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Sperm competition theory can be used to generate the hypothesis that men alter the quality of their ejaculates as a function of sperm competition risk. Using a repeated-measures experimental design, we investigated whether men produce a higher-quality ejaculate when primed with cues to sperm competition (i.e., imagined partner infidelity), relative to a control prime. Men (n = 45) submitted two masturbatory ejaculates—one ejaculate sample for each condition (i.e., sperm competition and control conditions). Ejaculates were assessed on 17 clinical parameters. The results did not support the hypothesis: Men did not produce higher-quality ejaculates in the sperm competition condition relative to the control condition. Despite the null results of the current research, there is evidence for psychological and physiological adaptations to sperm competition in humans. We discuss methodological limitations that may have produced the null results, and present methodological suggestions for research on human sperm competition.
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Original Article
Do Men Produce Higher Quality
Ejaculates When Primed With
Thoughts of Partner Infidelity?
Michael N. Pham
1
, Nicole Barbaro
1
, Andrew M. Holub
1
, Christopher J. Holden
1
,
Justin K. Mogilski
1
, Guilherme S. Lopes
1
, Sylis C. A. Nicolas
1
, Yael Sela
1
,
Todd K. Shackelford
1
, Virgil Zeigler-Hill
1
, and Lisa L. M. Welling
1
Abstract
Sperm competition theory can be used to generate the hypothesis that men alter the quality of their ejaculates as a function of
sperm competition risk. Using a repeated measures experimental design, we investigated whether men produce a higher quality
ejaculate when primed with cues to sperm competition (i.e., imagined partner infidelity) relative to a control prime. Men (n¼45)
submitted two masturbatory ejaculates—one ejaculate sample for each condition (i.e., sperm competition and control condi-
tions). Ejaculates were assessed on 17 clinical parameters. The results did not support the hypothesis: Men did not produce higher
quality ejaculates in the sperm competition condition relative to the control condition. Despite the null results of the current
research, there is evidence for psychological and physiological adaptations to sperm competition in humans. We discuss meth-
odological limitations that may have produced the null results and present methodological suggestions for research on human
sperm competition.
Keywords
sperm competition, humans, ejaculate quality, evolutionary psychology
Date received: July 12, 2017; Accepted: January 14, 2018
Sperm competition occurs when a female copulates with two or
more males within a sufficiently brief time period, resulting in
sperm of the different males competing to fertilize ova (Parker,
1970). Among humans, a common context for sperm compe-
tition is female infidelity (Shackelford, 2003; Smith, 1984). A
paternally investing man whose regular partner pursues extra-
pair copulations is at risk of cuckoldry (i.e., unwitting invest-
ment of resources into offspring to whom he is not genetically
related). The costs of cuckoldry may have driven the evolution
of male sperm competition tactics—strategic adjustments in
psychology, behavior, and physiology that increase the like-
lihood of sperm competition success. Because men have finite
resources for survival and reproduction (e.g., sperm produc-
tion), men judiciously deploy sperm competition tactics: Men
attend to specific sperm competition cues and adjust their
sperm competition tactics accordingly (Baker & Bellis, 1993;
Goetz et al., 2005; Shackelford, 2003).
A widely documented sperm competition tactic across taxa
is ejaculate adjustment (Smith, 1984). Males of various species
alter the quality of their ejaculates across several parameters,
including number of sperm and percentage of motile sperm, as
a function of sperm competition risk (Simmons & Fitzpatrick,
2012). Males at greater sperm competition risk produce larger
volume ejaculates to increase the probability that their sperm—
and not the sperm of rival males—fertilize ova (Wedell, Gage,
& Parker, 2002). Among many avian species, for example,
males at greater sperm competition risk produce ejaculates with
a greater number of sperm at the next copulation (Nicholls,
Burke, & Birkhead, 2001; Pizzari, Cornwallis, Løvlie, Jakobs-
son, & Birkhead, 2003).
1
Oakland University, Rochester, MI, USA
Corresponding Author:
Todd K. Shackelford, Department of Psychology, Oakland University,
654 Pioneer Drive, Rochester, MI 48309, USA.
Email: shackelf@oakland.edu.
Evolutionary Psychology
January-March 2018: 1–7
ªThe Author(s) 2018
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Cuckoldry is likely to have been a recurrent adaptive prob-
lem for ancestral men over human evolutionary history (Baker
& Shackelford, 2017). Estimates indicate approximately
3.1%of children are genetically unrelated to their social
father (Voracek, Haubner, & Fisher, 2008), with estimates
as high as 29.8%for men with low paternity confidence
(Anderson, 2006). Given the substantial costs imposed on
men as a result of cuckoldry, it is reasonable to suggest that
men may have evolved solutions to address problems of
sperm competition including cognitive, behavioral, and phy-
siological adaptations responsive to sperm competition risk
(see Baker & Bellis, 1993; Kilgallon & Simmons, 2005; Pham
& Shackelford, 2014).
Psychological and behavioral adaptations to sperm compe-
tition in humans include adjustments of mate retention beha-
viors and copulatory behaviors (Pham & Shackelford, 2014;
Shackelford & Goetz, 2007). More physically attractive
females present greater sperm competition risk than less attrac-
tive females, and therefore, men partnered to women who pres-
ent a greater risk of sperm competition (i.e., they are more
attractive) copulate more frequently with them (Pham et al.,
2014) and perform more semen displacing behaviors during
copulation (Goetz et al., 2005). Men partnered to women who
spend more time around potential male rivals also report
greater interest in copulating with their partner (Pham &
Shackelford, 2013). These findings suggest that men are atten-
tive to cues of sperm competition and show corresponding
cognitive and behavioral changes.
Additional research with humans suggests that men can
adjust their ejaculate as a function of sperm competition risk
(Baker & Bellis, 1995). Men who spend a greater proportion of
time apart from their regular partner since the couple’s last
copulation (i.e., indexing greater sperm competition risk) eja-
culate a greater number of sperm at the couple’s next copula-
tion (Baker & Bellis, 1993). Men produce masturbatory
ejaculates containing a greater percentage of motile sperm
when viewing pornography depicting two men and one woman
(i.e., a cue to sperm competition) than when viewing porno-
graphy depicting three women (i.e., absence of sperm compe-
tition; Kilgallon & Simmons, 2005). Men viewing images of
more attractive women produce higher quality masturbatory
ejaculates than when viewing images of less attractive women
(Leivers, Rhodes, & Simmons, 2014). Joseph, Sharma, Agar-
wal, and Sirot (2015) found that men produce lower quality
masturbatory ejaculates when viewing images of a woman to
whom they have previously masturbated than when viewing
images of a novel woman. These findings from Joseph et al.
are consistent with the “topping off” model proposed by Baker
and Bellis (1993), which posits that men adjust their ejaculates
to maintain an optimal population of viable sperm in their
partner’s reproductive tract across time. Men therefore appear
capable of adjusting the quality of their ejaculates in response
to sperm competition risk.
Results from Joseph et al. (2015), Kilgallon and Simmons
(2005), and Leivers, Rhodes, and Simmons (2014) demonstrate
the effect of visual stimuli on human ejaculate adjustment.
Although visual stimuli often elicit the most intense sexual
responses for men (e.g., Julien & Over, 1988; Leivers et al.,
2014; Rupp & Wallen, 2008), written depictions of sexual
stimuli have been shown to be psychologically and behavio-
rally arousing for both men and women (Schmidt, Sigusch, &
Scha¨fer, 1973) and may also elicit genital arousal in men
(Julien & Over, 1988). Contemporary research by Starratt,
McKibbin, and Shackelford (2013) demonstrates the potential
utility of written stimuli for assessing sperm competition out-
comes. Starratt et al. found that men experimentally primed
with thoughts of their long-term partner’s infidelity report
greater interest in copulating with their partner—a sperm com-
petition tactic that reflects men’s motivation to urgently place
their sperm into competition with rival sperm that may be in
their partner’s reproductive tract (Pham & Shackelford, 2013;
Shackelford, Goetz, McKibbin, & Starratt, 2007; Shackelford
et al., 2002). Given that previous research has demonstrated
experimental effects of infidelity priming manipulations
(Starratt, McKibbin, & Shackelford, 2013), it is reasonable to
expect that when primed with thoughts of partner infidelity
(and possible cuckoldry) men may adjust their ejaculates to
increase their chances of success in sperm competition with
rival sperm. We therefore hypothesized that men will produce
a higher quality ejaculate when primed with thoughts of partner
infidelity than when primed with a control stimulus.
Method
Participants
Participants were recruited via advertisements posted on bulle-
tin boards on the campus of a Midwestern University in the
United States and throughout the surrounding community. The
advertisements included contact information (i.e., laboratory
phone number and e-mail address) and participation inclusion
criteria. Men were eligible to participate if they (1) had not had
a vasectomy, (2) had never sought treatment for infertility, (3)
were aged 18–35 years, and (4) were currently in a committed,
sexually active relationship lasting at least 3 months with a
woman aged 18–35 years.
Potential participants contacted the laboratory to schedule
three in-person sessions—one intake session, followed by two
masturbatory sessions. The original data set included responses
from 66 men, with ages varying from 18 to 34 years
(M¼22.77; standard deviation [SD]¼3.83), mostly not mar-
ried (90.9%), and with relationship length varying from 6 to
123 months (M¼33.15; SD ¼26.62; Mdn ¼27 months). Only
data from men who provided ejaculate samples for both con-
ditions were included in analyses. The final sample included
45 men, with ages ranging from 18 to 33 years (M¼23.3;
SD ¼3.6), mostly unmarried (86.7%), and with relationship
length ranging from 6 to 123 months (M¼35.5; SD ¼26.8; Mdn
¼27 months). Given the moderate effect sizes found in previous
related research (Joseph et al., 2015; Kilgallon & Simmons, 2005;
Leivers et al., 2014), our design and sample size were adequate
to detect a moderate effect.
2Evolutionary Psychology
Materials and Procedure
All procedures were approved by the institutional review board
of the university where data were collected. Data collection
occurred between April 2013 and May 2016. Participants
arrived to the laboratory at a scheduled time to complete Ses-
sion 1 (intake session) and were escorted to a private room by a
researcher. Participants were given a written consent form and
were shown a video describing the procedures of the study.
Consenting participants were randomly assigned to receive
either the experimental condition or the control condition first.
The researcher then provided the participant with detailed
instructions and the necessary materials to produce and trans-
port masturbatory ejaculates.
Participants received a sealed envelope containing written
scenarios unique to their assigned condition for the next mas-
turbatory session. Participants were instructed not to open the
envelope until immediately before they began to masturbate. In
the experimental, sperm competition (hereafter “infidelity”)
condition, participants were instructed to masturbate while
imagining the following scenario:
Imagine thatyour romanticpartner confessed to you earlier today
that she cheated on you 2 days ago by having sex with a man that
she recently met. She assured you that it was only a one-night
stand and that it will not happen again.Despite being upset about
her infidelity, you decide to give her another chance. Now, you
and your romantic partner are going to have sex for the first time
since she admitted that she cheated on you. Focus on what you
think that first sexual experience would be like after her confes-
sion. Your task is to think only about this sexual experience
while you masturbate. Do not allow other thoughts or fantasies
to distract you during your masturbation. Focus only on what it
would be like to have sex with your romantic partner after learn-
ing that she had recently cheated on you.
In the control, non-sperm-competition (hereafter
“gambling”) condition, participants were instructed to mastur-
bate while imagining the following scenario:
Imagine that your romantic partner confessed to you earlier
today that she lost a considerable amount of money when she
went gambling 2 days ago. She assured you that it was only a
one-time mistake and that it will not happen again. Despite
being upset about her gambling; you decide to give her another
chance. Now, you and your romantic partner are going to have
sex for the first time since she admitted that she recently lost a
lot of money gambling. Focus on what you think that first
sexual experience would be like after her confession. Your task
is to think only about this sexual experience while you mastur-
bate. Do not allow other thoughts or fantasies to distract you
during your masturbation. Focus only on what it would be like
to have sex with your romantic partner after learning that she
had recently lost a lot of money gambling.
The gambling condition depicts a scenario in which the
participant has been betrayed by their partner (similar to the
infidelity condition) but in a nonsexual context. Participants
received materials needed to produce and transport the mastur-
batory ejaculate: nonlatex, nonspermicidal condom, plastic
twist tie, screw top specimen container, biohazard Ziploc bag,
and aluminum foil. Following the guidelines provided by the
World Health Organization (2010), participants were instructed
to abstain from sexual activity for 48 hr (but not longer than
7 days) prior to each of their scheduled masturbatory sessions.
Participants were able to reschedule their subsequent sessions
or to request replacement materials (e.g., if the condom broke)
without penalty. Participants were compensated US$25 at the
conclusion of Session 1. We also secured several measures
unrelated to the current research during Session 1. A list of
these measures is available upon request.
Prior to arriving to the laboratory for Session 2 (the first
masturbatory session), participants chose a private location in
which to masturbate. Participants opened the sealed envelope
and read the written instructions and then masturbated while
thinking about the provided scenario. Participants were asked
to not use any materials that we did not provide (e.g., porno-
graphy, lubricant). Participants were also instructed to mas-
turbate without the help of their partner. Prior to ejaculation,
participants placed a condom on their penis and then ejacu-
lated into the condom. Participants then removed the condom,
ensuring no spillage. The opening of the condom was sealed
with a plastic twist tie, placed in the specimen container,
which was then placed in the biohazard Ziploc bag, and
wrapped in aluminum foil. Participants transported the pack-
aged ejaculate under their armpit to retain the warmth and
thus the viability of the ejaculate and arrived to the laboratory
within 1 hr of ejaculation.
Upon arriving to the laboratory, participants gave their
packaged ejaculate to a researcher who immediately analyzed
the semen (see below). The researcher then provided the parti-
cipant with another set of materials—identical to the materials
provided at Session 1—to produce and transport their ejaculate
for Session 3. Participants were given a sealed envelope that
included the instructions for the condition that they did not
receive for Session 2. Participants received US$25 at the con-
clusion of Session 2. The procedures for Session 3 (the second
masturbatory session) were identical to the procedures for Ses-
sion 2 except that participants did not receive additional mate-
rials. Participants received US$25 at the conclusion of Session
3. Time between Session 2 and Session 3 ranged from 2 to 28
days, with an average of time of 7 days between masturbatory
sessions.
Ejaculate quality was assessed using the Semen Quality
Analyzer (SQA-V; Medical Electronic Systems, Los Angeles,
California, USA)—a fully automated machine that analyzes
semen along 17 clinical parameters (see Table 1) using
electro-optical technology, signal conversion, and the applica-
tion of proprietary algorithms. Upon receipt of the participant’s
masturbatory ejaculate at Sessions 2 and 3, a researcher
pipetted the entire ejaculate from the condom and measured
the volume of the ejaculate (in milliliter) using the volumetric
markings on the pipette. The ejaculate was then pipetted into a
Pham et al. 3
sterile specimen container. A test strip (Medical Electronic
Systems) was dipped momentarily into the ejaculate to assess
pH and white blood cell count. The ejaculate was syringed into
a proprietary measurement capillary, which was inserted into a
chamber in the SQA-V (Medical Electronic Systems) for auto-
matic analysis. After completion of the automated semen anal-
ysis, all materials that directly contacted the ejaculate were
discarded in a biohazard waste container.
Results
The semen analysis generated 4 dichotomous parameters (e.g.,
pH, viscosity) and 17 continuous parameters (e.g., concentra-
tion of sperm with rapid progressive motility, percentage of
morphologically normal progressive sperm; see Table 1). Out-
liers (i.e., any data greater than 3.0 SDs from the mean of that
variable) were identified and removed using casewise deletion.
A Shapiro–Wilk’s test was conducted, which tests the null
hypothesis that a sample distribution was drawn from a nor-
mally distributed population (Shapiro & Wilk, 1965). Skew-
ness and kurtosis for each continuous parameter were assessed.
For small samples (n< 50; see Table 1), z-scores greater than
1.96 for either skewness or kurtosis suggest a nonnormal dis-
tribution (George & Mallery, 2010). The Shapiro–Wilk’s test
indicated a normal distribution for seven and four parameters
of the infidelity and gambling conditions, respectively. The
sample was nonnormally distributed for all other parameters,
and therefore nonparametric tests were conducted.
Wilcoxon signed-rank tests were conducted to identify dif-
ferences in each ejaculate parameter between the infidelity and
gambling conditions. The Wilcoxon (1945) signed-rank test
assesses rank differences in scores between samples and is
adequate for both normally and nonnormally distributed sam-
ples. Because each ejaculate parameter had a nonnormal dis-
tribution in at least one condition, we conducted Wilcoxon
signed-rank tests for tests of all ejaculate parameters. The
results indicated that the pairwise difference between the infi-
delity and gambling conditions was not significant for any of
the tested ejaculate parameters (see Table 1). We also con-
ducted Wilcoxon signed-rank tests controlling for number of
abstinence days. We considered the unstandardized residuals of
each parameter for each condition. The results indicated that
the pairwise difference between the infidelity and gambling
conditions was marginally significant (p< .10, gambling con-
dition showed higher scores) for quantity of morphologically
normal progressive sperm (Parameter 2) and sperm motility
index (Parameter 14).
Discussion
The current study examined whether men adjust their ejaculate
quality as a function of sperm competition risk. We hypothe-
sized that men would produce a higher quality ejaculate when
primed with thoughts of partner infidelity (infidelity condition)
than when primed with a control stimulus (gambling condi-
tion). The results did not indicate differences in ejaculate qual-
ity across any of the 17 clinical parameters tested, and
therefore, the study hypothesis was not supported. Men in the
current study did not produce a higher quality ejaculate when
Table 1. Definitions, Means and SDs, Ranks, Main Effects, and pValues for Parameters Between Conditions.
Parameter Definition
Infidelity Gambling
Wilcoxon Signed-Rank
Test (IG)
Ranks
ZpMSDnM SDnPos. Neg.
01 Concentration of progressive sperm that are shaped normally
a
11.0 14.0 42 10.3 9.7 39 17 20 0.69 0.488
02 Quantity of progressive sperm that are shaped normally 18.2 20.8 38 25.1 28.8 39 16 17 0.05 0.957
03 Percentage of progressive sperm that are shaped normally 29.9 13.6 42 29.3 9.1 38 18 18 0.13 0.900
04 Concentration of rapid progressive motile sperm (a)
b
6.7 10.5 40 7.5 10.1 41 15 16 0.34 0.732
05 Percentage of rapid progressive motile sperm (a) 14.7 18.7 42 15.0 16.7 41 23 14 0.83 0.407
06 Concentration of slow progressive motile sperm (b) 9.1 7.8 42 9.9 10.7 41 15 24 0.83 0.406
07 Percentage of slow progressive motile sperm (b) 15.6 10.9 42 16.8 10.9 41 23 16 0.59 0.553
08 Quantity of progressive motile sperm (a þb) 36.4 40.0 39 41.9 45.4 41 15 21 0.79 0.428
09 Percentage of nonprogressive motile sperm (c) 13.5 7.5 42 15.1 8.6 41 20 18 0.57 0.572
10 Concentration of motile sperm (a þbþc) 24.0 20.4 43 24.1 19.7 42 15 25 0.87 0.382
11 Quantity of motile sperm (a þbþc) 54.0 52.7 39 60.0 59.4 41 15 21 0.79 0.432
12 Percentage of motile sperm (a þbþc) 42.8 23.5 43 43.7 23.8 44 21 21 0.29 0.769
13 Percentage of not moving or dead sperm (d) 56.2 22.8 42 53.1 21.3 41 19 20 0.47 0.635
14 SMI
c
78.6 85.9 42 89.6 105.4 45 18 22 0.07 0.941
15 Sperm concentration within the sample 56.9 36.7 43 57.2 47.3 42 16 25 0.73 0.468
16 Velocity of the fastest moving cells (microns/sec) 8.6 4.2 41 9.1 3.3 38 15 16 0.28 0.782
17 Volume of the semen sample 2.6 1.4 45 2.4 1.1 44 17 24 1.17 0.240
Note:n¼number of cases per analysis. a ¼rapid progressive motility (forward), b ¼slow progressive motility (curved), c ¼nonprogressive motility (circles), and
d¼dead or not moving. SMI ¼sperm motility index; SD ¼standard deviation.
a
Millions/ml.
b
Motility is subdivided into four categories.
c
SMI is an index such that higher scores reflect higher overall quality of sperm motility.
4Evolutionary Psychology
primed with thoughts of partner infidelity than when primed
with a control stimulus.
There are several possible explanations for the null results in
the current study. Condom use during masturbation may have
prevented proper tests of the hypothesis. The World Health
Organization (2010) recommends that men ejaculate directly
into nonlatex, nonspermicidal condoms to collect copulatory
samples but directly into wide mouth nontoxic containers
(usually glass or plastic) to collect masturbatory samples.
Men in the current research were instructed to ejaculate with
the condom on their penis to collect masturbatory samples.
This method ensures less spillage compared to ejaculating
into a container. The use of condoms, however, may be less
sexually arousing to men due to decreased physical sensitiv-
ity. The null results therefore may be attributable to a “floor
effect” across both conditions in which men may have been
aroused just enough to ejaculate but not aroused enough to
reveal differences between conditions. Future research should
require men to ejaculate directly into a wide mouth container
to decrease the potential negative effects of condom use on
men’s sexual arousal.
The use of written stimuli in the current study presented
unique costs and benefits. Although it allowed for greater con-
trol of variables by limiting variance that could be introduced
by audiovisual stimuli, the use of written stimuli may also have
constrained ecological validity and may not have constituted a
salient manipulation. Using written stimuli assuaged some of
the ethical concerns with priming partner infidelity in a roman-
tic relationship. Because written stimuli have demonstrated the
capability of eliciting sexual arousal (e.g., Schmidt et al., 1973)
and because written erotica has been widely consumed
throughout human history (Black, 2009), we anticipated that
the written stimuli would be an effective manipulation that
would also not constitute a long-term threat to participants’
relationships. Men frequently report sexual arousal to fantasies
that have no basis in reality nor any possibility of coming true
(e.g., extradyadic fantasies; Hicks & Leitenberg, 2001), so the
present manipulation was an attempt to minimize relationship
disturbances from imagining fictitious partner infidelity, while
still activating psychological mechanisms sensitive to sperm
competition in men. However, men are the primary consumers
of visual pornography (Malamuth, 1996) and may require
visual stimuli (rather than written stimuli) to achieve signifi-
cant changes in arousal and consequent adjustment of ejaculate
quality. Prior research documenting ejaculate adjustment in
humans has used experimental manipulations of visual stimuli
(e.g., Kilgallon & Simmons, 2005; Leivers et al., 2014), rather
than written stimuli, as in the current study. Given research
suggesting sex differences in mental processing (e.g., Hamann,
Herman, Nolan, & Wallen, 2004) and attending to sexual and
erotic stimuli of varying modalities (e.g., Dekker, Everaerd, &
Verhelst, 1985; Lykins, Meana, & Straus, 2008), subsequent
studies may make use of visual or audiovisual stimuli to shed
further light on the present results. Future researchers are
encouraged to use visual stimuli (e.g., pornography) to
manipulate psychological mechanisms that may have evolved
to solve adaptive problems of sperm competition.
The stimuli may also have lacked efficacy due to the pre-
sentation environment. Having participants read the vignettes
and produce ejaculates outside of the laboratory introduced
additional variance that could not be controlled (e.g., the use
of unauthorized stimuli, the comfort of the surroundings).
Environments could have differed (both within and between
participants) along a number of factors (e.g., temperature)
known to impact semen parameters (see World Health Organi-
zation, 2010, for environmental factors that may affect ejacu-
late sample quality). Whether participants diligently read the
scenarios (and thus properly introduced the appropriate stimuli)
cannot be stated with the level of certainty as in a laboratory
procedure. Further, because ejaculates were produced outside
of the laboratory, samples may have been degraded by the time
they were analyzed. Participants did verify that ejaculates were
produced and delivered to the laboratory within 1 hr; however,
it is possible that participants’ responses did not accurately
reflect actual time of ejaculation. Although it would have been
preferred to have participants produce samples in the labora-
tory using a specimen container or seminal retention device
(Schoenfeld, Amelar, Dubin, & Skwerers, 1978), the protocols
detailed in the present research were implemented as ethical
and biosafety controls at the university level.
Finally, our sample consisted primarily of nonmarried,
young men in relationships of relatively short duration. The
risk of cuckoldry—and, thus, the infidelity manipulation—may
therefore not be as salient as it might have been for married
men or men in more committed relationships of longer dura-
tion. For some of these men, conception might even constitute a
reason for termination of the relationship, and thus paternity
uncertainty may not be a relevant consideration. Future
researchers are encouraged to test the ejaculate adjustment
hypothesis in a sample of married men or men in longer term,
more committed relationships.
Alternatively, the results of the current research may indi-
cate the absence of mechanisms in men that evolved to solve
adaptive problems of sperm competition. The results may also
indicate that men may not be attentive to the particular sperm
competition cue (i.e., partner infidelity) that was assessed in the
current study. Despite the null results of the current research,
however, there is evidence for psychological and physiological
adaptations to sperm competition in humans (Pham &
Shackelford, 2014) and a substantial body of research addres-
sing nonhuman sperm competition (reviewed in Simmons &
Fitzpatrick, 2012). Many sperm competition studies in nonhu-
mans, however, involve procedures that cannot be replicated
using human participants (e.g., manipulating the number of
males with whom a female copulates and then measuring
which sperm of the different males fertilize ova; Price, Dyer,
& Coyne, 1999). Logistical and ethical constraints have
resulted in a relative lack of experimental sperm competition
research on humans compared to sperm competition research
on nonhumans. A parsimonious interpretation of the null
results is therefore that they are a consequence of one or more
Pham et al. 5
methodological artifacts. The current research nevertheless
contributes important methodological information to the sperm
competition literature more generally and highlights methodo-
logical considerations for researchers investigating adaptations
to human sperm competition.
Declaration of Conflicting Interests
The author(s) declared no potential conflicts of interest with respect to
the research, authorship, and/or publication of this article.
Funding
The author(s) received no financial support for the research, author-
ship, and/or publication of this article.
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Pham et al. 7
... The current study reports the results of novel analyses for a subset of data from a larger project (Pham et al., 2018). The sample included 41 men attending a Midwestern university in the U.S., with ages ranging from 18 to 33 years (M = 23.30; ...
... The sample included 41 men attending a Midwestern university in the U.S., with ages ranging from 18 to 33 years (M = 23.30; SD = 3.60; 71.1% of the sample was Caucasian, with Asian representing the second-most commonly reported ethnicity at 6.7%; see DeLecce, Fink, Shackelford, & Abed, 2020a;DeLecce et al. 2020b, DeLecce, Shackelford, Fink, & Abed, 2020cPham et al., 2018). By self-report, participants had not had a vasectomy and had never sought infertility treatment. ...
... In a previous study (Pham et al., 2018), two ejaculate samples from each participant were produced in experimental (more sexually arousing) and control (less sexually arousing) conditions. More specifically, participants were primed with partner infidelity in the experimental condition and were primed with the idea of sex with their partner in a context that did not involve infidelity in the control condition. ...
Article
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Males of some species use mate retention behavior and investment in ejaculate quality as anti-cuckoldry tactics concurrently while others do so in a compensatory fashion. Leivers, Rhodes, and Simmons (2014) reported that men who performed mate retention less frequently produced higher-quality ejaculates, suggesting that humans use these tactics compensatorily. We conducted a conceptual replication of this research in a sample of 41 men (18–33 years; M = 23.33; SD = 3.60). By self-report, participants had not had a vasectomy and had never sought infertility treatment. We controlled for several covariates known to affect ejaculate quality (e.g., abstinence duration before providing an ejaculate) and found no statistically significant relationships between mate retention behavior and four components of ejaculate quality: sperm velocity, sperm concentration, slow motility, and ejaculate volume. The present results provide little support for the hypothesis that human males deploy mate retention behavior and ejaculate quality investment compensatorily. We discuss the limitations of this study and highlight the need for research to address questions about the nature of anti-cuckoldry tactic deployment in humans, especially concerning investment in ejaculate quality.
... The current study reports novel analyses of a subset of data from a larger project (Pham et al., 2018). The original dataset included responses from 66 men, with ages ranging 18 to 34 years (M ¼ 22.77; SD ¼ 3.83). ...
... Only data from men who provided two ejaculate samples (see Procedures) and completed the intelligence measure were included in the current analyses. Thus, the final sample included 41 men attending a Midwestern university in the United States, with ages ranging 18 to 33 years (M ¼ 23.33; SD ¼ 3.60; 78.0% of the sample was Caucasian, with Asian representing the second-most commonly reported ethnicity at 7.3%; see Pham et al., 2018). By self-report, participants had not had a vasectomy, had never sought infertility treatment, and were currently in a committed, heterosexual, sexually active relationship for at least 6 months (range 6 to 123 months; M ¼ 33.54; SD ¼ 25.57). ...
... After completion of the automated analysis, all materials that directly contacted the ejaculate were discarded in a biohazard waste container. In a previous study, the two ejaculate samples from each participant were examined for differences using a Wilcoxon signedrank test, which revealed no significant differences (see Pham et al., 2018). We also correlated each parameter across the two ejaculates provided by each participant. ...
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Genetic quality may be expressed through many traits simultaneously, and this would suggest a phenotype-wide fitness factor. In humans, intelligence has been positively associated with several potential indicators of genetic quality, including ejaculate quality. We conducted a conceptual replication of one such study by investigating the relationship between intelligence (assessed by the Raven Advanced Progressive Matrices Test-Short Form) and ejaculate quality (indexed by sperm count, sperm concentration, and sperm motility) in a sample of 41 men (ages ranging 18 to 33 years; M ¼ 23.33; SD ¼ 3.60). By self-report, participants had not had a vasectomy, and had never sought infertility treatment. We controlled for several covariates known to affect ejaculate quality (e.g., abstinence duration before providing an ejaculate) and found no statistically significant relationship between intelligence and ejaculate quality; our findings, therefore, do not match those of Arden, Gottfredson, Miller et al. or those of previous studies. We discuss limitations of this study and the general research area and highlight the need for future research in this area, especially the need for larger data sets to address questions around phenotypic quality and ejaculate quality.
... This study reports novel analyses of a subset of data from a larger project (Pham et al., 2018). The original data set included responses from 66 men, with ages ranging 18-34 years (M ¼ 22.77; SD ¼ 3.83). ...
... Although 66 men completed the intake survey, errors in coding, ejaculate sample processing, and participant compliance produced a final sample of 45 men who completed the intake session and both ejaculate sessions. Thus, the final sample included men attending a university in the Midwestern United States, with ages ranging 18-33 years (M ¼ 23.30; SD ¼ 3.60; 71.1% of the sample was Caucasian, with Asian representing the second most commonly reported ethnicity at 6.7%; see Pham et al., 2018). By selfreport, participants had not had a vasectomy, had never sought treatment for infertility, and were currently in a committed, heterosexual, and sexually active relationship for at least 6 months (range 6-123 months; M ¼ 35.50; ...
... In a previous study, the two ejaculate samples from each participant were produced in experimental (more sexually arousing) and control (less sexually arousing) conditions, then examined for differences using a Wilcoxon signed-rank test, which revealed no significant differences in any ejaculate parameter (see Pham et al., 2018). We correlated each parameter across the two ejaculates provided by each participant. ...
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Research in nonhuman animals (including insects, birds, and primates) suggests a trade-off in males between investment in competitive traits and investment in ejaculate quality. Previous research reported a negative association between perceived strength and ejaculate quality, suggesting that this trade-off also applies to human males. We conducted novel analyses of data secured as part of a larger project to assess the relationship between competitive traits (shoulder-to-hip ratio, handgrip strength, and height) and ejaculate quality (indexed by sperm morphology, sperm motility, and sperm concentration) in a sample of 45 men (ages ranging 18–33 years; M = 23.30, SD = 3.60). By self-report, participants had not had a vasectomy and had never sought treatment for infertility. We controlled for several covariates known to affect ejaculate quality (e.g., abstinence duration before providing an ejaculate) and found no statistically significant relationships between competitive traits and ejaculate quality; our findings therefore do not accord with previous research on humans. We highlight the need for additional research to clarify whether there is a trade-off between investment in competitive traits and investment in ejaculate quality in humans.
... The current study reports novel analyses of a subset of data from a larger project (Pham et al., 2018). The original dataset included responses from 66 men, with ages ranging 18-34 years (M = 22.77; SD = 3.83; individuals were not screened for pre-existing conditions, such as psychopathology or special educational needs). ...
... The final sample included 45 men, aged between 18 and 33 years (M = 23.3; SD = 3.6; see Pham et al., 2018). Participants had not had a vasectomy, had never sought treatment for infertility, and were currently in a committed, heterosexual, sexually active relationship for at least six months (range 6-123 months; M = 35.5; ...
... Participants also completed a survey after providing each ejaculate that included a query about abstinence duration prior to producing that ejaculate. We also obtained several body measurements (e.g., height, weight, and several measurements not related to the current study; data reported by Pham et al., 2018). ...
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Some research has reported relationships between personality dimensions and ejaculate quality, but this research has methodological limitations. In the current study, we investigated the relationships between six major personality dimensions and ejaculate quality in a design that offered several methodological improvements over previous research. Forty-five fertile men provided two masturbatory ejaculates and completed a measure of personality (HEXACO-60) assessing honesty-humility, emotionality, extraversion, agreeableness, conscientiousness, and openness to experience. Agreeableness was the only personality dimension associated with ejaculate quality, after controlling statistically for participant age, Body Mass Index (BMI), and abstinence duration, and this association was negative. However, once the covariates of BMI, age, and abstinence duration were included in a hierarchical regression (along with the six personality dimensions), agreeableness was no longer a statistically significant predictor of ejaculate quality, although the direction of the relationship remained negative. The current study adds to previous research documenting that psychological attributes—including major dimensions of personality—may be associated with ejaculate quality. We highlight limitations of the current research and identify directions for future study.
... Further, as a potential mechanism of cryptic female choice, exciting new research has demonstrated that sperm chemoattractants produced by eggs may facilitate gamete-mediated 'mate choice' in humans [101]. The study of adaptations to human sperm competition has traditionally bordered the fields of behavioural ecology and evolutionary psychology [102]. Now it seems that this fascinating area of research is reaching into the realm of reproductive biology in relation to the role that the FRT, specifically oviductal fluid and sperm chemoattractants, plays in determining postmating 'mate choice' outcomes. ...
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Although initially lagging behind discoveries being made in other taxa, mammalian sperm competition is now a productive and advancing field of research. Sperm competition in mammals is not merely a ‘sprint-race’ between the gametes of rival males, but rather a race over hurdles; those hurdles being the anatomical and physiological barriers provided by the female reproductive tract, as well as the egg and its vestments. With this in mind, in this review, I discuss progress in the field while focusing on the female perspective. I highlight ways by which sperm competition can have positive effects on female reproductive success and discuss how competitive outcomes are not only owing to dynamics between the ejaculates of rival males, but also attributable to mechanisms by which female mammals bias paternity toward favourable sires. Drawing on examples across different species—from mice to humans—I provide an overview of the accumulated evidence which firmly establishes that sperm competition is a key selective force in the evolution of male traits and detail how females can respond to increased sperm competitiveness with increased egg resistance to fertilization. I also discuss evidence for facultative responses to the sperm competition environment observed within mammal species. Overall, this review identifies shortcomings in our understanding of the specific mechanisms by which female mammals ‘select’ sperm. More generally, this review demonstrates how, moving forward, mammals will continue to be effective animal models for studying both evolutionary and facultative responses to sperm competition. This article is part of the theme issue ‘Fifty years of sperm competition’.
... In parallel with previous research on mate retention (Buss et al., 2008;Lopes et al., 2016), this sample included only individuals in a romantic relationship with the opposite sex for at least 3 months. We invited prospective participants through: (1) Amazon Mechanical Turk (MTurk): interested and eligible individuals could access and complete an on-line survey, and those who completed the survey received U$2.50; (2) the Psychology Department Subject Pool at a large Midwestern U.S. university: interested and eligible individuals were provided a link to an online survey, and those who completed the survey received partial course credit upon completion (Lopes et al., 2017); and (3) advertising on bulletin boards on the campus of the same Midwestern U.S. university: interested individuals contacted the researcher, and those who met the participation criteria and agreed to participate were escorted to a private room to answer an in-person survey and received U$25 at the conclusion of the survey (Pham et al., 2018). The current article reports novel analyses of a subset of data from different projects. ...
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Mate retention behaviors are designed to reduce the risk of partner infidelity or relationship defection. In the current research, we used k-means cluster analysis to identify distinct strategies of mate retention behaviors. Participants were 637 individuals (56.3% male) in a romantic relationship with an opposite-sex individual for at least 3 months (M = 78.7; SD = 95.8) and aged between 18 and 70 years (M = 29.3, SD = 10.5). Participants completed the Mate Retention Inventory–Short Form. The results suggested three distinct mate retention clusters or strategies: (1) Disengaged (infrequent use of both benefit-provisioning and cost-inflicting behaviors), (2) Exhaustive (frequent use of both benefit-provisioning and cost-inflicting behaviors), and (3) Benevolent (frequent use of benefit-provisioning and infrequent use of cost-inflicting behaviors). The results also indicated, for example, that men more than women use a benevolent strategy, women more than men use a disengaged strategy, and men using an exhaustive strategy report being less physically intimate with their partners than men using a benevolent strategy. We discuss the results with reference to evolutionary hypotheses of mate retention, and we highlight limitations of the current research and important directions for future research.
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Full-text available
Objectives: The phrase "level of sperm competition" is used only vaguely in the primate literature. There is also little distinction between the important elements of frequency and intensity of sperm competition, largely because the two current forms of measurement (socio-sexual system and relative testes size) are both proxies which allow neither precision nor fine distinctions. Both measures have critics, socio-sexual system in particular being branded subjective, misleading, and changeable. Testes size is considered the more reliable despite its validation resting on correlations with the other, less reliable, proxy. Recently, genetic paternity studies have been mooted to provide a potentially superior third measure of sperm competition but so far lack a formal interpretive framework. Here we use the published and relatively comprehensive genetic field studies of the Hominoidea to develop such a framework. Materials and methods: Formulae are derived to convert paternity data into a direct measure of the frequency, intensity, and overall level of sperm competition. We then compare these measures with relative testes size at the study, species, and phylogenetic levels. Results: A significant correlation between level of sperm competition and relative testes size was obtained at each level. These correlations provide independent support for the continuing use of testes size as a proxy measure when such a measure is sufficient. However, they also suggest that paternity data and our formulae yield a viable alternative measure. Discussion: This alternative measure based on paternity data has a number of advantages. Not only is it a potentially direct measure of the level of sperm competition but it also allows the roles of frequency and intensity to be studied separately when of interest.
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EOFFREY P ARKER IN- TRODUCED the idea of sperm competition in 1970. Based on his work on insects, PARKER de- fined sperm competition as the competition be- tween the sperm of two or more males for the fer- tilization of one or more ova (PARKER 1970a, Sperm competition occurs when the sperm of two or more males simultaneously occupy the reproductive tract of a female and thereby compete to fertilize an egg. Sperm competition has been documented or in- ferred to exist in many species, including humans and birds. Female infidelity creates the primary context for sperm competition. In animals that practice social monogamy and in which there is substantial paternal investment, males incur costs associated with a fe- male partner's infidelity. A principle cost is investing resources in genetically unrelated offspring. Female sexual infidelity and the resulting sperm competition generated several adaptive problems for males over evolutionary history. In humans and in birds, these adaptive problems include preventing female infideli- ty, correcting female infidelity, and anticipating fe- male infidelity. I review empirical work suggesting that males have evolved physiological and psycholog- ical mechanisms designed to solve these problems. Female infidelity; sperm competition, evolutionary psychology, social monogamy. Abstract
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Sperm competition occurs when a female copulates with two or more males within a sufficiently brief time period, resulting in sperm of the different males competing to fertilize ova. Sperm competition has been documented or inferred to occur across several species. We address the evidence for sperm competition in humans by reviewing literature indicating apparently convergent adaptations to sperm competition in humans and non-humans. We discuss future research directions, and conclude that the evidence for anatomical, biological, physiological, and behavioral adaptations to human sperm competition provides compelling evidence that sperm competition has been a recurrent feature of human evolutionary history.
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Nonhuman males attend to the number of potential sexual rivals in the local environment to assess sperm competition risk. Males of these species sometimes perform more frequent in-pair copulations to increase the likelihood of success in sperm competition. Here, we extend this research to humans, Homo sapiens. We secured self-report data from 393 men in a committed, sexual, heterosexual relationship. The results indicate that men whose in-pair partner has more male coworkers and friends (i.e., potential sexual rivals) also perform more frequent in-pair copulations, but only among men who perceive their partner to be particularly attractive relative to assessments of partners by other men in the sample. This research is the first to empirically investigate the number of potential male rivals in the local environment as a cue to sperm competition risk in humans. Discussion addresses limitations of the current research and highlights directions for future research. (PsycINFO Database Record (c) 2014 APA, all rights reserved).
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Men who spend a greater proportion of time apart from their female partner since the couple's last copulation are at greater "objective" sperm competition risk. We propose a novel cue to sperm competition risk: the time she spends with her male friends. Four hundred and twenty men in a committed, heterosexual, sexual relationship completed a questionnaire. The results indicate that men at greater objective sperm competition risk report less time desired until the couple's next copulation, greater interest in copulating with their partner, and greater anger, frustration, and upset in response to their partner's sexual rejection, but only among men whose partner spends more time with her male friends. These results remain after controlling statistically for the participant's age and their partner's age. We discuss limitations of the current research, and discuss how research in human sperm competition can inform social issues, including men's partner-directed sexual coercion.
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Males in many species differentially allocate sperm and seminal fluid depending on certain social variables, including perceived sperm competition and female reproductive status. In some species, males reduce their investment in sperm quantity or quality upon repeated matings with the same female and increase such investment when mated to a novel female. We tested for effects of stimulus habituation and novelty on ejaculated semen parameters in humans. We analyzed ejaculates produced through masturbation with stimulation from sexually explicit films. When males were exposed successively to the same female six times, we saw no change in ejaculate parameters between the first and sixth exposures to the same female. However, ejaculate volume and total motile sperm count significantly increased when males were exposed to a novel female. Time to ejaculation also decreased significantly upon exposure to a novel female. Thus, our results suggest that human males ejaculate more quickly and invest more in ejaculates with novel females.
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Secondary sexual traits in males are recognized as having arisen in order to gain access to reproductive opportunities, through their effects on the outcome of male–male competition and female choice. The phenotype-linked fertility hypothesis proposes that ejaculate quality is honestly advertised via secondary sexual traits. Alternatively, if males have limited resources to allocate to both pre- and postcopulatory traits, males possessing attractive phenotypic or behavioral traits may produce poorer quality ejaculates. Sperm competition theory also predicts that the female phenotype will influence ejaculate quality, with males increasing investment as female attractiveness increases. However, the extent to which the male and female phenotypes interact in affecting ejaculate quality has not been widely studied. Here, we examine how male and female phenotypes influence ejaculate quality in humans. Eighty-one men, for whom we had a composite measure of overall male mate value, produced a semen sample in response to images of either highly attractive or less attractive women. We found a significant relationship between male mate value and ejaculate quality that was context dependent. Sperm motility and concentration increased with male mate value but only when men viewed images of highly attractive women. Context dependence may contribute, in part, to the often conflicting patterns of variation found in studies that test the phenotype-linked fertility hypothesis.
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Recent research indicates that men may have evolved mechanisms dedicated to detecting and responding to the risk of partner infidelity. Because activation of these “anti-cuckoldry” mechanisms depends on partner infidelity, or the perception of partner infidelity, existing evidence for such mechanisms relies on correlational data. The current study tests several predictions regarding men’s anti-cuckoldry mechanisms in an experimental design. As predicted, the results demonstrated: (1a) experimental activation of men’s anti-cuckoldry mechanisms by presenting them with a vignette depicting a female partner’s sexual infidelity; (1b) no activation of men’s anti-cuckoldry mechanisms by presenting them with a vignette depicting a sexual encounter without female infidelity; (2) experimental activation of men’s anti-cuckoldry mechanisms was influenced by their perceived risk of partner infidelity; and (3) women were not influenced by the partner infidelity manipulation.