Early Psychosocial Stress Predicts Extra-pair Copulations

Evolutionary Psychology
www.epjournal.net – 2007. 5(1): 184-201
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Original Article
Early Psychosocial Stress Predicts Extra-pair Copulations
Nicole Koehler, School of Anatomy and Human Biology, The University of Western Australia, 35 Stirling
Hwy, Crawley WA 6009, Australia. Email: nkoehler@anhb.uwa.edu.au (Corresponding author)
James S. Chisholm, School of Anatomy and Human Biology, The University of Western Australia, 35
Stirling Hwy, Crawley WA 6009, Australia. Email: jchisholm@anhb.uwa.edu.au
Abstract: Cheating on a mate, known as an extra-pair copulation (EPC), is considered
unacceptable by most individuals. Nonetheless many individuals engage in such risky
behaviors. Because individuals with high, as opposed to low, levels of early psychosocial
stress are more risk prone and more likely to engage in opportunistic matings, we predicted
that individuals reporting EPCs, one of many types of opportunistic mating (e.g., one-night
stand, consecutive short-term relationships etc), have higher levels of early psychosocial
stress than those who do not. Two types of EPCs were examined: EPC-self (EPC-S),
having sex with someone other than one’s mate, and EPC-other (EPC-O), having sex with
someone else’s mate. In a sample of 229 women and 161 men, significantly higher levels
of early psychosocial stress were found amongst those reporting an EPC-S than those
reporting none, irrespective of EPC-Os. Furthermore, the more EPC-Ss men, but not
women, reported the higher their early psychosocial stress. Early psychosocial stress was
not associated with EPC-Os irrespective of EPC-Ss. Participants were also classified into
one of four groups (no EPCs, EPC-O only, EPC-S only, or EPC-S&O) which significantly
interacted with early psychosocial stress. Results are discussed from adaptationist and
mechanist perspectives and why early psychosocial stress was higher in individuals
reporting EPC-Ss irrespective of EPC-Os, but not EPC-Os irrespective of EPC-Ss, than
those not reporting the EPC of interest.
Keywords: genetic benefits, gratification, opportunistic mating, risk-taking.
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Introduction
Extra-pair copulations (EPCs) are defined as having a sexual relationship with
someone other than a pair-bonded mate (i.e., a spouse or a boy/girlfriend). There are two
types of EPCs: EPC-self (EPC-S), which is having a sexual relationship with someone

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Early psychosocial stress and extra-pair copulations
other than one’s currently pair-bonded mate (i.e., he/she is having an affair) and EPC-other
(EPC-O), which is having a sexual relationship with someone who is already in a pair-
bonded relationship (i.e., he/she is the person someone is having an affair with).
Furthermore, individuals can also simultaneously engage in an EPC-S and an EPC-O.
Although 68 – 83% of people disapprove of having extramarital sexual
relationships (Medora and Burton, 1981; Singh, Walton, and Williams, 1976; Thornton and
Young-DeMarco, 2001; Weis and Slosnerick, 1981; Wiederman, 1997) their occurrence is
still quite common (see Table 1.). Furthermore 6% of British women reported that their
last copulation was an EPC (Bellis and Baker, 1990). The inconsistencies between
attitudes and behaviors about EPCs raises the question as to why individuals engage in
such behavior despite these negative views. From an evolutionary perspective the answer
may be different depending on the individual’s gender. Males may engage in EPCs to
increase offspring quantity by using EPCs as another form of short-term mating, whereas
females may engage in EPCs to increase offspring quality by mating with males of superior
genetic quality (Hrdy, 1981; Jennions and Petrie, 2000) while obtaining direct benefits
from a pair-bonded mate (Smith, 1984). However, it must be kept in mind that females
may also obtain direct benefits from an EPC (i.e., protection, food, monetary rewards)
(Smith, 1984). Evidence for the idea that females may engage in EPCs to obtain genetic
benefits comes from the animal literature (e.g., Hasselquist, Bensch, and von Schantz,
1996; Kempenaers, Verheyen, van den Broeck, Burke, van Broeckhoven, and Dhondt,
1992). For example, great reed warbler females pair-bonded with low genetic quality
mates engaged in EPCs with neighboring high genetic quality males, indicated by their
larger song repertoires, presumably to obtain genetic benefits for their offspring such as
increased offspring survival, which is positively associated with large song repertoires
(Hasselquist et al., 1996).

Researchers have not only examined extra-pair relationships in animals, but have
also looked at extra-pair relationships in humans via self-report data. Women not using
oral/hormonal contraceptives report greater sexual interest in extra-pair men and fantasize
more about them than their pair-bonded mate when conception is likely, near ovulation,
than when conception is unlikely, during the luteal phase of their menstrual cycle
(Gangestad, Thornhill, and Garver, 2002). Furthermore, women are more sensitive to
indicators of genetic quality around ovulation (Johnston, Hagel, Franklin, Fink, and
Grammer, 2001; Penton-Voak and Perrett, 2000; Penton-Voak, Perrett, Castles, Kobayashi,
Burt, Murray, et al., 1999; but see also Koehler, Rhodes, and Simmons, 2002; Koehler,
Rhodes, Simmons, and Zebrowitz, 2006), are more likely to engage in EPCs around
ovulation and are less likely to use contraception during EPCs than in-pair matings (Bellis
and Baker, 1990). Men with high, as opposed to low, symmetry, a trait conjectured to
signal genetic quality (Møller and Swaddle, 1997; Thornhill and Gangestad, 1999), are also
more likely to report having been an EPC partner (Gangestad and Thornhill, 1997). Taken
together, these findings are consistent with the idea that women may engage in EPC-Ss to
obtain genetic benefits from another mate while obtaining direct benefits from a pair-
bonded mate. Furthermore, the genetic diversity hypothesis, which suggests that mating
with multiple mates increases the genetic diversity of offspring, has also been put forward
as a possible explanation as to why males and females might engage in EPCs with multiple
mates (Kempenaers, Congdon, Boag, and Robertson, 1999; Smith, 1984; Yasui, 1998).
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Early psychosocial stress and extra-pair copulations
Bearing in mind that both men and women can benefit by engaging in EPCs raises
the question as to why more individuals do not engage in such behavior. As with most
behaviors accompanied with a benefit there is also a cost. In the case of EPCs individuals
are faced with the cost of being discovered by their pair-bonded mate which could lead to
the dissolution of their relationship or in extreme cases could lead to spousal homicide
because of sexual jealously, which is more commonly inflicted by men than women
(Chimbos, 1998; Daly and Wilson, 1988; Daly, Wilson, and Weghorst, 1982; Serran and
Firestone, 2004).

Table 1. Prevalence of EPCs reported by women and men.

Sample
Age
EPC
Study
Location
Period
Type of EPC
Gender
N
characteristics
(years)
prevalance
Essock-Vitale & McGuire, 1988
Los Angeles, CA General population
Lifetime
Extra-marital sex
Female
283
35 – 45
23.0%
Johnson et al., 2001
Britain
General population
Last 12 months
Concurrent relationship Female
6399
16 – 24
15.2%
25 – 34
7.6%
35 – 44
6.7%
16
–
44
9.0%
Male
4762
16 – 24
20.8%
25 – 34
15.3%
35 – 44
9.8%
16 – 44
14.6%
Kinsey et al., 1948
U.S.A.
General population
Lifetime
Extra-marital sex
Male
3833
16 – 20
35.4%
21 – 25
24.4%
26 – 30
26.0%
31 – 35
28.4%

36
–
40
27.8%
41 – 45
23.5%

46 – 50
25.2%
51 – 55
22.3%
56
–
60
22.1%
Kinsey et al., 1953
U.S.A.
General population
Lifetime
Extra-marital sex
Female
6927
16 – 20
6.0%
21 – 25
9.0%
26 – 30
14.0%
31 – 35
17.0%
36 – 40
17.0%
41 – 45
16.0%
46 – 50
11.0%
51 – 55
6.0%
56 – 60
4.0%
Pietropinto & Simenauer, 1977
U.S.A.
General population
Lifetime
Concurrent relationship Male
4066
18 – 29
53.7%
30 – 39
51.1%
40 – 54
50.8%
55 – 65
37.0%
18 – 65
49.0%
Simmons et al., 2004
Australia
Mainly students
Lifetime
Concurrent relationship Female
194
18 – 24
13.0%
25 – 50
46.0%
17 – 51
22.2%
Male
222
18 – 24
20.0%
25 – 50
44.0%
17 – 51
27.9%
Spira et al., 1992
France
General population
Female
10127
18 – 24
5.0%
35 – 49
51.7%
Male
9928
18 – 24
6.9%
35 – 49
33.7%
Wiederman, 1997
U.S.A.
General population
Lifetime
Extra-marital sex
Female
1288
20 – 29
10.9%
30 – 39
14.2%
40 – 49
19.3%
50 – 59
10.9%
60 – 69
7.6%
?70 0.5%
?20
11.6%
Male
884
20 – 29
14.9%
30 – 39
14.3%
40 – 49
29.3%
50 – 59
28.7%
60 – 69
34.0%
?70 13.2%
?20
22.7%



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Early psychosocial stress and extra-pair copulations
Given that EPCs are risky and that not all individuals engage in such behaviors
raises the question whether some individuals are simply more likely to engage in risky
behavior in general. The answer is yes, because previous research has shown that
psychosocial stress during childhood and/or adolescence is associated with risky behaviors
such as tobacco, alcohol and illicit drug use (Chen, Dunne, and Han, 2006; Felitti et al.,
1998; Moran, Vuchinich, and Hall, 2004; Rodgers et al., 2004; Stock, Bell, Boyer, and
Connell, 1997; Zierler et al., 1991), driving while intoxicated (Rodgers et al., 2004),
pathological gambling (Petry and Steinberg, 2005), risky sexual behavior (e.g., less
frequent contraception use [Hill, Ross, and Low, 1997; Mosher and Bachrach, 1987; Stock
et al., 1997], engaging in sexual activity at a younger age, with more partners and after a
shorter duration of meeting their partner [Testa, VanZile-Tamsen, and Livingstone, 2005],
exchanging sex for money or drugs [Parillo, Freeman, Collier, and Young, 2001]), as well
as getting involved in fights and truancy from school (Bereczkei and Csanaky, 1996).
Furthermore, individuals with high, as opposed to low, levels of early psychosocial stress
are hypothesized to engage in more opportunistic matings such as EPCs (Belsky, Steinberg
and Draper, 1991; Draper and Harpending, 1982, 1988).
But what theories can explain why some individuals are more likely to engage in
risky behaviors such as EPCs than others? Life history theory together with the
psychology of time preference may help answer this question (see Figure 1.). We will
firstly examine the answer to this question from the perspective of life history theory
although the two theories are highly intertwined.
Briefly, life history theory is the study of life cycles and life history traits (e.g., age
at maturation, age at first birth, longevity etc) in the context of evolutionary ecology (e.g.,
Hill and Kaplan, 1999; Stearns, 1989, 1992). Importantly, life history theory is concerned
with trade-offs individuals face amongst components of fitness. Because resources are
always limited resources used for one task (e.g., somatic functions) cannot be utilized for
another function (e.g., reproduction) and thus trade-offs arise. The most dominating trade-
off individual’s face is that between current and future reproduction. Thus, in
environments of high risk and uncertainty (i.e., mortality rates are high) the optimal
reproductive strategy is to reproduce as early and as often as possible (i.e., a quantitative
reproductive approach), which would be adaptive to minimize the probability of lineage
extinction, whereas the opposite is optimal in environments of low risk and uncertainty
(i.e., a qualitative reproductive approach). Consistent with this view, Draper and
Harpending (1982) theorized that father-absent, but not father-present, children learn that
unstable paternal care is appropriate (males) / that paternal care is unimportant (females)
and will thus follow a developmental trajectory indicative of a quantitative, as opposed to a
qualitative, reproductive style. A few years later Belsky et al. (1991) extended Draper and
Harpending’s theory by focusing on a more generalized view of a child’s upbringing,
specifically attachment behavior. They proposed that children learn about the availability
and predictability of resources within their environment, others’ trustworthiness, and the
stability of close interpersonal relationships during their first 5 to 7 years of life.
Specifically, they proposed that those reared in an environment were resources are scarce
and unpredictable are thought to follow a quantitative reproductive strategy by speeding up
biological maturation and forming short-term rather than long-term heterosexual
relationships, whereas those reared in environments were resources are available and
predictable are thought to follow a qualitative reproductive strategy by delaying biological
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Early psychosocial stress and extra-pair copulations
maturation and forming enduring pair-bonds. Chisholm (1993) reinterpreted Belsky et al.’s
theory in terms of early psychosocial stress from a life history theory perspective about the
effect of local mortality rates. He suggested that children perceive information about local
mortality rates such that those reared in environments of high, but not those in
environments of low, mortality will follow a developmental path indicative of a
quantitative reproductive strategy to minimize the chances of lineage extinction. A few
years later Ellis and colleagues (Ellis, 2004, 2005; Ellis et al., 2003; Ellis, McFadyen-
Ketchum, Dodge, Pettit, and Bates, 1999) suggested that paternal parental investment is
important above and beyond all early psychosocial stressors (i.e., sexual abuse, quality of
family life) in determining the developmental path individuals will follow. In sum, given
that Draper and Harpending, Belsky et al., Chisholm and Ellis theorized that individuals
who have experienced early psychosocial stress are likely to pursue a quantitative
reproductive strategy suggests that these individuals may be more likely to engage in EPCs,
a type of opportunistic mating, leading to an increase in offspring especially amongst men.
The psychology of time preference, which is also known as intertemporal choice,
impulsiveness, delayed gratification, self-control, impatience and discount rates (Chisholm,
1999a, 1999b; Lancaster, 1994), can also explain EPC behavior, together with life history
theory. Briefly, according to the psychology of time preference individuals in
environments of high risk and uncertainty (i.e., those with early psychosocial stress) are
less likely to delay gratification and thus take smaller immediate rewards in favor of larger
delayed rewards because in risky and uncertain environments there is the possibility that
the larger delayed reward may never be obtained. Mischel (1958, 1961) examined the
delay of reinforcement father-present and father-absent young children preferred and found
that father-absent children were more likely to favor receiving a smaller immediate reward
instead of a delayed larger reward than father-present children. From the perspective of the
psychology of time preference, EPC activity may be more likely in those with high, as
opposed to low, levels of early psychosocial stress because they are more impulsive in
general.
Taken together, the ideas that individuals with high, as opposed to low, levels of
early psychosocial stress are more likely to engage in impulsive and risky behaviors, that
there is some risk involved in engaging in EPCs, and that early psychosocial stress is
theoretically positively associated with opportunistic mating, suggests that individuals who
report EPCs may have higher levels of early psychosocial stress than those who do not.
Thus, the first aim of our study was to examine this possibility by comparing early
psychosocial stress scores of individuals that reported no EPCs to those that reported at
least one EPC-S and at least one EPC-O. Our second aim was to examine the relationship
between the number of EPC partners individuals report and their level of early
psychosocial stress to determine whether those with higher, as opposed to lower, levels of
early psychosocial stress engage in such risky behaviors more often. Specifically, we
hypothesized that the higher individuals’ early psychosocial stress scores the more EPC-
S/EPC-O partners they will report.
Allowing for the possibility that individuals could have engaged in both an EPC-O
and an EPC-S either simultaneously or consecutively, we also classified participants into
one of four groups: (1) no EPCs reported; (2) EPC-O only reported; (3) EPC-S only
reported; and (4) both EPC-S and EPC-O (here after referred to as EPC-S&O) reported.
On the view that having engaged in any type of EPC (i.e., groups 2 – 4) is more risky than
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Early psychosocial stress and extra-pair copulations
not, we predicted that individuals belonging to EPC groups 2 – 4 would have higher levels
of early psychosocial stress than those who have not engaged in an EPC. On the view that
engaging in an EPC-S might be more risky than engaging in an EPC-O, given that in an
EPC-S one’s own pair-bonded relationship is at stake should one be discovered, we
hypothesized that individuals reporting an EPC-S only would have higher levels of early
psychosocial stress than those reporting an EPC-O only. Furthermore, considering that
individuals who are in the EPC-S&O category would have engaged in an EPC-S, which
may be the riskier of the two types of EPCs, we hypothesized that individuals reporting
both types of EPCs will have higher levels of early psychosocial stress than those reporting
an EPC-O only but should not differ from those reporting an EPC-S only because having
an EPC-O should add nothing to the risk of having an EPC-S. All analyses were conducted
separately for women and men because the reasons (not necessarily conscious) for
engaging in an EPC may be different depending on an individual’s gender.

Figure 1. Outcomes of early psychosocial stress from adaptationist and mechanist
perspectives. See text for details.


Hypotheses/Models:
Father-absence (Draper & Harpending, 1982)
Insecure attachment (Belsky et al., 1991)
All early psychosocial stressors
Lack of paternal investment (Ellis, 2004)
ADAPTATIONIST EXPLANATION
MECHANIST EXPLANATION
Interpreted from life history theory
Psychology of short time
(Chisholm, 1993)
preference (i.e., impulsiveness)
(Mischel, 1958, 1961)

Quanitative reproductive approach

Short-term mating
- short-term relationships

- one night stands
- EPCs
Decrease probability of lineage extinction



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Early psychosocial stress and extra-pair copulations
Materials and Methods
Participants

The participants were part of a larger study which investigated the relationship
between early psychosocial stress and life history traits, sexual behavior and mate choice
who were recruited from university communities (The University of Queensland: N = 292;
The University of Western Australia: N = 192; Curtin University of Technology: N = 132),
from the general community via advertisements mailed to various Western Australian
medical centers (N = 1), among family friends (N = 5), work colleagues (N = 19), by word
of mouth by participants (N = 79), by placing questionnaires at an annual University of
Western Australia Expo in September 2005 (N = 19) and from sources unknown (N = 19)1.
All participants were given the opportunity to enter a draw to win one of ten $50 personal
checks as compensation for their time. Questionnaires were returned by 326 women and
233 men. Women (N = 229) and men (N = 161) who fulfilled the following criteria were
included in our study: heterosexual, sexually active, at least 18 years of age and no known
fertility problems. These women and men had a mean age of 28.3 years (SD = 10.7; range
= 18 – 64 years) and 26.8 years (SD = 9.6; range = 18 – 64 years) respectively.

Procedure

Extra-pair copulations. Participants were asked whether they had engaged in an
EPC-S and an EPC-O. EPC-Ss were assessed by asking participants whether they had ever
had sex with someone while they themselves were in a relationship with someone else (i.e.,
they had an affair) and if so with how many individuals. EPC-Os were assessed by asking
participants whether they had ever had sex with someone they knew was involved in a
relationship with someone else (i.e., the individual they had sex with was having an affair
with them) and if so with how many individuals.
Early psychosocial stress. Early psychosocial stress was assessed by asking
participants to indicate the frequency/occurrence of any of the following stresses between
the ages of 0 and 7 years: psychological abuse (three questions), physical abuse (two
questions), sexual abuse (by household members: four questions; by non-household
members: four questions), father’s or father figure’s abuse of mother (four questions),
substance abuse by household members (two questions), household member imprisonment
(one question), the presence of mentally ill/suicidal household members (two questions),
the occurrence of parental divorce/separation (one question) (taken from Anda, Chapman,
Felitti, Edwards, Williamson, Croft, et al., 2002; Dietz, Spitz, Anda, Williamson,
McMahon, Santelli, et al., 1999; Dong, Anda, Dube, Giles, and Felitti, 2003; Dong, Anda,
Felitti, Dube, Williamson, Thompson, et al., 2004; Felitti et al., 1998); the death of various
family members (mother, father, siblings, other family members living with the
participant), absence of the mother “for what seemed like a long time”, and absence of the
father “for what seemed like a long time” (taken from Chisholm, 1999a). Unlike previous
studies (e.g., Anda et al., 2002), the distinction between abuse by household and non-
household members was made because the effects of sexual abuse are more severe if the

1 Some participants did not indicate where they heard about the study during the recruitment procedure.
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Early psychosocial stress and extra-pair copulations
abuser was close to the child (e.g., father) than less close (Kendall-Tackett, Williams, and
Finkelhor, 1993). Participants were also asked to completed nine 7-point ratings scales
(taken from Kim and Smith, 1998a, 1998b) about self-mother (with “self” pertaining to
themselves), self-father, mother-father relationships and the general quality of family life
between birth and up to and including age 7 years. These ages were chosen to reflect
Belsky et al.’s (1991) proposed sensitive period for attachment formation. Participants also
had the option of checking “not applicable” to items that did not apply to them (e.g.,
absence of siblings).
Blank responses to any items used to assess early psychosocial stress were coded as
lack of exposure to the item of interest, which was the most conservative option so that any
potential findings cannot be attributed to falsely classifying participants as being exposed to
a certain stressor. Participants who reported anything but a “never” response, with the
exception of blank responses, for questions about psychological abuse, physical abuse,
sexual abuse (household), sexual abuse (non-household), and violence towards mother
were given a score of 1 per category whereas “never” responses for all questions within a
category were scored as 0 for that category. The presence of substance abuse, mental
illness, criminal activity, parental divorce/separation, death of mother, death of father,
death of sibling, death of a family member, separation from mother, and separation from
father, all of which were assessed via one question with the exception of substance abuse
and mental illness were given a score of 1 per category whereas the absence of events
including non-applicable questions (e.g., not having a father and therefore not responding
to questions referring to a father) were scored as 0. For the nine rating scales, ratings
below 4 were scored as 1 (i.e., the presence of psychosocial stress), whereas ratings
between 4 and 7, including non-applicable responses, were scored as 0. These scores per
category/rating were then summed so that participants could obtain a maximum early
psychosocial stress score of 24, indicating that they had been exposed to at least one form
of stress per category/rating.
Results
Early psychosocial stress scores ranged from 0 to 18 in women (M = 4.49, SD =
3.78) and 0 to 17 in men (M = 3.76, SD = 3.21) when the scoring technique of assigning a
score of 1 per exposed category and 0 per unexposed category was applied. Just under
10% of participants (Women: 10.0%; Men: 11.8%) reported no early psychosocial stresses
at all (i.e., a score of 0).
Fifty-seven women and 43 men reported an EPC-S and 64 women and 53 men
reported an EPC-O. As can be seen in figure 2, women and men who reported an EPC-S
experienced significantly higher levels of total early psychosocial stress than those who did
not report an EPC-S, t227 = 2.54, p = .006 and t159 = 3.71, p ? .001 respectively (both one-
tailed). Individuals who reported an EPC-O also had higher levels of total early
psychosocial stress than those did not report an EPC-O, but these differences did not reach
significance in women, t227 = 1.55, p = .061, nor in men, t159 = 1.30, p = .098 (both one-
tailed).



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Early psychosocial stress and extra-pair copulations

Figure 2. Mean total early psychosocial stress as a function of the presence (no vs. yes) of
EPC-S and EPC-O relationships, irrespective of EPC-Os and EPC-Ss respectively, in
women and in men. SE bars are shown.
6.5
6.0
No
Yes
5.5
5.0
4.5
4.0
3.5
3.0
2.5
Mean total early psychosocial stress
2.0
Women
Men
Women
Men
EPC-S
EPC-O
Type of EPC relationship and participant gender


Women reported a mean of 1.83 EPC-Ss (SD = 1.69, range = 1 – 8, N = 53) and a
mean of 1.90 EPC-Os (SD = 2.33, range = 1 – 13, N = 60), whereas men reported a mean of
1.57 EPC-Ss (SD = 0.74, range = 1 – 4, N = 42) and a mean of 1.35 EPC-Os (SD = 0.72,
range = 1 – 5, N = 49)2. Women’s total early psychosocial stress scores were not
significantly correlated with the number of EPC-Ss reported, (r = -0.05, p = .370, one-
tailed) nor to the number of EPC-Os reported (r = 0.19, p = .068, one-tailed). Men’s total
early psychosocial stress scores correlated significantly with the number of EPC-Ss
reported (r = 0.40, p = .004, one-tailed). On the other hand, like women, men’s total early
psychosocial stress scores and the number of EPC-Os reported were not significantly
correlated (r = 0.09, p = .262, one-tailed).

Considering that individuals can engage in an EPC-S and an EPC-O simultaneously
or consecutively we also assessed the relationship between early psychosocial stress and
EPCs by classifying participants into four groups: (1) no EPCs (Women: N = 144; Men: N
= 95); (2) EPC-O only (Women: N = 28; Men: N = 23); (3) EPC-S only (Women: N = 21;
Men: N = 13); and (4) EPC-S&O (Women: N = 36; Men: N = 30). Separate ANOVAs with
EPC group as a between subjects factor were separately conducted on women’s and men’s
early psychosocial stress scores with six planned comparisons (see Table 2. and Figure 3.
for details). Early psychosocial stress was significantly affected by EPC group for women
(F3,225 = 3.09, p = .028) and for men (F3,157 = 5.81, p = .001).

2 The number of EPC-S and EPC-O relationships were not reported by all participants.
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Early psychosocial stress and extra-pair copulations


Figure 3. Mean total early psychosocial stress as a function of EPC group for a) women
and b) men. SE bars are shown.

a)
8
7
6
5
4
3
2
1
Mean total early psychosocial stress 0
no EPCs
EPC-O only
EPC-S only
EPC-S&O
EPC group

b)
8
7
6
5
4
3
2
1
Mean total early psychosocial stress 0
no EPCs
EPC-O only
EPC-S only
EPC-S&O
EPC group


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