pb.01, prep=.983], with normally
cycling participants demonstrating a greater increase in tip
earnings at the fertile phase relative to the other two phases
(+135.63; 95% confidence limits from +111.20 to +160.07)
than pill users (+47.62; 95% confidence limits from +11.50
to +83.74).
Although the sample size of individuals was small
(N=18), the sample size of work shifts was much larger
(N=296), and these work shifts reflected a sample of about
5300 lap dances (about 18 per work shift).
5. Discussion
We found strong ovulatory cycle effects on tip earnings,
moderated by whether the participants were normally
cycling. All women made less money during their menstrual
periods, whether they were on the pill or not. However, the
normally cycling women made much more money during
estrus (about US$354 per shift)--about US$90 more than
during the luteal phase and about US$170 more than during
the menstrual phase. Estrous women made about US$70 per
hour, luteal women made about US$50 per hour, and
menstruating women made about US$35 per hour. By
contrast, the pill users had no midcycle peak in tip earnings.
As in other previous research, the pill eliminates peakfertility
effects on the female body and behavior by putting
the body in a state of hormonal pseudopregnancy (e.g.,
Gangestad, Simpson, Cousins, Garver-Apgar, & Christensen,
2004; Gangestad et al., 2005; Macrae, Alnwick, Milne,
& Schloerscheidt, 2002). This also results in pill users
making only US$193 per shift compared to normally cycling
women making US$276 per shift--a loss of more than US
$80 per shift.
This is the first direct economic evidence for the existence
of estrus in contemporary human females. Under the
"revealed preference" doctrine in behavioral decision theory
(Camerer, 2003; Hensher, Louviere, & Swait, 1999), real
consumer spending patterns reveal human preferences more
reliably than verbally stated judgments do, especially for
socially stigmatized products such as pornography or sex
work (Salmon & Symons, 2001). When women and men
interact intimately over the course of several minutes through
conversation and body contact, women apparently either
"signal" or "leak" cues of their fertility status, and these cues
influence spending patterns by male consumers. These
results argue against the view that human estrus evolved to
be lost or hidden from males (e.g., Strassmann, 1981; Turke,
1984). Indeed, the standard argument for women's
"extended sexuality" (sexual receptivity outside the fertile
estrus phase) is that it evolved to help women extract material
resources from males (Gangestad et al., 2005; Thornhill,
2006), and some evidence suggests that women outside
estrus place a higher value on male wealth relative to other
male traits (Haselton & Miller, 2006). By this reasoning,
nonestrous women in their extended sexuality phase should
be better adapted to maximize tip earnings through displays
of (nonfertile) sexual receptivity such as lap dancing. The fact
that tip earnings peak during estrus suggests that men can
detect female fertility more accurately than the "concealed
ovulation" model suggested--but not so accurately that tips
during the luteal and menstrual phases drop to zero (as they
might if men found women generally unattractive during
low-fertility parts of the cycle). As in so many coevolutionary
arms races between the sexes (Arnqvist & Rowe, 2005), this
outcome is not a clear victory for either sex.
These cycle effects are notable because in previous
research on gentlemen's clubs (e.g., Barton, 2006; Deshotels
& Forsyth, 2006; Pasko, 2002), summarizing thousands of
hours of interviews, dancers are never reported as noticing
cycle effects on tip earnings (thus making it less likely that
participant expectations or demand characteristics can
explain the results). Yet, dancers have rich opportunities to
learn how to maximize tip earnings. Learning optimal
performance in any economic game requires immediate cash
feedback across many iterations of the game (Camerer,
2003), and this is exactly what lap dancers get, every few
minutes, in every shift, throughout the average of 6.4 years
of dance experience. For this reason, we suspect that cyclic
shifts in women's attractiveness are driving our tip earnings
results--rather than the well-documented shifts in sexual
receptivity, proceptivity, or selectivity for good genes (e.g.,
Feinberg et al., 2006; Gangestad et al., 2002, 2004, 2005;
Haselton & Miller, 2006; Thornhill, 2006). Although an
estrous increase in sexual receptivity and proceptivity toward
clients who exhibit good-gene cues may bias dancers to
approach certain men, it is unclear how this bias would lead
to greater tip earnings during estrus. Indeed, it seems that the
optimal strategy for obtaining tips is to focus on men who are
profligate, drunk, and gullible rather than those who are
intelligent, handsome, and discerning.
This study has several limitations. The sample size of
participants is small (N=18), although we gathered many
data points per participant, which allowed us to use a
statistically powerful repeated-measures design (including
296 work shifts reflecting about 5300 lap dances). Although
the modest number of participants does not increase type I
errors (i.e., false positives) in our statistical tests, it may
reduce the generalizability of the results across populations
--although it is unclear why different populations of
sexually mature, normally cycling, human females would
show different ovulatory cycle effects on tip earnings, if they
work in the same industry.
Another limitation is that our key measures (tip
earnings, menstrual cycle phases, hormonal contraception
379G. Miller et al. / Evolution and Human Behavior 28 (2007) 375­381
use) were self-reported, to maximize participant anonymity
and confidentiality in this stigmatized and suspicious
population (see Thompson et al., 2003). Future studies
could use larger samples, could identify ovulation more
precisely using the luteinizing hormone surge as measured
in urine samples, and could investigate whether these
effects hold in other less-stigmatized types of sex work
and service and entertainment industries, in other settings
and cultures.
A final limitation is that our study did not identify the
precise proximal mechanisms that influence tip earnings.
These might include the previously documented shifts in
body scent, facial attractiveness, soft-tissue body symmetry,
waist-to-hip ratio, and verbal creativity and fluency--or
they might include shifts in other phenotypic cues that have
not yet been studied. We can, however, exclude some
possible mediators based on previous exotic dancer
research. Tip earnings are unlikely to be influenced by
cycle shifts in stage-dance moves, clothing, or initial
conversational content because these cues just do not vary
much for professional dancers (Barton, 2006; Beasley,
2003). The tip earnings pattern in Fig. 1 is similar to the
pattern of estradiol levels across the cycle (with a main
estrous peak and a secondary midluteal peak); hence, it is
plausible that estradiol levels might mediate the tipearning
effects.
Perhaps, most importantly, from an evolutionary viewpoint,
further research could clarify whether women have
evolved special adaptations to signal estrus through such
cues--or whether the cues are "leaking" to sexually
discriminating men as unselected side effects of cycle
physiology. Distinguishing between estrous "signals" and
"leaked cues" may be difficult in practice because estrous
females (seeking extra-pair copulations with good-gene
males) and extra-pair males (offering good genes) may
have shared interests in female fertility signals being
"conspiratorial whispers" that are accurate but inconspicuous
(Pagel, 1994). In serially monogamous species such as
ours, women's estrous signals may have evolved an extra
degree of plausible deniability and tactical flexibility to
maximize women's ability to attract high-quality extra-pair
partners just before ovulation, while minimizing the
primary partner's mate guarding and sexual jealousy. For
these reasons, we suspect that human estrous cues are likely
to be very flexible and stealthy--subtle behavioral signals
that fly below the radar of conscious intention or
perception, adaptively hugging the cost­benefit contours
of opportunistic infidelity.
Acknowledgments
We thank Rosalind Arden, Bernadette Barton, Juliana
Beasley, Steve Gangestad, Vlad Griskevicius, Martie
Haselton, Catherine Salmon, Ursina Teuscher, Randy
Thornhill, Rhiannon West, and several anonymous reviewers
for helpful feedback on this article.
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