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Altruists Attract

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Explaining human cooperation continues to present a challenge because it goes beyond what is predicted by established theories of kinship and reciprocal altruism. Little attention has been paid to the sexual selection hypothesis that proposes that cooperation can act as a display that attracts mates. The costs of cooperating are then offset not by kinship or reciprocation but by increased mating success. Here we present results from a series of experiments which show that, as predicted by the sexual selection hypothesis, people preferentially direct cooperative behavior towards more attractive members of the opposite sex. Furthermore, cooperative behavior increases the perceived attractiveness of the cooperator. Economically costly behaviors can therefore bring benefits through mate choice and sexual selection should be regarded as an evolutionary mechanism capable of promoting cooperation.
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Evolutionary Psychology
www.epjournal.net – 2007. 5(2): 313-329
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Original Article
Altruists Attract

Daniel Farrelly, School of Biology (Psychology), Newcastle University, Newcastle-upon-Tyne, UK. Email:
daniel.farrelly@sunderland.ac.uk (Corresponding author) (Current address: Department of Psychology,
University of Sunderland, Sunderland, UK.)

John Lazarus, School of Biology (Psychology), Newcastle University, Newcastle-upon-Tyne, UK.

Gilbert Roberts, School of Biology (Psychology), Newcastle University, Newcastle-upon-Tyne, UK.

Abstract: Explaining human cooperation continues to present a challenge because it goes
beyond what is predicted by established theories of kinship and reciprocal altruism. Little
attention has been paid to the sexual selection hypothesis that proposes that cooperation can
act as a display that attracts mates. The costs of cooperating are then offset not by kinship
or reciprocation but by increased mating success. Here we present results from a series of
experiments which show that, as predicted by the sexual selection hypothesis, people
preferentially direct cooperative behavior towards more attractive members of the opposite
sex. Furthermore, cooperative behavior increases the perceived attractiveness of the
cooperator. Economically costly behaviors can therefore bring benefits through mate
choice and sexual selection should be regarded as an evolutionary mechanism capable of
promoting cooperation.

Keywords: Altruism, cooperation, sexual selection, attractiveness.

¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯¯
Introduction

Why are humans so cooperative? The question continues to challenge because
human cooperation goes beyond what is predicted by established theories of kinship
(Hamilton, 1964) and reciprocal altruism (Trivers, 1971). For example, people often
cooperate in one-shot Prisoner’s Dilemma games despite the rational choice being to defect
(e.g. Colman, 2003; Fehr and Fischbacher, 2003). This has led to a number of new ideas,
including ‘indirect reciprocity’ (Alexander, 1987; Nowak and Sigmund, 1998), in which,
through reputation building, cooperative acts are reciprocated by individuals other than the
recipient, and ‘strong reciprocity’ (Gintis, 2000) - a tendency to cooperate with others and
punish defectors, possibly as a result of cultural group selection (Boyd, Gintis, Bowles, and

Altruists Attract
Richerson, 2003). It has also been proposed that cooperation and altruism sometimes
function as sexually selected displays that attract mates, either directly or through
reputation (Miller, 2000; Roberts, 1998; Tessman, 1995; Zahavi, 1995). This last proposal
has received little empirical testing and is the focus of our paper.
Sexual selection is responsible for traits ranging from complex songs to elaborate
ornamentation, which are advertised to potential mates (Andersson, 1994). Like many such
traits, altruism is, by definition, costly and therefore deleterious to survival. However, just
as these other traits function in courtship, so altruism could function as a sexual display
(Miller, 2000; Roberts, 1998; Tessman, 1995; Zahavi, 1995). Investment in altruistic
displays would then reap rewards not through reciprocation or increased survival of
relatives but through enhanced mating success. As in other cases of sexually-selected
characters, the benefits of attending to the display of altruism could come through those
that accrue from mating with high quality individuals. Altruism is a valuable trait in both
males and females, in the long-term relationships of humans, since it allows the efficient
sharing of resources - gained by either adult in the partnership - between both adults and
their offspring, in response to need.
Models based on Zahavi’s Handicap Principle (Zahavi, 1995) show that altruism
can be favored when it acts as a costly signal of quality (Gintis, Smith, and Bowles, 2001;
Lotem, Fishman, and Stone, 2003). Signalers (altruists) benefit through improving their
reputation, which attracts mates; mates benefit through obtaining information about partner
quality; and signal honesty is assured through condition dependence. An important feature
of the signaling hypothesis is that it can explain acts of altruism that are not reciprocated,
either directly or indirectly. It is important to appreciate the difference between reciprocity
and signaling here. Reciprocal altruism, whether direct or indirect, involves two individuals
benefiting others at some cost to themselves. Unless one is to argue that mating is an
altruistic act that reduces one’s fitness, then preferring to mate with an altruistic individual
cannot be considered to be an example of reciprocity, whether direct or indirect. To extend
Haldane’s famous remark, kinship can explain rescuing drowning people if they are
relatives (Hamilton, 1964); reciprocal altruism if they return the favor (Trivers, 1971);
indirect reciprocity if a third party returns the favor (Alexander, 1987) and signaling if the
rescuer is judged more attractive (Roberts, 1998).
There is suggestive evidence that cooperative individuals are more attractive:
kindness is a highly preferred trait in mating partners (Buss, 1989; Jensen-Campbell,
Graziano, and West, 1995), Meriam hunters who share costly turtle meat have greater
reproductive success (Smith and Bliege Bird, 2000; Smith, Bird, and Bird., 2003), and
there is evidence for ‘showing off’ in some sub-groups of male Ache hunters (Wood and
Hill, 2000). There is also evidence for sex and attractiveness effects on altruism: males on
their own give more to female than to male beggars (Goldberg, 1995) and attractive people
are offered more in Ultimatum and Dictator games (Solnick and Schweitzer, 1999). Recent
reviews have considered the role that costly signaling may play in explaining cooperative
behavior (Fehr and Fischbacher, 2003; Gurven, 2004; Henrich et al., 2005; Smith et al.,
2003) and one of these concluded that there was no experimental evidence for altruistic
costly signaling (Fehr and Fischbacher, 2003). Here we provide evidence for altruistic
costly signaling in the context of sexual selection. Our experiment examines the influence
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Altruists Attract
of the quality of potential mates on the signaling of cooperative and altruistic motivation.
We are not concerned with the signaling of the ability to give (i.e. resource wealth) and
therefore hold this factor constant in the experiments.
We tested four key predictions of the sexual selection hypothesis that altruism or
cooperation is a signal to potential mates using a series of experimental cooperation games.
First, participants should be more cooperative with individuals of the opposite sex. Second,
participants should be more cooperative when opposite sex partners are more attractive,
and therefore of higher mate quality. Third, the strength of the signal should increase with
the strength of sexual selection. Therefore, as females are the choosier sex, males are under
stronger sexual selection and should therefore show a stronger cooperative bias in the first
two predictions. Finally, for the signal to benefit the signaler, receivers should be more
likely to mate with signalers. Thus more cooperative partners should be judged to be more
attractive.

Materials and Methods

Our experiment involved asking participants to play a series of cooperative game
scenarios with a series of partners differing in sex and attractiveness. Partners were
presented as facial images in a web-based interface (faces being key components of
attractiveness judgements, e.g. Penton-Voak et al., 1999). Participants were led to believe
they were interacting with other participants in the experiment, although in fact the partners
were virtual and their responses pre-programmed. Also, participants were told that they
would be able to contact any individual they would interact with in the study privately in
the future, via e-mail (however this was not true; participants were informed of this
deception at the end of the study). This possibility of future interactions created an
environment in which mate choice displays could be expected to occur.
There were 231 participants; 85 males (mean age=24.4, SD=6.4) and 146 females
(mean age=22.8, SD=5.2). Participants were students at Newcastle University and took part
voluntarily. They were recruited via e-mails to several degree programmes and posters
placed around the university campus. As an incentive for participating, there were cash
prizes of £25 for each of two participants, selected at random. The ethics committee of
Newcastle University approved the experimental protocol. All participants gave informed
consent online before the experiment began.
The entire experimental procedure was conducted on-line. The facial pictures
(which participants were told were of other participants) were from a database of pictures
supplied by Ian Penton-Voak, University of Stirling. These pictures were frontal, in colour
and individuals were photographed with neutral expressions. The pictures had previously
been rated for attractiveness on a scale of 1-5; we used the top and bottom three rated
pictures for both sexes. These pictures were randomly assigned to three sets, with each set
containing a high attractive male (MH), a low attractive male (ML), a high attractive
female (FH) and a low attractive female (FL). Each participant received a single, randomly
assigned, picture set.
Participants played four games that were chosen to investigate different aspects of
cooperation and altruism. In the mutualism game, participants do better by donating
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Altruists Attract
(cooperating), regardless of what their partner does. This leaves no room for displaying
altruism, and the game tests whether participants have the understanding and motivation to
behave in their economic interests. In the one-shot Prisoner’s Dilemma (PD) game, the
rational decision is defection (e.g. Colman, 1998), but cooperation is the rational choice for
those who have a sense of fairness (Fehr and Fischbacher, 2002) and who trust the partner
also to cooperate. An alternative explanation for cooperating in this game is that it displays
a cooperative tendency towards the partner even if this is costly. In the Dictator game, it is
again rational to offer nothing, so giving may reflect a sense of fairness, or generosity. We
also adapted the Dictator game to a scenario in which participants were told their offers
would go to charity rather than the partner. Again, this allows displays of altruism, but
differs in that the beneficiary is not the partner, emphasizing the signaling aspect of
altruism. We refer to these two versions of the Dictator game as the standard Dictator (SD)
and charity Dictator (CD) games. As well as the rational choices stated above it is always
possible that participants have not understood the games and act under misguided notions
of the consequences of their actions. However, the games were straightforward and
instructions clear.
Participants played these four games against four other individuals (one each of the
MH, ML, FH and FL images), playing all four games with each individual. The
instructions for all the games were presented to the participants at the start. In the
mutualism game, participants started with an imaginary amount of £10 and were told that if
they donated £10 to a public fund and their partner did too, then both would receive £20 in
return. They were also told that if only one or neither player donated, then each would
either keep or have their £10 returned to them. In the one-shot PD game, participants again
started with an imaginary amount of £10 and were told that if they donated £10 to a public
fund and their partner did as well, each would receive £14; if one player donated and the
other did not, the donor received only £7 whereas the other player received £17; and if
neither player donated they would each keep their initial £10. In the SD game, participants
were told: “In this game, you should imagine the following scenario: You are out shopping
one afternoon. You spot a £10 note on the ground. After you pick up the note, an individual
walks up to you and says, ‘Aah!! You beat me to it! I spotted the note too, and was about to
pick it up myself.’ How much would you offer them (anything from £0 to £10)?” The CD
game used the same instructions, except that the penultimate sentence read: “After you pick
up the note, an individual walks up to you and says, ‘Aah!! You beat me to it! I spotted the
note too, and was about to pick it up myself and give it to the charity I am collecting for. It
would help us greatly’” (in both the SD and CD game, participants were told that the
individual referred to in the scenario was the same as the individual in the current photo).
Participants were then told that the four individuals they would interact with had
already taken part in the study and had made the decisions about what they would do in all
four games without any knowledge of whom they were playing with. Participants then
played all four games with each of the four partners, with the order of both the game type
and the partner identity being randomized. There were full reminders of the instructions for
each game available on the screen when participants were playing them. Participants were
then asked the following question: “Please rate this individual out of 10 for how you
perceive them to be for the following attributes (1 = Low, 10 = High).” There were four
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Altruists Attract
attributes in total and they were ‘intelligence’, ‘attractiveness’, ‘cooperativeness’ and
‘interesting’. Only the ratings of attractiveness were relevant here, the remaining
characteristics being included to disguise the aims of the study and reduce demand
characteristics. In each game, participants were asked to make a decision (either ‘give’ or
‘keep’ in the mutualism and PD game, or to invest between £0 and £10 in the two Dictator
games). The picture of the current partner was on the screen throughout each game. Once
all the games had been completed, participants were presented with the decisions that each
partner had supposedly made in these games.
Participants were allocated randomly to one of two conditions; in one condition all
partners had been highly cooperative when they played the game (74 females, 50 males)
and in the other condition they had been less cooperative (72 females, 35 males) (Table 1).
Finally, participants rated each partner again for intelligence, attractiveness,
cooperativeness and how interesting they thought they were. Afterwards, participants
received a full de-briefing about the experiment in an e-mail.

Table 1: The decisions of partners for each cooperativeness condition.

Game
Highly cooperative
Less cooperative
Mutualism Invest
Keep
One-off Prisoner’s Dilemma
Invest
Keep
Standard Dictator
Offer £5
Offer £2
Charity Dictator
Offer £5
Offer £2

Due to the within-subject design of the experiments, paired tests were used where
appropriate. Non-parametric tests were used when data diverged from a normal distribution
or when analyzing ratings. All significance levels are for two-tailed tests.

Results

In the mutualism game, all participants chose to donate. In the one-shot PD game,
neither males nor females showed a sex bias in their tendency to cooperate (Wilcoxon
matched pairs test between the sums of cooperative moves to female versus male partners
by female participants: Z=-0.744, N-ties=53, p=0.457, and by male participants: Z=-0.860,
N-ties=24, p=0.390). However, both sexes cooperated more with more attractive members
of the opposite sex (Fig. 1): males were more likely to cooperate with an FH as opposed to
an FL partner (McNemar test: binomial, N=85, p=0.04), while females were more likely to
cooperate with an MH than with an ML partner (McNemar test: ?2=4.694, N=146, p=0.03).
In neither sex was there a significant effect of attractiveness on cooperation towards
partners of their own sex (female participants with FH versus FL partners: ?2= 0.39,
N=146, p=0.532; male participants with MH versus ML partners: binomial, N=146,
p=0.286).



Evolutionary Psychology – ISSN 1474-7049 – Volume 5(2). 2007. -317-





Altruists Attract
Figure 1: Percentage of cooperative choices made in a one-shot Prisoner’s Dilemma game
for (a) female and (b) male participants. Bars show percentage of cooperative choices when
interacting with male and female partners of high and low attractiveness.

a. Female participants

b. Male participants

In the SD game, males showed a strong preference for giving to females rather than
other males (Wilcoxon Matched Pairs Test: Z=4.564, N-ties=33, p<0.0005; Fig. 2a) but
females showed no sex preference (Z=-0.062, N-ties=37, p=0.95; Fig. 2a). Both sexes
Evolutionary Psychology – ISSN 1474-7049 – Volume 5(2). 2007. -318-





Altruists Attract
showed a preference for giving to more attractive members of the opposite sex (male
participants with FH versus FL partners: Z=2.594, N-ties=24, p=0.009; female participants
with MH versus ML partners: Z=3.723, N-ties=39, p<0.0005; Fig. 3a), yet showed no
attractiveness preference when giving to their own sex (male participants with MH versus
ML partners: Z=0.548, N-ties=17, p=0.583; female participants with FH versus FL
partners: Z=1.559, N-ties=26, p=0.119; Fig. 3a).

Figure 2: Sex preferences in offers in (a) the Standard Dictator and (b) the Charity
Dictator Game. The figure shows the mean differences (± standard error) in the offers made
to female versus male partners, calculated as the sum of offers made to high and low
attractiveness females minus the sum of offers made to high and low attractiveness males,
for male and female participants respectively.

a. Standard Dictator Game
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Altruists Attract

b. Charity Dictator Game
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Altruists Attract
Figure 3: Mean differences in offer (high-low, ± standard error) between high and low
attractive male partners (HM and LM) and between high and low attractive female partners
(HF and LF) for male and female participants respectively in (a) the Standard Dictator and
(b) the Charity Dictator game.

a. Standard Dictator Game
Evolutionary Psychology – ISSN 1474-7049 – Volume 5(2). 2007. -321-





Altruists Attract

b. Charity Dictator Game

Much higher offers were given in the CD than the SD game (Z=-12.199, N-
ties=200, p<0.0005, Fig. 4). Nevertheless, there was a similar pattern: as in the SD game,
males gave more to females than to males (Z=-2.321, N-ties=33, p=0.020; Fig. 2b),
although unlike the SD game females also gave more to female partners than to males (Z=-
3.608, N-ties=51, p<0.0005; Fig. 2b). Attractiveness effects were similar to the SD game
with females preferring high over low attractive males (Z=-4.000, N-ties=45, p<0.0005;
Fig. 3b) and males preferring high over low attractive females (Z=-2.371, N-ties=28,
p=0.018; Fig. 3b). Again neither sex showed an attractiveness bias when interacting with
their own sex (females Z=-1.933, N-ties=33, p=0.053; males Z=-1.045, N-ties=21, p=0.296;
Fig. 3b).











Evolutionary Psychology – ISSN 1474-7049 – Volume 5(2). 2007. -322-





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