the cognitive and communicative demands of cooperation - cwi

Chapter 1The Cognitive and Communicative Demands ofCooperation

Peter Gardenfors

Abstract I argue that the analysis of different kinds of cooperation will benefit froman account of the cognitive and communicative functions required for that coopera-tion. I review different models of cooperation in game theory—reciprocal altruism,indirect reciprocity, cooperation about future goals, and conventions—with respectto their cognitive and communicative prerequisites. The cognitive factors consideredinclude recognition of individuals, memory capacity, temporal discounting, prospec-tive cognition, and theory of mind. Whereas many forms of cooperation require nocommunication or just simple communication, more advanced forms that are uniqueto humans presuppose full symbolic communication.

1.1 The span of cooperation in game theory

The evolution of cooperation remains an enigma. In surprisingly many situations,bothanimals and humans fail to behave as predicted by game theory. For example,humans cooperate more in prisoner’s dilemma games than would be expected fromlogical analysis. Several explanations for this mismatch have been suggested, rang-ing from the claim that animals and humans are not fully rational to the claim thatthe kind of rationality presumed in classical game theory is not relevant to evolution-ary arguments. The purpose of this chapter is to argue that the behavior of variousagents must be judged in relation to their cognitive and communicative capacities.In my opinion, these factors have not been sufficiently considered in game theory.

There are many ways of defining cooperation. A broad definition is that it consistsof joint actions that confer mutual benefits.1

Peter GardenforsLund University Cognitive Science, Sweden e-mail: [email protected]

1 A stronger criterion, focused on human cooperation, is formulated by Bowles and Gintis [8]: “Anindividual behavior that incurs personal costs in order to engage in a joint activity that confers ben-

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2 Peter Gardenfors

A narrower definition concerns situations in which joint action poses a dilemma,so that, in the short run, an individual would be better off not cooperating, eventhough in the long run cooperation may be preferable [66, p. 358]. The typical ex-ample of such a situation is some variant on the prisoner’s dilemma game. In thischapter, the focus will be on the narrower concept.

Conventionally, cooperation has been studied from two perspectives. First, in tra-ditional game theory, cooperation has been assumed to take place between individu-als of the species Homo economicus, who are ideally rational. Homo economicus ispresumed to have a perfect theory of mind: that is, the capacity to judge the beliefs,desires, and intentions of the other players. In a Bayesian framework, this amountsto assuming that all the other players are Bayesian decision makers. In the case ofHomo sapiens, that theory of mind is well developed, at least in comparison to otheranimal species, but it is far from perfect [27].

Second, cooperative games have been studied from an evolutionary perspec-tive. The classical accounts are found in Axelrod and Hamilton [2] and MaynardSmith [52]. Here, the “players” are the genes of various animal species. The genesare assumed to have absolutely no rationality nor any cognitive capacities. How-ever, their strategies can slowly adapt, via the mechanisms of natural selection, overrepeated interactions and generations. In the evolutionary framework, the theory ofmind of the “players” is either not accounted for or not considered relevant.

The differences in cognitive and communicative demands between these two per-spectives are seldom discussed. For example, in Lehmann and Keller’s [45] classifi-cation of models for the evolution of cooperation and altruism, only two parametersrelated to cognition and communication are included: a one-period “memory” pa-rameter m defined as the probability that “an individual knows the investment intohelping of its partner at the previous round” and a “reputation” parameter q definedas the probability that “an individual knows the image score of its partner” [45, p.1367].

I believe that the analysis of different kinds of cooperative games would benefitfrom a richer account of the cognitive and communicative functions required forthe cooperation. The cognitive factors to be considered include recognition of indi-viduals, memory capacity, temporal discounting, prospective cognition, and theoryof mind. The relevant communication ranges from simple signaling to full sym-bolic communication. I will argue that, whereas many forms of cooperation requireonly very limited cognitive capacities and no communication, more advanced forms,unique to humans, presuppose a high awareness of future needs, a rich understand-ing of the minds of others, and symbolic communication. For example, the reputa-tion mechanism used in Nowak and Sigmund’s [58] model of indirect reciprocityassumes that the individuals can recognize each other, remember previous interac-tions, have at least some aspects of a theory of mind, and possess a communicationsystem that, among other things, can refer to individuals in their absence and de-scribe the roles of donor and receiver in interactions.

efits exceeding these costs to other members of one’s group.” It should be noted that ‘joint’ activityneed not imply that the actions are performed simultaneously: they can be done in sequence.

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1.2 Cognitive requirements for different kinds of cooperation

In this section, I will outline different kinds of cooperation. I will take as my pointof departure the forms of cooperation that have been studied within game theory—in particular evolutionary game theory—though I will also borrow some themesfrom animal behavior. The paradigmatic game in my analysis will be that of theprisoner’s dilemma, mainly in its iterated form, although I will occasionally referto other games. A reason to focus on variations of the prisoner’s dilemma is thatsuch games frequently arise in biological/ecological settings. Biological players of-ten cooperate considerably more than the total defection predicted by any strictlyrationalistic analysis: a finding that generates much of the interest in studying thesegames. A bio-cognitively oriented analysis faces the problem of identifying thosefactors that promote cooperation. I believe that they include varying forms of cog-nition and communication.

I shall present the different kinds of cooperation I discuss roughly in order ofincreasing cognitive demands. My list is far from exhaustive; I have merely selectedsome forms of cooperation where the cognitive and communicative components canreadily be identified, at least to some degree. Finer distinctions could be made forseveral of the items on my list; those distinctions are not, however, of interest here.

1.2.1 Flocking behavior

Flocking behavior is a cognitively low-level form of cooperation. Flocking is foundin many species, and its function is often to minimise predation. Simulation modelsof flocking (e.g. [65, 51, 40]) show that the sophisticated behavior of the flock canemerge from the behaviors of single individuals following simple rules. Only tworules seem to be required for birds to fly in a flock: (a) position yourself as closelyas possible to the centre of the flock and (b) maintain a certain minimum distancefrom your neighbors. Such rules require no cognitive capacities beyond basic visualperception. (Other senses such as echolocation could, in principle, be used as well).

1.2.2 In-group versus out-group

One simple way for a player to increase cooperation in an iterated prisoner’sdilemma game is to divide the other individuals into an in-group and an out-group.Then, the basic strategy is to cooperate with everybody in the in-group while de-fecting against everybody in the out-group. The only cognitive requirements of thisstrategy is that you be able to separate members of the in-group from the out-group.In nature this is often accomplished via olfaction: for example, bees from a differenthive have a different smell and are therefore treated with aggression. MeanwhileDunbar [17] speculates that dialects have evolved to serve as markers of the in-

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group among humans: a more advanced version of the same idea. The ubiquity andstrength of in-group mechanisms throughout the animal kingdom provide good rea-son to suspect that the mechanisms are at least partially genetically determined [43].

Consider a special case. The spatial prisoner’s dilemma games studied by, amongothers, Nowak and May [57], Lindgren and Nordahl [50], Hauert [35], Brandt etal. [9] can be seen as a form of in-group cooperation. In these games, players areorganised spatially so that one can only interact with one’s neighbors. The results ofthese studies show that the spatial structure is manipulated so as to promote cooper-ation. Cooperators survive by forming clusters, creating spatially defined in-groupsthat defect against outliers.

It seems fairly clear that the evolutionary origin of in-group formation is kin-ship selection. In a kin group, the outcome of a prisoner’s dilemma game cannot bejudged solely by the benefits accruing to single players. This is because of the ge-netic relatedness of the individuals; in many cases, kinship mechanisms make coop-eration the only evolutionarily stable strategy. In other words, a prisoner’s dilemmagame that is seemingly irrational or less than fully rational at the individual agentlevel may be perfectly cooperative when the genes are taken as the players. Kingroups can form kernels from which larger cooperative groups can subsequently de-velop [49, p. 351]. The in-group requires some form of marker, be it just physicalproximity, to distinguish the in-group from the out-group. Evolutionary biologists(e.g. [89]) stress that such markers should be hard to fake in order to exclude freeriders from the in-group.

Another way of generating an in-group is to identify a common enemy. Westet al. [87] present empirical evidence that when a group playing an iterated pris-oner’s dilemma game competes with another group, within-group cooperation willincrease. Bernhard et al. [4] present an anthropological study of two groups inPapua New Guinea; it supports the same conclusion. The downside of an in-group-versus-out-group mechanism, as Hamilton [33] notes, is that what favors coopera-tion within groups will, at the same time, favor hostility between groups. A parallelexample can be found among apes. A troop of chimpanzees in the Gombe forest inTanzania became divided into a northern and a southern group. The chimpanzeesin the two groups, who had earlier played and groomed together, started fightinglethally against each other [12].

1.2.3 Reciprocal altruism

In an iterated prisoner’s dilemma game, one player can retaliate against another’sprevious defection. Trivers [82] argues that this possibility can make cooperationmore attractive and lead to what he calls reciprocal altruism. In their seminal paper,Axelrod and Hamilton [2] used computer simulations to show that cooperation canevolve through multiple iterations of the prisoner’s dilemma game. A Nash equilib-rium strategy, such as the well-known Tit-for-Tat, can lead to reciprocal cooperativebehavior among individuals who encounter each other frequently. The minimal cog-


nitive capacities required for this strategy are the ability to recognize the individualone interacts with and a memory of previous outcomes (see below for further spec-ification of these criteria). Trust may be an emotional correlate of reciprocity thatstrengthens reciprocal behavior. Frank [22] suggests that such an emotion may be-come selected for and thus genetically grounded in a species that engages in recipro-cal altruism. On this point, Van Schaik and Kappeler [85, p. 17] write that “we mustexpect the presence of derived psychological mechanisms that provide the proxi-mate control mechanisms for these cooperative tendencies.... [T]he evidence sug-gests a critical role for unique emotions as the main mechanisms. Extreme vigilancetoward cheaters, a sense of gratitude upon receiving support, a sense of guilt uponbeing detected at free riding, willingness to engage in donation of help and zeal todole out altruistic punishment at free riders – all these are examples of mechanismsand the underlying emotions that are less developed in even our closest relatives.”Meanwhile Trivers [83, p. 76] writes about his 1971 paper on reciprocity that itsmost important contribution was that “it laid the foundation for understanding howa sense of justice evolved.”

If the players in an iterated prisoner’s dilemma game choose their moves syn-chronously, then the size of available memory does not seem to affect which strate-gies are successful [1, 48, 49, 36]. However, for so-called alternating iterated pris-oner’s dilemma games in which players take turns choosing their moves, it seemsthat the best strategies depend on the size of memory for previous moves [23, 56].

Field studies of fish [64], vampire bats [88], and primates [70], among others,have revealed the presence of reciprocal altruism. Still, the evidence for reciprocalaltruism in non-human species is debated, and the results of some laboratory ex-periments seem to argue against it [77]. In line with this, Stevens and Hauser [78]argue that the cognitive demands of reciprocal altruism have been underestimated(see also [34]). They claim that reciprocal altruism will be evolutionarily stable ina species only if (a) the temporal discounting of future rewards is not too steep, (b)members of the species have sufficient ability to discriminate the value of the re-wards so as to judge that what an altruist receives back is comparable to what it hasgiven, and (c) there is sufficient memory capacity to keep track of interactions withseveral individuals.

Temporal discounting refers to the tendency to discount the future value of areward, the further removed it is in time. Studies reveal that the rates of temporaldiscounting differ drastically between species [42, 15]. Humans have the lowestrate, a prerequisite for the prospective planning that will be discussed below. Newstudies on apes reveal that, in food contexts, apes are at level with humans [69, 61].What factors (ecological, cognitive, neurological, etc.) explain the discounting rateof a particular species is an interesting problem that should receive more attentionwithin evolutionary game theory.

According to Stevens and Hauser [78], cognitive demands explain why recipro-cal altruism is difficult to establish in species other than Homo sapiens. Weighingagainst their argument, one may present the three types of reciprocity proposed byDe Waal and Brosnan [13, pp. 103-104]: (1) Individuals reciprocate on the basis ofsymmetrical features of dyadic relationships (kinship, similarities in age, or same-

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ness of gender). De Waal and Bosnan say that this is the cognitively least demandingform: “it requires no scorekeeping since reciprocation is based on pre-existing fea-tures of the relationship. [. . .] All that is required is an aversion to major, lasting im-balance in incoming and outgoing benefits” (2) Next comes attitudinal reciprocity,where individual willingness to cooperate depends on the attitude the partner hasrecently shown. Cognitively, the “involvement of memory and scorekeeping seemsrather minimal, as the critical variable is general social disposition rather than spe-cific costs and benefits of exchanged behavior” [13, p. 104]. (3) Finally there iscalculated reciprocity, in which “individuals reciprocate on a behavioral one-on-onebasis with a significant time interval.” This is the cognitively most demanding formof reciprocity, since it “requires memory of previous events, some degree of score-keeping, [and] partner-specific contingency between favors given and received” [13,p. 104]. Reciprocity as discussed by Stevens and Hauser [78] is most similar to thisthird form. It is not surprising that it would be the most difficult to achieve. Whenanalyzing to what extent different species exhibit reciprocal altruism, De Waal andBrosnan’s [13] distinctions must be kept in mind.

Meanwhile, attitudinal reciprocity as defined by de Waal and Brosnan [13] in-volves cognitively fairly minimal capacities: individuals must be recognized overtime and their attitude as a partner remembered. Understanding the attitude of apartner requires some kind of theory of mind, with at least the capacity to under-stand the desires of the other (see [27]).

Understanding the attitude of a partner is a good general mechanism for improv-ing cooperation. Charness and Dufwenberg [11, p. 1580] write of “guilt aversion” asa way to explain why individuals behave more cooperatively than predicted by gametheory in prisoner’s-dilemma-type (and similar) games. To wit: “decision makers ex-perience guilt if they believe they let others down.” By considering the expectationsof your partner(s) and having an emotional predisposition to avoid letting a part-ner down, your payoff matrix in a prisoner’s dilemma game changes, and you endup being much more cooperative than traditional game theory would predict. ForCharness and Dufwenberg (ibid.) guilt aversion requires beliefs about the beliefsof others. However, this claim—about the theory of mind of the decision maker—seems to be too strong for the mechanism they are trying to explain. Surely it issufficient that the decision maker understand the desires of the other just far enoughto realize that the other is let down. This caveat aside, the notion of guilt aversionseems key to analyzing many forms of human cooperative behavior, and it meritsfurther investigations, both theoretical and experimental. The question arises as towhat extent guilt aversion can be shown to exist in other species. At the least, thecapacity to understand the desires of others should be within the cognitive reach ofmany species.


1.2.4 Indirect reciprocity

Reciprocal altruism can be rendered in slogan form: “scratch my back and I’llscratch yours.” As we have seen, this principle makes sound evolutionary sense.However, in humans one often finds more extreme forms of altruism: “If I help you,somebody else will help me.” Such cooperation has been called indirect reciprocity,and it seems to be unique to humans. Nowak and Sigmund [58] show that, at leastunder certain conditions, indirect reciprocity can be given an evolutionary ground-ing (see also [46]) . However, as we shall see, their explanation depends on verystrong assumptions concerning the communication of the inter-actors (which mayexplain why indirect reciprocity is only found in humans).

In Nowak and Sigmund’s [58] account of indirect reciprocity, any two playersare meant to interact at most once with each other. This is, of course, an idealisingassumption; but it has the useful effect that the recipient of a defection (cheat) in aprisoner’s dilemma game cannot retaliate. In this way, all strategies that have beenconsidered for the iterated prisoner’s dilemma game are excluded. So the questionbecomes: under what conditions can cooperation evolve as an evolutionarily stablestrategy (ESS) by means of indirect reciprocity?

Nowak and Sigmund note that indirect reciprocity (as we saw, earlier, with atti-tudinal reciprocity) seems to require some kind of “theory of mind”. An individualwatching a second individual (the donor) helping (or failing to help) a third (thereceiver) (or not helping a third in need) must be able to judge that the donor doessomething “good” (“bad”) to the receiver.2 The form of intersubjectivity requiredfor such comparisons is related closely to empathy [63, 27].

The key concept in Nowak and Sigmund’s evolutionary model of indirect reci-procity is that of the reputation of an individual.3 An individual’s reputation i isbuilt up by members of the society observing i’s behavior towards third parties andthen spreading this information to other members of the society. To wit, gossip be-comes a way of achieving societal consensus about reputation [17, 74]. In this waythe reputation for i being a ‘selfless” helper can be known by more or less all themembers of the group. The level of i’s reputation can then be used by any individualwhen deciding whether or not to assist i in a situation of need.4 Reputation is not, ofcourse, something immediately visible to others in the way of such status markersas a raised tail among wolves. Instead each individual must keep a private accountof the reputation of all others with whom she interacts. Semmann et al. [71] providea nice experimental demonstration that building a reputation through cooperation isvaluable for future social interactions, not only within but also beyond one’s socialgroup.

2 This is in contrast to kinship-based altruism, where what an individual experiences as “good” canbe that which improves her own reproductive fitness.3 A precursor to this concept, foreshadowing it, is Sugden’s [80] notion of “good standing.”4 The in-group behavior discussed in Section 2.2 can be understood as a limiting case of indirectreciprocity whereby all in-group members are treated as having a good reputation and all out-groupmembers as having a bad one. Turned the other way around, indirect reciprocity becomes a flexibleway of determining the membership of the in-group.

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It is important to distinguish in these interactions between justified and unjusti-fied defections. If a potential receiver has defected repeatedly in the past, the donormay be justified in defecting: e.g., as a form of punishment. However, the donorruns a risk that his own reputation may drop. To prevent this, the donor should com-municate his reason for defecting: i.e., the receiver has a bad reputation.

A range of more sophisticated strategies is available for distinguishing betweenjustified and unjustified defections. Nowak and Sigmund [58] call a strategy firstorder if the assessment of an individual i in the group depends only on i’s ownactions. A strategy is second order if it depends on the reputation of the receiverand third order if it depends additionally on the reputation of the donor. Nowak andSigmund argue that only eight of the possible strategies—all of which depend on thedistinction between justified and unjustified defection—are evolutionarily stable.

Thus, the success of indirect reciprocity depends heavily on determining repu-tation. This means that indirect reciprocity, in Nowak and Sigmund’s [58] model,requires several cognitive and communicative mechanisms. The minimal cognitiverequirements are (a) recognising the relevant individuals over time, (b) rememberingand updating the reputation scores of these individuals, and (c) judging by means ofempathy whether a particular donor action is “good” or “bad”. The minimal com-municative requirements are (d) being able socially to identify individuals in theirabsence, e.g. by name; (e) being able to express that “x was good to y” and “y wasbad to x”. No non-human communication system seems able to handle these re-quirements. It should come as no surprise that Homo sapiens is the only speciesexhibiting indirect reciprocity.5 The communication system need not be a human-style language with full syntax; a form of protolanguage along the lines discussed byBickerton [5] is sufficient. The communication could be symbolic, but it is possiblethat a system based on miming [16, 90] would be adequate.

The trust that is built up through reciprocal altruism is dyadic: that is, constitutinga relation between two individuals. In contrast, reputation is an emergent socialnotion involving everybody in the group. Nowak and Sigmund [58, p. 1296] write:

Indirect reciprocity is situated somewhere between direct reciprocity and public goods. Onthe one hand it is a game between two players, but it has to be played within a larger group.

As noted above, a potential donor should signal that a potential recipient has a‘bad” reputation before defecting, in order not to lose reputation himself. This isanother requirement on communication: the ability to form expressions of the type“y has a bad reputation”. Nowak and Sigmund’s [58] model considers only twokinds of reputation: “good” and “bad.” It goes without saying that, in real groups,communication about reputation is far more nuanced.

Of course, the need for communication is dependent on the size of the groupthat is facing the prisoner’s dilemma situations. In a small, tightly connected groupwhere everybody sees everybody else most of the time, no reputation mechanism is

5 In contrast, Van Schaik and Kappeler [85, p. 15] write: “the three basic conditions for reputationare individual recognition, variation in personality traits, and curiosity about the outcome of in-teractions involving third parties.” In my opinion, however, these criteria are too weak, since theyplace no requirement on reputation to be communicated.


needed. One can look at the way within-group ranking is established. If I observethat x has higher rank than y and I know that y has higher rank than I do, there isno need for (aggressive) interaction or for communication to establish the relativeranking. However, in large and loosely connected groups, these mechanisms arenot sufficient, and some form of communication about non-present individuals isrequired. This “displacement” of the message is one of the criteria that Hockett [41]uses to identify symbolic language (see also [25]).

Still other mechanisms may exist that influence the reputation of an individual.In many situations, people are willing to cooperate but also to punish free riders.Punishment behavior is difficult to explain because it is costly, and the coopera-tive non-punisher benefits [21]. Thus, in general, punishment should decrease infrequency.6

Barclay [3] presents evidence that the reputation of a punisher increases, andthat people are more willing to cooperate with punishers. In this way, punishmentbehavior may be rewarded in the long run (see also [72, 37, 38, 55] and can stabilizecooperative behavior in iterated prisoner’s dilemma games [49].

For all its cost, punishment behavior seems to be something that all human pop-ulations are willing to use, to varying degrees.

Using computer simulations, Hauert et al. [37] show that, if participants not onlyhave the choice to cooperate, to defect, or to punish, but also to stand aside, defectorswill eventually be eliminated from the population. Surprisingly, if standing aside isnot an option, then, in the long run, the defectors take over. Hauert at al. concludethat compulsory joint enterprises are less likely to lead to cooperation than voluntaryones.

These results clearly show that Nowak and Sigmund’s idealizing assumptionabout one-shot interactions is too strong. In real life it is important to be able, atleast sometimes, to retaliate. No evidence exists for altruistic punishment amonganimals in nature ([85], although De Waal and Brosnan [13] report experimentallyinduced refusal to cooperate that is costly for the non-cooperative individual).

1.2.5 Cooperation to achieve future goals

Cooperation often occurs over an extended timespan and requires a form of plan-ning. One must distinguish between immediate planning that concerns present needsand prospective planning that concerns future ones. Humans can predict that theywill be hungry tomorrow and so should save some food. They can imagine that thewinter will be cold and so should start building a shelter already in the summer.The cognition of most other animals appears concerned almost exclusively with thehere and now, while humans live both here and now and in the future. The centralcognitive aspect of prospective planning is the capacity to represent future needs.Prospective thinking is sometimes also called “mental time travel” (e.g. [67]).

6 In line with Frank [22], Fehr and Gachter [21] say that negative emotions towards defectors arethe proximate causes of altruistic punishment.

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Bischof [6] and Bischof-Kohler [7] argue that non-human animals cannot antic-ipate future needs or drive states (see also [32]). Their cognition is therefore boundto their present motivational state (see also [79]). This so-called Bischof-Kohler hy-pothesis had seemed supported by the previously available evidence on planningin non-human animals. However, recent experimental results on non-human pri-mates [54, 67, 61] as well as recent observations of spontaneous planning behav-iors [59] demonstrate that the hypothesis no longer can be upheld.

For most forms of cooperation one may find among animals, mental represen-tations for the goal of the cooperation are not needed. If the goal is immediatelypresent in the environment—for example as food to be eaten or an antagonist to befought—the collaborators need no mutual representation of the goal before acting.If, on the other hand, the goal is distant in time or space, a mutual representationof it must be produced before cooperative action can be taken. So building a shareddwelling requires coordinated planning of how to obtain the building materials andadvanced collaboration during the construction. In general, cooperating on futuregoals requires that the conceptual spaces of the individuals be coordinated.

A mutual representation of a future goal changes a situation from a cooperativedilemma into a game where the cooperative strategy is the equilibrium solution. Forexample, if my group lives in a very dry area, each individual (or each family) willbenefit from digging a well. However, if my neighbor digs a well, I may defect,and take all my water from his well instead of digging my own. Meanwhile, if no-body digs a well, we are all worse off than if everybody does so. Now, if somebodysuggests that we cooperate in digging a communal well, then such a well, by be-ing potentially much deeper, could yield much more water than all the individualwells taken together. Once such cooperation is established, the incentive to defectmay either disappear or at least be significantly reduced, since everybody stands tobenefit more from supporting the common goal. In game-theoretic terms, digginga communal well may work out to be a new equilibrium strategy. This exampleshows how the capacity for sharing detached future goals can strongly enhance thevalue of cooperative strategies within the group. Strategies based on future goalsmay introduce new equilibria that are beneficial to all participants.

The cognitive requirements for cooperating on future goals crucially involve thecapacity to represent future drives or wishes: that is, they require prospective plan-ning. Even if water is currently plentiful, you can foresee the coming dry season.This representation, and not any current drive state, forms the driving force behinda wish to cooperate in digging a well.

What are the communicative requirements for such (shared) representations?Symbolic language is the primary tool by which agents can make their personalrepresentations known to each other. In previous work [10, 25, 26, 29], I have pro-posed a strong connection between the evolution of prospective cognition and theevolution of symbolic communication. In brief, symbolic language makes it pos-sible to cooperate on future goals in an efficient way. Again, such communicationinvolves any complex syntactic structures; protolanguage [5] will serve perfectlywell.


An important feature of the use of symbols in cooperation is the way they set thecooperators free from the goals that are immediately available in the environment.This requires that present goals can be suppressed, which depends upon the exec-utive functions of the frontal brain lobes [44]. Future goals and the means to reachthem can then be picked out and shared through symbolic communication. Thiskind of sharing gives humans an enormous cooperative advantage in comparisonto other species. I propose that symbolic communication and cooperation on futuregoals have co-evolved (cf. the “ratchet effect” discussed by Tomasello [81, pp. 37-40]). That said, without the prior presence of prospective cognition, the selectivepressures that resulted in symbolic communication would not have emerged.

1.2.6 Commitments and contracts

Commitments and contracts are special cases of cooperating about the future. Theyrely on an advanced theory of mind that allows for joint beliefs.7 Joint beliefs formthe basis for much of human culture. They make many new forms of cooperationpossible. For example, one might promise to do something, which only means thatyou intend to do it. On the other hand, when you commit to another person to per-form an action, the other person normally wants you to do it and intends to checkthat you do so. Furthermore, the two parties share joint beliefs concerning theseintentions and desires [19]. Unlike promises, commitments cannot arise unless theparties have such joint beliefs and possess prospective cognition. It has been ar-gued in several places (e.g. [81, 25, 27]) that the capacity for joint beliefs is onlyfound in humans. Interestingly, studies show that three-year-olds already have someunderstanding of joint commitments [31].

For similar reasons, contracts cannot be established without joint beliefs andprospective cognition. Deacon [14] argues that marriage is the first example ofcooperation where symbolic communication is needed. It should be obvious thatmarriage is a form of contract. Even though I do not know of any evidence thatmarriage agreements came first, I still find this idea interesting in the discussionof early prospective cognition. Any sort of pair-bonding agreement implicitly de-termines which future behaviors are allowed and not allowed. These expectationsconcerning future behavior do not only involve the pair, but also the other membersof the social group who are supposed to support the relationship, and not to disturbit by cheating. Anybody who breaks the agreement risks punishment by the entiregroup. Thus in order to maintain such bonds as marital bonds, they must be linkedto social sanctions.

7 For an analysis of different levels of a “theory of mind,” see Gardenfors [25], ch. 4 andGardenfors [28].

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1.2.7 Cooperation based on conventions

In human societies, many forms of cooperation are based on conventions. Conven-tions assume common beliefs (often called common knowledge). If two cars meeton a narrow gravel road in Australia, then both drivers know that this co-ordinationproblem has been solved numerous times before by passing on the left. Both knowthat both know this, both know that both know that both know this, etc., and so theypass on the left without any hesitation [47]. Many conventions are established with-out explicit communication; but, of course, communication makes the presence of aconvention clearer.

Conventions function as virtual governors in a society. Successful conventionscreate equilibrium points that, once established, tend to be stable. The conventionof driving on the left forces me to “get into step” and drive on the left. The same ap-plies to language itself: a new member of any society must adjust to the predominantlanguage of the community. The collective meaning of all the linguistic utterancesemerges from individuals’ individual meanings.8 There is, of course, a potentiallyinfinite number of possible “equilibrium” languages in a society. The point is that,once a language has developed, it will both strongly constrain and have strong ex-planatory effects regarding the behavior of individuals in the society.

Similarly, the existence of money depends on a web of conventions. Money re-quires cooperation in the form of a mutual agreement to accept e.g. certain deco-rative pieces of paper as a medium of exchange for goods or labor. As a memberof a monetary society, or as a visitor to one, I must “get into step” and accept theconventional medium of exchange as legal tender. Money is an example of “socialsoftware” [62] that improves the cooperation within a society.

Actually, there are many similarities between language and money as tools forcooperation. Humans have been trading goods for as long as they have existed.When a monetary system does emerge, it makes certain transactions more efficient.The same applies to language: hominids were communicating long before they hada language, but language makes the exchange of information more efficient. Theanalogy carries further. When money is introduced into a society, a stable system ofprices normally emerges. Similarly, when linguistic communication is introduced,individuals come to share a relatively stable space of meanings: i.e., componentsof their private worlds that, as communicators, they can publicly exchange betweeneach other. In this way, language fosters a common structure to the private worldsof the individual members in a society (see [30]).

In general, the social framework we construct, in the form of e.g. our institutions,increases the possibilities for establishing contracts, conventions, and other mattersof the social good.

8 This emergence of a social dimension to language is analysed in detail in Gardenfors [24] andGardenfors and Warglien [30].


1.2.8 The cooperation of Homo economicus

Homo economicus, the rational human, is an idealization, typically assumed to haveperfect reasoning powers. He is fully logically consistent, a perfect Bayesian whenit comes to probability judgements, and a devoted maximiser of utility. He possessesa complete theory of mind: he can put himself fully into the shoes of his opponents(as they can with him) and see them as fellow perfect Bayesians reasoning aboutthe possible equilibrium strategies in the game in which he is playing. When hecommunicates, his messages have a logically unambiguous meaning. He cooperates,if and only if such cooperation maximizes his utility according to the rules of thegame.

Similar idealizations are made in analysing cooperation, using epistemic and de-ontic logics or so called belief-desire-intention logics. The semantics is, typically,formulated in terms of possible worlds. This leads, of course, to unrealistic assump-tions of strict logical consistency, unbounded self-reflective powers, and unlimitedrecursive knowledge attribution—as Verbrugge [86, p. 657] points out. The tradi-tional logical approaches cannot be used as realistic models of human cooperation.Several suggestions have recently been presented for how one might generalize thelogical approach to provide such models [84, 86].

The problem with Homo economicus is that he seldom faces up to the complex-ities of reality, where the rules of the game are not well defined, where the possibleoutcomes and available strategies may vary from minute to minute, where the util-ities of the various outcomes are uncertain if not unknown, where reasoning timeand memory resources are limited, where communication is vague and prone tomisunderstandings, and where his opponents are idiosyncratic and have all mannerof cognitive shortcomings. Trivers [83, p. 79] writes: “I know of no species, hu-mans included, that leaves any part of its biology at the laboratory door; not priorexpectations, nor natural proclivities, nor ongoing physiology, nor arms or legs, norwhatever.”

Game theory has, almost exclusively, studied strategies within a fixed and clearlydefined game. In nature, it is more realistic to speak of strategies that can be used ina variety of different but related games. How should one rationally act when meet-ing an opponent face to face, given all kinds of biological constraints, knowing thatone’s opponent is not fully rational, but not knowing what the opponent’s exact lim-itations? Game theory without biology is esoteric at best; with biology, it becomesas complicated as life itself (see e.g. [20]).

1.3 The effects of communication on cooperation

I want to distinguish between, on the one hand, communication that involves achoice (conscious or not) by some individual to send some signal; and, on the other

14 Peter Gardenfors

hand, mere signaling without such intention.9 Thus a caterpillar, though by its brightcolors signals that it is poisonous, is not communicating. By this definition, com-munication becomes, in itself, a trust-based cooperative strategy. (Of course, com-munication can be abused: what the sender communicates may be a deliberate lieintended to mislead the receiver.)

When a communicator sends a message, the receiver may not, initially, be ableto interpret the intended meaning of the message. However, over iterated interac-tions, a message may develop a fairly fixed meaning. Lewis [47] introduced a game-theoretic account to conventions of meaning, according to which signals transmit in-formation as a result of an evolutionary process. A variety of computer simulationsand robotic experiments (e.g. [75, 76]), have shown how a stable communicationsystem can emerge out of iterated interactions between (natural or) artificial agents,even though nobody determines any rules for the communications. The greater thenumber of “signalers” and “recipients” involved, the stronger the convergence ofreference and the faster the convergence is attained.

The addition of communication to a prisoner’s-dilemma-type game changes thespace of strategies in the game. In game theory, it has widely been assumed thatsignals that “cost nothing” are of no significance to any games that are not purely ofcommon interest; even while evolutionary biologists have emphasized signals thatare designed to be so costly that they (effectively) cannot be faked [89]. In contrast,Skyrms [73] shows how, in the “stag hunt” game and in another bargaining game,costless signals have dramatic effects on the game dynamics compared to the samegames. The mere presence of signals creates new equilibria and shifts the basins ofattraction of old equilibria. To wit: even if communication is not strictly necessaryfor all of the forms of cooperation listed above, adding it to a game may drasticallychange the game’s structure.

Such an effect was first pointed out by Robson [68] with respect to a populationof defectors in an evolutionary setting of the prisoner’s dilemma game. A signal thatis not used by the population at large can be used by mutant members as a kindof secret handshake to identify an in-group. Mutants can then cooperate with thosewho share the secret handshake and defect against the others. They would performbetter than “pure” defectors and invade the population. Without the availability ofsignaling, “pure” defection would be the evolutionarily more stable strategy. VanSchaik and Kappeler [85, p. 9] argue that, even without anything as sophisticated asa secret handshake, “communication about the intentions of each player before theinteractions and negotiation with them about payoffs is likely to make reciprocitymuch more stable than under the conditions of prisoner’s dilemma games. Thuscommunication before engaging in risky cooperation is frequently observed in pri-mates.”

In traditional game theory, the moves in a game are assumed to be fixed. Eitherthe players cannot communicate at all, or else they have access to full linguistic com-munication. From an evolutionary perspective such extremes are not very realistic.In many natural settings, the inter-actors can communicate in some at least limited

9 This is in contrast to e.g. Hauser [39], who uses “communication” to also include all forms ofsignals.


way, and there is almost always the possibility to create a new signal, which imme-diately multiplies the number of possible moves in the game. Taking these factorsinto account means that a game-theoretic analysis done originally along traditionallines becomes immensely more complicated. Already the simple communicationstrategies analysed by Skyrms [73] bear witness of this.

An animal can communicate in various ways without employing much in the wayof cognitive capacity. A system of signals can reach a communicative equilibriumwithout actively exploiting any cognitive capacities of the participants. However,as soon as one introduces the theory of mind and the strong tendency to assignmeanings to signals that one finds in humans, the game-theoretic situation becomesyet more complex again.

To give but one example of the powerful effects of communication: Mohlin andJohannesson [53] investigated the effects of communication on a dictator variationof the prisoner’s dilemma game, where the dictator decides how much of an allot-ted amount to give to a recipient. They compared one-way written communicationfrom an anonymous recipient to no communication, and found that communicationincreases donations by more than 70 percent. In order to eliminate any “relationshipeffect” from communication, another test condition was designed, to allow commu-nication from a third party. donations were still about 40 percent higher than in thecondition with no communication, suggesting that even the impersonal content ofcommunication affects donations.

Charness and Dufwenberg’s [11] notion of guilt aversion provides one possiblerationale for the effects of this “endogenous communication”: the dictator suffersfrom guilt if he hurts the recipients relative to what they believe that they will re-ceive. By taking guilt into account, the dictator can be seen to exhibit some under-standing of the minds of his subjects, or at least of their desires. Communicationmay affect the dictator’s beliefs and, in this way, influence any allocation to therecipient. One might reasonably expect that, on such an account, communicationfrom a bystander will have less effect than a direct message from a recipient, evenan anonymous one.

1.4 Conclusion

Traditional analysis of cooperation in game theory assumes fully rational and fullyintersubjective participants. In contrast, evolutionary game theory assumes no ra-tionality and no intersubjectivity, but relies on evolutionary mechanisms for theestablishment of cooperation. However, for most forms of animal and human co-operation, the truth falls somewhere in between. I have shown how different formsof cooperation build on different cognitive and communicative capacities. The rea-son why certain forms of cooperation are so far found only among humans is thatthose forms of cooperation presuppose a high awareness of future needs, a rich un-derstanding of the minds of others, and availability of symbolic communication.

16 Peter Gardenfors

The somewhat eclectic list of forms of cooperation and their cognitive and com-municative prerequisites, as presented above, is summarized in the following table:

Type of cooperation Cognitive demands Communicative demands

Flocking behavior Perception None

In-group—out-group Recognition of groupmember

None

Reciprocal altruism(attitudinal reciprocity)

Individual recognition,minimal memory, un-derstanding the desiresof others

None

Reciprocal altruism(calculated reciprocity)

Individual recognition,memory, slow temporaldiscounting, value com-parison

None

Indirect reciprocity

Individual recognition,memory, slow temporaldiscounting, value com-parison, minimal theoryof mind (empathy)

Proto-symbolic (mime-tic) communication

Cooperation aboutfuture goals

Individual recognition,memory, prospectiveplanning, value com-parison, more advancedtheory of mind

Symbolic communica-tion (protolanguage)

Commitment and contractIndividual recognition,memory, prospectiveplanning, joint belief

Symbolic communica-tion (protolanguage)

Cooperation based onconventions

Theory of mind that al-lows common knowl-edge

None, but enhanced bysymbolic communica-tion

The cooperation ofHomo economicus

Full theory of mind, per-fect rationality

Perfect symbolic com-munication

Table 1: The cognitive and communicative demands of different forms of coopera-tion.

The different kinds of cooperation presented here are ordered according to increas-ing cognitive demands. They certainly do not exhaust the field of possibilities, andthere is a real need to make finer divisions within the forms presented. Nevertheless,


the table clearly shows how cognitive and communicative constraints are importantfactors when analysing evolutionary aspects of cooperation.

Only humans are known to clearly exhibit reciprocal altruism, indirect reci-procity, cooperation on future goals, and cooperation based on conventions.10 Thiscorrelates well with the cognitive factors that are required for these forms of coop-eration.

The analysis, or a more detailed version of it, can be used to make predictionsabout the forms of cooperation to be found in different species. For example, if amonkey species can be shown to be able to recognize members of one’s own troop,and to remember and evaluate the results of previous encounters with individualmembers, even though two monkeys of that species cannot communicate about athird member, then the prediction is to expect reciprocal altruism but not indirectreciprocity between the members of the species. In this way, a causal link fromcognitive capacities to cooperative behaviors can be established. Conversely, butperhaps less reliably, if a species does not exhibit a particular form of cooperativebehavior, it can be expected not to possess the required cognitive or communicativecapacities.

The analysis can also be turned into a tool for predictions about the evolutionof cognition in hominids. Osvath and Gardenfors [60] argue that the first traces ofprospective planning are to be found during the Oldowan culture 2.5 million yearsago. It is reasonable to assume that the forms of cooperation toward future goals thatdepend on this form of cognition had already been established by then. Conversely,if the archaeological record of some hominid activities provides evidence for certaintypes of cooperation, one can reasonably assume that the hominids had developedthe cognitive and communicative skills necessary for that form of cooperation. Thismay, in turn, generate predictions concerning brain structure and other matters ofphysiology.

AcknowledgementsThis chapter is an expanded and updated version of an article by the same title

that was published electronically in the Festschrift Hommage a Wlodek for WlodekRabinowicz in January 2007 (see http://www.fil.lu.se/HommageaWlodek/). It alsooverlaps with Gardenfors [28]. I would like to thank two anonymous referees as wellas Barbara Dunin-Keplicz, Jan van Eijck, Martin van Hees, Kristian Lindgren, ErikOlsson, Mathias Osvath, Rohit Parikh, Erik Persson, Wlodek Rabinowicz, RobertSugden, Rineke Verbrugge, and Annika Wallin. I would also thank the members ofthe seminars in cognitive science and philosophy at Lund University and the mem-bers of the project group on Games, Action and Social Software at the NetherlandsInstitute for Advanced Study for very helpful comments on drafts of the paper. I alsowant to thank the Netherlands Institute for Advanced Study for excellent workingconditions during my stay there in October 2006 and the Swedish Research Coun-cil for general support that made this research possible. This paper was originallywritten as part of the EU project Stages in the Evolution and Development of Sign

10 As mentioned above, whether other species engage in reciprocal altruism is open to some con-troversy.

18 Peter Gardenfors

Use (SEDSU). The updated version has been prepared with support from the Lin-neaus environment Cognition, Communication and Learning that is financed by theSwedish Research Council.

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