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A mutualistic approach to morality:
The evolution of fairness by
partner choice
Nicolas Baumard
Institute of Cognitive and Evolutionary Anthropology, University of Oxford,
Oxford OX2 6PN, United Kingdom; and Philosophy, Politics, and Economics
Program, University of Pennsylvania, Philadelphia, PA 19104
nbaumard@gmail.com
Jean-Baptiste André
Laboratoire Ecologie et Evolution, UMR 7625, CNRS – Ecole Normale
Supérieure, 75005 Paris, France
jeanbaptisteandre@gmail.com
http://jb.homepage.free.fr
Dan Sperber
Institut Jean Nicod, ENS, EHESS, CNRS, 75005 Paris, France; and
Department of Cognitive Science and Department of Philosophy, Central
European University, 1051 Budapest, Hunga ry
dan@sperber.fr http://www.dan.sperber.fr
Abstract: What makes humans moral beings? This question can be understood either as a proximate “how” question or as an ultimate
“why” question. The “how” question is about the mental and social mechanisms that produce moral judgments and interactions, and has
been investigated by psychologists and social scientists. The “why” question is about the fitness consequences that explain why humans
have morality, and has been discussed by evolutionary biologists in the context of the evolution of cooperation. Our goal here is to
contribute to a fruitful articulation of such proximate and ultimate explanations of human morality. We develop an approach to
morality as an adaptation to an environment in which individuals were in competition to be chosen and recruited in mutually
advantageous cooperative interactions. In this environment, the best strategy is to treat others with impartiality and to share the costs
and benefits of cooperation equally. Those who offer less than others will be left out of cooperation; conversely, those who offer
more will be exploited by their partners. In line with this mutualistic approach, the study of a range of economic games involving
property rights, collective actions, mutual help and punishment shows that participants’ distributions aim at sharing the costs and
benefits of interactions in an impartial way. In particular, the distribution of resources is influenced by effort and talent, and the
perception of each participant ’s rights on the resources to be distributed.
Keywords: cooperation; fairness; economic games; evolutionary psychology; morality; partner choice
1. Introduction
Humans don’t just cooperate. They cooperate in a great
variety of quite specific ways and have strong views in
each case on how it should be done (with substantial cul-
tural variations). In collective actions aimed at a common
goal, there is a right way to share the benefits: Those
who have contributed more should receive more. When
helping others, there is a right amount to give. One may
have the duty to give a few coins to beggars in the street,
but one does not owe them half of one’s wealth, however
helpful it would be to them. When people deserve to be
punished, there is a right amount of punishment. Most
people in societies with a modern penal system would
agree that a year in jail is too much for the theft of an
apple and not enough for a murder. People have strong
intuitions regarding the right way to share the benefits of
activity, the right way to help the needy, and the right
way to punish the guilty. Do these intuitions, notwithstand-
ing their individual and cultural variability, have a common
logic, and, if so, to what extent is this logic rooted in evo lved
dispositions?
To describe the logic of morality, many philosophers
have noted that when humans follow their moral intuitions,
they behave as if they had bargained with others in order to
reach an agr eement abou t the distribution of the benefits
and burdens of cooperation (Gauthier 1986; Hobbes
1651; Kant 1785; Locke 1689; Rawls 1971; Scanlon
1998). Morality, these “contractualist” philosophers argue,
is about maximizing the mutual benefits of interactions.
The contract analogy is both insightful and puzzling. On
the one hand, it well captures the pattern of moral intui-
tions, and to that extent well explains why humans
cooperate, why the distribution of benefits should be
BEHAVIORAL AND BRAIN SCIENCES (2013) 36,59–122
doi:10.1017/S0140525X11002202
© Cambridge University Press 2013 0140-525X/13 $40.00 59
proportionate to each cooperator’s contribution, why the
punishment should be proportionate to the crime, why
the rights should be proportionate to the duties, and so
on. On the other hand, it provides a mere as-if explanation:
It is as if people had passed a contract – but since they
didn’t, why should it be so?
To evolutionary thinkers, the puzzle of the missing con-
tract is immediately reminiscent of the puzzle of the
missing designer in the design of life-forms, a puzzle essen-
tially resolved by Darwin’ s theory of natural selection. Actu-
ally, two contractualist philosophers, John Rawls and David
Gauthier, have argued that moral judgments are based on a
sense of fairness that, they suggested, has been naturally
selected. Here we explore this possibility in some detail.
How can a sense of fairness evolve?
2. Explaining the evolution of morality
2.1. The mutualistic theory of morality
2.1.1. Cooperation and morality
. Hamilton ( 1964a; 1964b)
famously classified forms of social interaction between an
“actor” and a “recipient” according to whether the conse-
quences they entail for actor and recipient are beneficial
or costly (with benefits and costs measured in terms of
direct fitness). He called behavior that is beneficial to the
actor and costly to the recipient (+/−) selfishness, behavior
that is costly to the actor and beneficial to the recipient
(−/+) altruism, and behavior that is costly to the actor
and costly to the recipient (−/−) spite. Following a
number of authors (Clutton-Brock 2002; Emlen 1997;
Gardner & West 2004; Krebs & Davies 1993; Ratnieks
2006; Tomasello et al. submitted), we call behavior that is
beneficial to both the actor and the recipient (+/+) mutual-
ism.
1
Cooperation is social behavior that is beneficial to the
recipient, and hence cooperation can be altruistic or
mutualistic.
Not all cooperative behavior, whether mutualistic or
altruistic, is moral behavior. After all, cooperation is
common in and across many living species, including
plants and bacteria, to which no one is tempted to attribute
a moral sense. Among humans, kin altruism and friendship
are two cases of cooperative behavior that is not necessarily
moral (which is not to deny that being a relative or a friend
is often highly moralized). Unlike kin altruism, friendship is
mutualistic. In both cases, however, the degree of coopera-
tiveness is a function of the degree of closeness – genealogi-
cal relatedness in the case of parental instinct (Lieberman
et al. 2007), affective closeness typically linked to the force
of common interests in the case of friendship (DeScioli &
Kurzban 2009;Roberts2005). In both cases, the parent or
the friend is typically disposed to favor the offspring or the
close friend at the expense of less closely related relatives
or less close friends, and to favor relatives and friends at
the expense of third parties.
Behavior based on parental instinct or friendship is
aimed at increasing the welfare of specific individuals to
the extent that this welfare is directly or indirectly ben-
eficial to the actor. These important forms of cooperation
are arguably based on what Tooby et al. (2008) have
described as a Welfare Trade-Off Ratio (WTR). The
WTR indexes the value one places on another person’s
welfare and the extent to which one is disposed, on that
basis, to trade off one’s own welfare against the welfare
of that person (for an example, see Sell et al. 2009). The
WTR between two individuals is predicted to be a function
of the basic interdependence of their respective fi tness (see
also Rachlin & Jones [2008] on social discounting). Choices
based on WTR considerations typically lead to favoritism
and are quite different from choices based on fairness
and impartiality. Fairness may lead individuals to give
resources to people whose welfare is of no particular inter-
est to them or even to people whose welfare is detri mental
to their own. To the extent that morality implies impartial-
ity,
2
parental instinct and friendship are not intrinsically
moral.
Forms of cooperation can evolve without morality, but it
is hard to imagine how morality could evolve without
cooperation. The evolution of morality is appropriately
approached within the wider framework of the evolution
of cooperation. Much of the recent work on the evolution
of human altruistic cooperation has focused on its conse-
quences for morality, suggesting that human morality is
first and foremost altruistic (Gintis et al. 2003; Haidt
2007; Sober & Wilson 1998). Here we focus on the evol-
ution and consequences of mutualistic cooperation.
Advances in comparative psychology suggest that, during
their history, humans evolved new skills and motivations
for collaboration (intuitive psychology, social motivation,
linguistic communication) not possessed by other great
apes (Tomasello et al., submitted). We arg ue that morality
NICOLAS BAUMARD is a post-doctoral fellow at the Uni-
versity of Pennsylvania. Inspired by contractualist the-
ories, his work is based on the idea that morality aims
at sharing the costs and benefits of social interactions
in a mutually advantageous way. This theory has led to
a book and a series of articles in evolutionary biology,
experiment psychology, cognitive anthropology, and
moral philosophy.
J
EAN-BAPTISTE ANDRÉ is a junior scientist at the
CNRS, working in the Ecology and Evolution Lab in
Paris. He is a theoretician in evolutionary biology.
Having earned his Ph.D. in microbial evolution, he is
currently developing evolutionary models on the foun-
dations of human cooperation and morality.
D
AN SPERBER is a French social and cognitive scientist.
He is Professor of Philosophy and Cognitive Science at
the Central European University, Budapest, and Direc-
teur de Recherche Emeritus at the Institut Jean Nicod
(CNRS, ENS, and EHESS, Paris). He is the author of
Rethinking Symbolism (Cambridge University Press,
1975), On Anthropological Knowledge: Three Essays
(Cambridge University Press, 1985), and Explaining
Culture: A Naturalistic Approach (Blackwell, 1996);
the coauthor with Deirdre Wilson of Relevance: Com-
munication and Cognition (1986; Wiley-Blackwell,
second revised edition, 1995) and Meaning and Rel-
evance (Cambridge University Press, 2012); the editor
of Metarepresentations: A Multidisciplinary Perspective
(Oxford University Press, 2000); the coeditor with David
Premack and Ann James Premack of Causal cognition:
A Multidisciplinary Debate (Oxford University Press,
1995), and, with Ira Noveck, of Experimental Prag-
matics (Palgrave Macmillan, 2004).
Baumard et al.: A mutualistic approach to morality
60
BEHAVIORAL AND BRAIN SCIENCES (2013) 36:1
may be seen as a consequence of these cooperative inter-
actions and emerged to guide the distribution of gains
resulting from these interactions (Baumard 2008; 2010a).
Note that these two approaches are not mutually incompa-
tible. Humans may well have both altruistic and mutualistic
moral dispositions. While a great deal of important research
has been done in this area in recent decades, we are still far
from a definite picture of the evo lved dispositions under-
lying human morality. Our goal here is to contribute to a
rich ongoing debate by highlighting the relevance of the
mutualistic approach.
2.1.2. The evolution of cooperation by partner
choice
. Corresponding to the distinction between altruistic
and mutualistic cooperation, there are two classes of
models of the way in which cooperation may have
evolved. Altruistic models describe the evolution of a dispo-
sition to engage in cooperative behavior even at a cost to
the actor. Mutualistic models describe the evolution of a
disposition to engage in cooperation that is mutually ben-
eficial to actor and recipient (see Fig. 1).
Mutualistic models are themselves of two main types:
those focusing on partner control and those focusing on
partner choice (Bshary & Noë 2003).
3
Earlier mutualistic
models were of the first type, drawing on the notion of reci-
procity as defined in game theory (Luce & Raiffa 1957; for
a review, see Aumann 1981) and as introduced into evol-
utionary biology by Trivers (1971).
4
These early models
used as their paradigm case iterated Prisoner’s Dilemma
games (Axelrod 1984; Axelrod & Hamilton 1981). Partici-
pants in such games who at any time fail to cooperate
with their partners can be penalized by them in subsequent
trials as in Axelrod’s famous tit-for-tat strategy, and this way
of controlling one’s partner might in prin ciple stabilize
cooperation.
In partner control models, partners are given rather than
chosen, and preventing them from cheating is the central
issue. By contrast, in more recently developed partner
choice models , individuals can choose their partners and
the emphasis is less on preventing cheating than in choos-
ing and being chosen as the right partner (Bull & Rice
1991; Noë et al. 1991; Roberts 1998).
5
Consider, as an illus-
tration, the relationship of the cleaner fish Labroides dimi-
diatus with client reef fish. Cleaners may cooperate by
removing ectoparasites from clients, or they may cheat by
feeding on client mucus. As long as the cleaner eats just
ectoparasites, both fish benefit from the interaction.
When, on the other hand, a cleaner fish cheats and eats
mucus, field observations and laboratory experiments
suggest that clients respond by switching partners, fleeing
to another cleaner, and thereby creating the conditions
for the evolution of cooperative behavior among cleaners
(Adam 2010; Bshary & Grutter 2005). Reciprocity can
thus be shaped either by partner choice or by partner
control only.
Mutually beneficial cooperation might in principle be
stabilized either by partner control or by partner choice
(or, obviously, by some combination of both). Partner
control and partner choice differ from each other with
respect to their response to uncooperativeness, which is
generally described as “defection” or “cheating.” In
partner-control models, a cooperator reacts to a cheating
partner by cheating as well, thereby either causing the
first cheater to return to a cooperative strategy or turning
the interaction into an unproductive series of defections.
In partner-choice models, on the other hand, a cooperator
reacts to a partner’s cheating by starting a new cooperative
relationship with another hopefully more cooperative
partner. Whereas in partner-control models, individuals
only have the choice between cooperating and not cooperat-
ing with their current partner, in partner-choice models,
individuals have the “outside option” of cooperating with
someone else. This difference has, we will see, major
implications.
6
The case of cleaner fish illustrates another important
feature of partner choice. In partner-choice models, the
purpose of switching to another partner is not to inflict a
cost on the cheater and thereby punish him. It need not
matter to the switcher whether or not the cheater suffers
as a consequence. A client fish switching partners is indif-
ferent to the fate of the cleaner it leaves behind. All it
wants in switching partners is to benefit from the services
of a better cleaner. Still, cheating is generally made costly
by the loss of opportunities to cooperate at all, and this
may well have a dissuasive effect and contribute to stabilize
cooperation. The choice of new partners is particularly
advantageous when it can be based on information about
their past behavior. Laboratory experiments show that
reef fish clients gather information about cleaners’ behavior
and that, in response, cleaners behave more cooperatively
in the presence of a potential client (Bshary & Grutter
2006).
The evolution of cooperation by partner choice can be
seen as a special case of social selection, which is a form
of natural selection where the selective pressure comes
from the social choices of o ther individuals (Dugatkin
1995; Nesse 2007;West-Eberhard197 9). Sexual selection
by female choice is the best-known type of social selec-
tion. Female bias for mating with or namented males
selects for more elaborate m ale displays, and the advan-
tages of having sons with extreme displays (and perhaps
advantages from getting good genes) select for stronger
preferences (Grafen 1990). Similarly, a s ocially wide-
spread preference for reliable partners selects for psycho-
logical dispositions that foster reliability. When we talk of
social selection in the rest of this article, we always refer to
the special case of the social selection of dispositions to
cooperate.
Figure 1. Evolutionary models of cooperation
Baumard et al.: A mutualistic approach to morality
BEHAVIORAL AND BRAIN SCIENCES (2013) 36:1 61