Helping Others to Find Long-Term and Short-Term Mates: A Test of Inclusive Fitness, Reciprocal Altruism, and Parental Investment Theories

Evolutionary Psychology
www.epjournal.net – 2007. 5(4): 716-732
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Original Article
Helping Others to Find Long-Term and Short-Term Mates: A Test of
Inclusive Fitness, Reciprocal Altruism, and Parental Investment Theories

Peter K. Jonason, Psychology, New Mexico State University, Las Cruces, NM, USA Email:
pjonason@nmsu.edu (Corresponding author)

Pamela L. Izzo, History, New Mexico State University, Las Cruces, NM, USA
Gregory D. Webster, Psychology, University Illinois at Urbana-Champaign, Urbana-Champaign, IL, USA
Abstract: Individuals prefer helping some people more that others when it comes to
finding a mate, and these preferences depend on whether long- or short-term mates are
considered. Study 1 (N = 108) examined three theoretical frameworks (inclusive fitness,
reciprocal altruism, and parental investment) for understanding why individuals would be
more willing to help some individuals find mates instead of others. College participants
reported how willing they were to help different types of individuals (e.g., sister, stranger)
find a mate. When considering willingness to help others find a long-term mate, people
preferred kin over nonkin, supporting an inclusive fitness model. However, when
considering willingness to help others find short-term mates, people preferred helping
people their own age, supporting a reciprocal altruism model. Study 2 (N = 143) replicated
this age-cohort effect. Although rates of willingness to help others find mates were
generally low, people were more likely to help others find a long-term mate than a short-
term one.

Keywords: Helping; long-term mates; short-term mates; parental investment; inclusive
fitness; reciprocal altruism.
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Introduction
Picture yourself in the position to play matchmaker. You know a myriad of
individuals whom you could match. You have family members and non-family members;
you have those who are similar in age as you and those who are not; and you have the
choice between male and female relatives. How then do you decide who to help and who
not to help?

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Evolutionary matchmaking
Helping behavior has been an area of major theoretical discussions in evolutionary
biology (Brown and Pimm, 1985) and social psychology (Berscheid and Reis, 1998). One
theory that has emerged to explain helping behavior is inclusive fitness theory or kin-
selection theory (Hamilton, 1964; Oli, 2003; Van den berghe and Barash, 1977). In this
study we will specifically be addressing willingness to help not altruism. While both are
similar phenomenon they are not identical. Altruism is defined in terms of fitness and
requires a cost to the altruist, whereas helping behavior deals with observable features and
typically does not address fitness (Brown and Pimm, 1985). While altruism and helping
behavior are hallmarks of evolutionary psychology so is mating research. To date few
researchers have attempted to link these two.

Inclusive fitness as a theory of helping behavior
Inclusive fitness theory predicts, and numerous authors have found, that closer
genetic relatedness increases willingness to help (Burnstein, Crandall, and Kitayama, 1994;
Stewart-Williams, 2007) and helping (Essock-Vitale and McGuire, 1985; Korchmaros and
Kenny, 2006). By helping those relatives who are genetically close to them, individuals
increase their fitness both in terms of survival but also in terms of reproduction (Rushton,
Russell, and Wells, 1984). For instance, a grandparent is likely to share genes with her/his
grandchild and is thus more likely to help the grandchild to increase her/his own inclusive
fitness (Mace and Sear, 2005). This is especially the case for maternal grandmothers
helping granddaughters as compared to grandmothers helping grandsons or grandfathers
helping grandchildren (Euler and Weitzel, 1996; Michalski and Shackelford, 2005). It is
assumed that people are not consciously aware of mechanisms of inclusive fitness. Genes
“encourage” their hosts to make copies of themselves via reproduction and helping those
who share other copies of those genes (Dawkins, 1976).

Most evolutionary research has supported an inclusive fitness approach to helping
behavior. Research on resource allocation within families has found that people share more
of their wealth with closer genetic relatives than more distant genetic relatives or non-kin
using both probated wills (Judge and Hrdy, 1992; Smith, Kish, and Crawford, 1987;
Webster, Bryan, Crawford, McCarthy, and Cohen, in press) and classroom experiments
(Webster, 2003, 2004). Other research has found that in emergency situations individuals
act more nepostically than in less dire situations (Shavit, Fischer, and Koresh, 1994). Other
evidence shows that individuals are more likely to help kin with poor health over nonkin
(Burnstein, Crandall, and Kitayama, 1994).
Little research, however, has been done on helping others find mates. In terms of
aiding conspecifics in their mating success, research has addressed how humans are
cooperative breeders. Cooperative breeding refers to the tendency to help others rear
offspring (Mace and Sear, 2005). Whereas cooperative breeding appears to be referring to
helping others breed, it is more accurately described as alloparenting or cooperative
parenting. In fact, when dealing with actual mating, researchers have focused on the
suppression of reproduction of daughters by mothers (Voland, 1998, for a review of parent-
offspring conflict see Trivers, 1974) not on reproduction itself. Voland (1998) noted that
human strategies, including helping, depend on different circumstances or domains. One of
these domains is helping others find long-term mates and short-term mates (LTMs and
STMs, respectively; see Li and Kenrick, 2006, for a review). Unlike cooperative breeding
and reproductive suppression, helping others find LTMs and STMs has been rarely
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addressed. Helping others find LTMs and STMs likely entails acts such as formally setting
individuals up on dates but also initiating more impromptu introductions.
When the costs are high, individuals tend to favor kin over nonkin; when costs are
low the individuals may help nonkin more (Burnstein, Crandall, and Kitayama, 1994;
Shavit, Fischer, and Koresh, 1994; Stewart-Williams, 2007). The pattern of helping
according to evolutionary psychologists is that when there is high risk involved in a given
course of action individuals act more nepostically; act more in favor of their own genes
over the genes of others. Thus the question of helping others relates to the balancing of
costs and benefits of that act of helping.
When helping others find mates, individuals are investing effort and potentially
losing their own reproductive opportunities to help kin and nonkin find mates, which are
examples of costs involved in helping others (Stewart-Williams, 2007). The benefit of
helping others to find long-term mates is that pairbond formation increases group cohesion
and decreases intragroup sexual conflict. As such, we predict that individuals should be
more willing to help others find LTMs over STMs.
However, an added benefit may exist for helping kin over nonkin because the
arranger may also reproductively benefit from the act of helping. This is similar to the
sexy-sons hypothesis (Gangestad and Simpson, 2000) in that when a mother helps her son
mate, by increasing his level of attractiveness, she also reproductively benefits. In fact,
similar sentiments have been expressed by Ackerman, Kenrick, and Schaller (2007), when
they stated that interactions among kin and nonkin had different functional consequences.
Those differences lie in the lack of shared genetic material among friends, thus helping
among them is not about genes but about ingroup functions. While short-term mating
opportunities may increase reproductive success it comes with the cost of decreased social
reputation. Individuals are more likely to adopt the sexual double standard for relatives
over non-relatives (Sprecher, 1989), suggesting that individuals are attuned to the sexual
acts of their kin and may actually be concerned more with the sexual acts of kin over
nonkin. All in all, arranging long-term relationships is more beneficial to the arranger than
arranging short-term relationship because the formation of bonds and probably the sharing
of responsibilities in child rearing lead to higher levels of reproductive success. Thus, we
predict that individuals will be more willing to help kin find LTMs over STMs.

Reciprocal altruism as a theory of helping behavior

A second and related theory of helping behavior is reciprocal altruism (Trivers,
1971, 1985). From an evolutionary perspective, reciprocal altruism may help explain why
people help nonkin in addition to kin (Fehr and Henrich, 2003; Gintis, 2000; Richerson,
Boyd, and Henrich, 2003). Over evolutionary time, natural selection may have favored
altruistic acts directed towards nonkin so long as such acts were likely to be reciprocated in
the future (Brown and Brown, 2006; Stewart-Williams, 2007). This theory rests on the
assumption that there is a system of mutual social contracts between individuals. Social
contracts facilitate helping behavior because they can promote feelings of indebtedness
among those who have been helped; indebtedness in turn facilitates the return of the favor.
Reciprocity and equity are important in friendships, both of which can create a system of
reciprocation (Robert, 2005; Silk, 2003; Tooby and Cosmides, 1996).

If a reciprocal altruism model of helping behavior explains why individuals help
others find LTMs and STMs, then one would expect that individuals should be more
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willing to help those of a similar age-cohort than those of a different age-cohort, because
those of the similar age-cohort are best suited to help because they have access to potential
others of the same age-cohort to arrange pairings.1 Those who are of different age-cohorts
are likely to not have access to numerous others to arrange such pairings. Alternatively, it
may be that individuals choose friends who are genetically similar (perhaps based on a
genetically assortative preference in friends) and thus are more likely to commit altruistic
acts towards them (Rushton, Russell, and Wells, 1984). However, Maner et al. (2002)
argued that we are more likely to help strangers who we perceive as genetically close to us:
often calling these people friends. It may also be that those in an age-cohort may have
fitness interdependence (Brown and Brown, 2006; Roberts, 2005). By fitness
interdependence, it is meant that individuals are tied together; their reproductive futures are
linked. For example, two individuals may benefit reproductively or socially by going out to
a nightclub together as opposed to going solo.
Helping others of an age-cohort find an LTM or an STM also comes with risks just
as it does for kin. Costs involved in helping others find a mate can be the loss of mating
opportunities themselves. By arranging a pairing between two others, the person is
neglecting their own reproduction and perhaps other more tangential facets of their lives.
Reciprocal altruism would predict that individuals play matchmaker because they are
hoping, to some degree, that the favor will be reciprocated. In fact, there is some evidence
that demonstrates that reciprocity is more likely among friends than kin (Essock-Vitale and
McGuire, 1985). We predict that individuals will be more willing to help those of a similar
age-cohort than those who are of a different age-cohort.

Parental investment as a theory of helping behavior
Lastly, parental investment theory (Trivers, 1972) predicts that, as a result of
different levels of obligatory investment in a sexual encounter, males and females should
differ in their mating behavior. For instance, since men can invest less in their offspring
than women, they are more likely to employ a short-term mating style as compared to
women (Schmitt, Shackelford, and Buss, 2001). The parental investment hypothesis would
predict that male relatives should be helped more than female relatives to find an STM.
Conversely, parental investment would predict that individuals should be more willing to
help their female relatives find LTMs over male relatives because the reproductive success
of females is increased more than males by the creation of pairbonds. Individuals may help
those who can benefit the most from their help in different mating domains. Males would
benefit more than females from help finding an STM, where females would benefit more
than males from help finding an LTM. In addition, the social costs of arranging a short-
term pairbond for women may be more costly for women than men and thus this pattern
would also emerge. We predict that individuals will be more willing to help their male
relatives than their female relatives to find STMs. We also predict that individuals will be
more willing to help their female relatives than their male relatives to find an LTM.


1 This is in contrast to common usage of the term matchmaker that evokes images of a well-connected older
woman who arranges pairings. While it is true that older people may have played a role in the formation of
long-term paribonds, it is the contention of the authors that most of the matchmaking that actually occurs is
among those of a similar age-cohort because they exist in similar if not the same peer groups.
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The current studies
Ironically, little attention has been paid to long-term relationships such as family
and friends in the context of helping behavior and instead has focused on short-term or
even non-existent relationships using college-student samples (Daly, Salmon, and Wilson,
1997). The irony lies in the fact that few studies addressing inclusive fitness have actually
used pre-existing long-term relationships like kin or friends and instead focus on
experimental methodologies to induce or reduce helping. Research on mating and helping
are hallmarks of evolutionary psychology, yet few have attempted to integrate these
domains.
The present studies test three evolutionary theories of helping behavior and how
they relate to individuals’ willingness to help others find both LTMs and STMs: inclusive
fitness, reciprocal altruism, and parental investment. Individuals may help others who share
their genes (inclusive fitness). Individuals may help those who they feel would be best
suited to return the favor (reciprocal altruism). Or individuals may be more inclined to help
males find STMs and females find LTMs (parental investment). This study will attempt to
test and differentiate between those theories in predicting willingness to help others find
LTMs and STMs.

Method
Study 1
Sample

One hundred eight participants (31% male) participated in this study for extra credit
in a psychology class. The mean age of the participants was 24 years (SD = 7.15; Range =
17 - 53). The sample consisted of single (25%), dating (55%), and married (20%)
participants.

Measures

Participants were asked how much (1 = not at all; 5 = very much) they were willing
to help a number of others (sister, brother, father, mother, friend, relative, cousin, parent,
neighbor, stranger, and acquaintance) find either short-term (casual sex) or long-term mates
(serious romantic relationship). Helping across each category of mating was calculated by
averaging all the relationship types (LTM Cronbach’s ? = .88; STM ? = .85).
To test for inclusive fitness effects, averaging items into indexes created six scales.
The helping of kin was composed of the items for willingness to help all those related to
the participant (brother, sister, father, mother, parents, relatives, and cousins) find an LTM
(? = .91) and an STM (? = .88). The willingness to help first-order kin was composed of
the items for those who were directly related to the participant (brother, sister, father,
mother, and parents) find an LTM (? = .87) and an STM (? = .83). The willingness to help
nonkin was composed of all items for those not related to the participants (friends,
strangers, neighbor, and acquaintance) find an LTM (? = .75) and an STM (? = .83). Two
single-item measures were used to measure willingness to help second-order relatives (i.e.,
cousins).
To test for reciprocal altruism effects, averaging items into indexes created four
scales. The age-cohort group was composed of all of those who were most likely to be of a
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similar age as the participant (friend, brother, sister, cousin, and acquaintance) on
willingness to help find an LTM (? = .76) and an STM (? = .82). The non-age-cohort group
(which is essentially a parent group due to limitations in the different relationship types
assessed) was created by averaging the items parents, mother, and father together (LTM ?
= .95; STM ? = .94).
To test for parental investment effects, averaging items into indexes created four
groups. Items for brother and father were averaged to create indexes for willingness to help
a male relative (LTM ? = .68; STM ? = .71). Items for mother and sister were averaged to
created indexes for willingness to help a female relative (LTM ? = .71; STM ? = .67).

Procedure

Participants received a packet that contained the materials. Participants first
completed an informed consent form. Participants were instructed to assume the individuals
who they will answer questions about are single (not involved in a romantic or sexual
relationship). They then completed the items regarding helping others find a long-term
dating partner. In efforts to reduce any carry-over effects a distractor task that involved the
participant counting backwards in threes from 35 was done next. Then participants
completed the items for willingness to help others find a short-term casual sex partner.
Lastly, they completed a brief demographics questionnaire and were debriefed.

Results

There were no main effects or interactions for sex of participant or dating status
(single, married, or dating) on the willingness to help anyone find an LTM or an STM.
Younger participants were less willing to help others find LTMs (r(102) = -.21, p < .05) or
STMs (r(102) = -.20, p < .05). Regardless of their relationship, participants were more
willing to help others find an LTM over an STM (t(101) = 5.85, p < .01, d = .45; MLTM =
2.59, SDLTM = .80, MSTM = 2.08, SDSTM = .81). This relationship generally held across all
relationship types as can be seen in Table 1. In addition, a correlation matrix is reported in
Table 2 that addresses bivariate correlations for willingness to help different individuals
find LTMs or STMs.

Testing inclusive fitness

Differing levels of relatedness may predict willingness to help others find both
LTMs and STMs. Degree of relatedness and mating type significantly interacted to predict
willingness to help (F(1, 101) = 40.50, p < .01, ?2 = .29). This interaction can be seen in
Figure 1. Participants were more willing to help kin find an LTM over nonkin (t(103) = -
3.41, p < .01, d = .38; MNonkin = 2.38, SDNonkin = .74, MKin = 2.73, SDKin = 1.07). Participants
were less willing to help kin find an STM compared to nonkin (t(103) = 2.53, p < .05, d =
.25; MNonkin = 2.21, SDNonkin = .95, MKin = 1.98, SDKin = .90).
Because second-order relative was only a single-item measure, it was excluded
from the interaction test and is reported here as a t test. Participants were more willing to
help their first-order relatives find an LTM than their cousins (t(103) = -1.87, p < .10, d =
.18; MFirst = 2.84, SDFirst = 1.18, MCousin = 2.63, SDCousin = 1.18) and nonkin (t(103) = -3.86,
p < .01, d = .47; MFirst = 2.84, SDFirst = 1.18, MNonkin = 2.38, SDNonkin = .74). Participants
were less willing to help their first order relatives find an STM than their cousins (t(103) =
4.60, p < .01, d = .45; MFirst = 1.89, SDFirst = .96, MCousin = 2.39, SDCousin = 1.23).
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Participants were more willing to help their cousin over nonkin find an LTM (t(106) =
2.72, p < .01, d = .18; MCousin = 2.64, SDCousin = 1.17, MNonkin = 2.39, SDNonkin = .73) and an
STM (t(104) = 1.98, p = .05, d = .12; MCousin = 2.41, SDCousin = 1.23, MNonkin = 2.22, SDNonkin
= .95).

Table 1. Descriptive and paired-sample t-test for willingness to help others find an LTM and an STM.
Relationship
STM Mean (SD)
LTM Mean (SD)
t d
Individuals
Relative
2.14 (1.21)
2.66 (1.12)
2.60*
0.45
Parent
1.60 (0.99)
2.39 (1.41)
5.71** 0.65
Mother
1.49 (1.01)
2.55 (1.48)
7.19** 0.84
Father
1.59 (1.12)
2.30 (1.45)
5.12** 0.55
Sister
2.09 (1.23)
3.25 (1.33)
7.35** 0.91
Brother
2.41 (1.31)
3.17 (1.25)
5.71** 0.59
Cousin
2.41 (1.23)
2.63 (1.15)
1.71
0.18
Friend
3.20 (1.27)
3.67 (1.08)
3.52** 0.40
Acquaintance
2.02 (1.14)
2.21 (1.04)
1.68
0.17
Neighbor
1.94 (1.50)
2.14 (0.93)
1.66
0.16
Stranger
1.72 (1.11)
1.49 (0.76)
-2.14* -0.24
Inclusive fitness groups
First-order
1.90 (0.96)
2.83 (1.16)
8.00** 0.87
All kin
2.00 (0.90)
2.71 (1.05)
6.85** 0.73
Non-kin
2.22 (0.95)
2.38 (0.72)
1.64
0.19
Reciprocal altruism groups
Age-cohort
2.97 (0.84)
2.42 (0.95)
5.55** 0.61
Different-age
1.56 (0.99)
2.41 (1.38)
6.60** 0.70
Parental investment groups
Male relatives
2.00 (1.07)
2.75 (1.19)
8.41** 0.66
Female relatives
1.80 (1.00)
2.90 (1.22)
6.41** 0.99
Note. N = 103 - 107, d = Cohen’s d, STM = Short-term mate. LTM = Long-term mate.
* p < .05, ** p < .01


No other comparisons were significant. Results supported an inclusive fitness
model for willingness to help find an LTM. The story for willingness to help others find
STMs seems to be more complicated than an inclusive fitness model would predict.





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Table 2. W illingnes s to help correlation matrix.
Sister
Brother
Mother
Father
Friend
Stranger
Cousin
Relative
Parent
Neighbor Acquaintance
Sister
--
0.64**
0.56**
0.40**
0.41**
0.22*
0.49**
0.48**
0.51**
0.15
0.22*
Brother
0.68**
--
0.42**
0.52**
0.60**
0.24*
0.59**
0.61**
0.55**
0.18
0.22*
Mother
0.51**
0.58**
--
0.82**
0.21*
0.21*
0.23*
0.35**
0.83**
0.08
0.04
Father
0.53**
0.56**
0.84**
--
0.27**
0.24*
0.25*
0.35**
0.89**
0.13
0.11
Friend
0.37**
0.33**
0.10
0.06
--
0.40**
0.60**
0.65**
0.23*
0.42**
0.44**
Stranger
0.19
0.13
0.27*
0.25*
0.23*
--
0.45**
0.56**
0.19
0.73**
0.66**
Cousin
0.54**
0.54**
0.44**
0.35**
0.44**
0.45**
--
0.84**
0.30*
0.46**
0.53**
Relative
0.53**
0.60**
0.44**
0.45**
0.45**
0.43**
0.85**
--
0.37**
0.57**
0.55**
Parent
0.47**
0.60**
0.91**
0.82**
0.16
0.28*
0.45**
0.51**
--
0.13
0.09
Neighbor
0.24*
0.15
0.16
0.15
0.39**
0.41**
0.46**
0.51**
0.18
--
0.70**
Acquaintance
0.09
0.10
0.07
-0.05
0.42**
0.49**
0.39**
0.42**
0.08
0.65**
--
Note. Thos e below the diagonal refer to willingnes s to help find a long-term mate. Those above the diagonal refer to willingnes s to
help find a short-term mate.
* p < .05, ** p < .01

Figure 1. Mean willingness to help others find long-term and short-term mates by relatedness.

5
e
l
p
4
h
g
n
e
s
s
to
3
illin
LTM
e
a
n
w
2
M
STM
1
First-order
Non-kin
Relatedness
Note. Interaction was significant (F(1, 101) = 15.69, p < .01, ?2 = .27).
Note. LTM = long-term mate, STM = short-term mate

Testing reciprocal altruism
It may be an age-cohort effect and not a relatedness effect that drives willingness to
help others find an STM and an LTM. To test this, those of the same cohort (sister, brother,
friend, cousin, and acquaintance) were compared to non-cohorts (parents, father, and
mother). Age-cohort and mating type significantly interacted to predict willingness to help
(F(1, 101) = 6.11, p < .05, ?2 = .06). This interaction can be seen in Figure 2. Participants
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were more willing to help their age-cohorts find an LTM (t(103) = -4.50, p < .01, d = .96;
MCohort = 2.98, SDCohort = 1.40, MNoncohort = 1.55, SDNoncohort = .85) and an STM (t(103) = -
8.30, p < .01, d = .79; MCohort = 2.41, SDCohort = .94, MNoncohort = 1.56, SDNoncohort = 1.02)
than those of a different age-cohort. Age of participants was negatively associated with
willingness to help age-cohorts find an LTM and an STM (both rs(101) = -.25, ps < .01).

Figure 2. Mean willingness to help others find a long-term or short-term mates by age groups
5
4
3
LTM
2
Mean willingness to help
STM
1
Age-cohort
Non age-cohort

Note. Interaction was significant (F(1, 101) = 6.11, p < .05, ?2 = .06).
Note. LTM = long-term mate, STM = short-term mate


Testing parental investment
Sex of relatives and mating pattern interacted to predict willingness to help (F(1,
101) = 15.21, p < .01, ?2 = .13) and can be seen in Figure 3. Participants were more willing
to help female relatives to find LTMs than male relatives and were slightly more willing to
help male relatives over female relatives to find STMs. Individuals were more willing to
help their male relatives find an STM over their female relatives (t(103) = 2.82, p < .01, d =
.30; MFemales = 1.78, SDFemales = 1.00, MMales = 2.00, SDMales = .10). Individuals were more
willing to help their female relatives find an LTM over their male relatives (t(103) = -2.40,
p < .05, d = .17; MFemales = 2.91, SDFemales = 1.23, MMales = 2.76, SDMales = .12). Results
supported a parental investment model. In one case, a significant sex difference emerged:
Female participants were more willing to help their female relatives find an STM (t(103) =
-2..55, p < .05, d = .58; MFemale = 1.94, SDFemale = 1.08, MMale = 1.42, SDMale = .65).
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Figure 3. Mean willingness to help others find long-term and short-term mates by sex of relative.
5
4
3
LTM
2
Mean willingness to help
STM
1
Male
Female
Sex of relative

Note. Interaction was significant (F(1, 101) = 15.21, p < .01, ?2 = .13).
Note. LTM = long-term mate, STM = short-term mate

Discussion

All three theories appear to explain why individuals help others to find mates. Of
these three however, the most convincing were the findings that individuals help kin find
LTMs over nonkin and that an age-cohort effect was associated with reciprocal altruism.
However, the non-age-cohort group was confounded in that it was simply a group of
parents. Individuals may be unwilling to help or think about their parents becoming
sexually or romantically involved and this might have affected the results. A further study
was conducted next to attempt to address this potential confound.

Study 2

Sample
There were one hundred forty two participants (29% male) in this study who
received extra credit in a psychology class. The sample consisted of single (28%), dating
(57%), and married (15%) participants. Participants were randomly assigned to an age-
Evolutionary Psychology – ISSN 1474-7049 – Volume 5(4). 2007. -725-

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