Cooperation is a hallmark of all social organisms. Social groups are, in effect, cooperative solutions to the day-to-day problems of survival and reproduction. For some or all members of the group, however, group living invariably incurs costs, which may be reflected in social subordination, restricted access to the best feeding or resting sites, the social suppression of reproduction, or increased ecological costs. Because individuals (or at least, their genes) are by definition in evolutionary competition with each other, this creates a paradox that is not easy to explain in Darwinian terms: Cooperation is a form of ALTRUISM in which one individual gives up something to the benefit of another (see also SOCIOBIOLOGY).
Evolutionary theory identifies three ways cooperation can evolve, which differ in the delay before the "debt" incurred by cooperating is repaid (see Bertram 1982). In mutualism, both individuals gain an immediate advantage from cooperating. This may be an appropriate explanation for many cases of group living where individuals gain mutually and simultaneously from living together (e.g., through increased protection from predators, group defense of a territory, etc). In reciprocal altruism, the debt is repaid at some future time, providing this is during the lifetime of the altruist. This may be an appropriate explanation for cases where individuals who are unrelated to each other form a coalition for mutual protection. The ally, even though in no immediate danger, will come to the aid of a beleaguered partner, on the implicit assumption that the partner will come to the ally's aid on some future occasion. And in kin selection, the debt is repaid after the death of the altruist because the extra fitness that accrues to the recipient contributes to the altruist's inclusive fitness, defined as the number of copies of a given gene contributed to the species' gene pool by an individual as a result of his or her own reproductive output plus the number contributed by his or her relatives as a direct result of that individual helping each relative to breed more successfully. Kin selection can only work when the two individuals are genetically related. It may provide an explanation for assistance freely given to relatives without prior demands for reciprocation.
Although cooperation and the exchange of services or resources occur widely among humans, such exchanges are not wholly altruistic, especially when the actor incurs a significant cost. A number of recent studies of humans have demonstrated that exchange of benefits occurs without preconditions for repayment when it involves relatives, but only with strict reciprocation when it involves nonrelatives, (for example, garden labor exchange among South American K'ekchi' horticulturalists and Nepalese hill farmers (Berté 1988; Panter-Brick 1989); alliance support among historical Vikings (Dunbar, Clark, and Hurst 1995); and exchange of information about good fishing grounds among contemporary Maine lobstermen (Palmer 1991).
Cooperation is an unstable strategy because it is susceptible to cheating by free riders. The ease with which selfish interests can undermine cooperativeness is most conspicuous in the case of common pool resources (e.g., forest resources, communally owned commons or oceanic fishing grounds). Although it may be obvious to everyone that a communal agreement to manage the use of these resources would benefit everyone because the resource would last longer, the advantages to be gained by taking a disproportionate share can be an overwhelming temptation. The result is often the complete destruction of the resource through overuse, the "tragedy of the commons" (see Ortsrom, Gardner, and Walker 1994). Tax avoidance and parking in no parking zones are everyday examples of a similar kind of cheating on socially agreed conventions.
The free rider problem is one of the most serious problems encountered by organisms living in large groups that depend on cooperation for their effectiveness. It acts as a dispersive force that, unless checked, leads inexorably to the disbanding of groups (and thus the loss of the very purpose for which the groups formed). The problem arises because the advantages of free riding are often considerable, especially when the risks of being caught (and thus of being punished or discriminated against) are slight. Perhaps as a result, strategies that help to detect or deter free riders are common in most human societies. These include being suspicious of strangers (whose willingness to cooperate remains in doubt), rapidly changing dialects (which helps identify the group of individuals with whom you grew up, who are likely to be either relatives or to bear obligations of mutual aid; see Nettle and Dunbar 1997), entering into conventions of mutual obligation (e.g., blood brotherhood, exchange of gifts, or formal treaties), and ostracizing or punishing those who cheat on the system (e.g., castigating hunters who eat all their meat rather than sharing it, as among !Kung San bushmen: Lee 1979).
In addition to these purely behavioral mechanisms (perhaps the product of CULTURAL EVOLUTION), there is also evidence to suggest that there may be dedicated "cheat detection" modules hardwired in the human brain. The evidence for this derives from studies that consider abstract and social versions of the Wason selection task (a verbal task about logical reasoning: see Table 1). Although most people get the answer wrong when presented with the abstract version of the Wason task, they usually get it right when the task is presented as a logically identical social contract problem that involves detecting who is likely to be cheating the system (see Cosmides and Tooby 1992). This is assumed to happen because we have a cognitive module that is sensitive to social cheats, but that cannot easily recognize the same kind of logical problem in another form.
The mechanisms involved in the evolution of cooperation have been of considerable interest to economists and other social scientists, as well as to evolutionary psychologists (see EVOLUTIONARY PSYCHOLOGY). GAME THEORY, in particular, provides considerable insights into the stability of cooperative behavior. The situation known as "prisoner's dilemma" has been the focus of much of this research. It involves two allies who must independently decide whether to cooperate with each other (to gain a small reward) or defect (to gain a very large reward) -- but with the risk of doing very badly if cooperation is met with defection. A computer tournament that pitted alternative algorithms against each other in an evolutionary game revealed that the very simplest rule of behavior is the most successful. This rule is known as "tit-for-tat" (or TfT): Cooperate on the first encounter with an opponent and thereafter do exactly what the opponent did on the previous round (cooperate if he cooperated, defect if he defected).
In more conventional face-to-face situations, cues provided by nonverbal behavior may be important in promoting both trust in another individual and the sense of obligation to others required for successful cooperation. Experiments have shown that simply allowing individuals to discuss even briefly which strategy is best greatly increases the frequency of cooperation. Allowing them to exert moral pressure or fines on defectors improves the level of group cooperativeness still further.
The Wason selection task (table 1) was developed as a test of logical reasoning. When presented with four cards bearing letters and numbers (as shown in column 1) and informed that "an even number always has a vowel on its reverse," the subject has to decide which card or cards to turn over in order to check the validity of the rule. This rule has the standard structure of the logical statement "If P (= even number), then Q (= vowel on reverse)." Because the four cards correspond to the statements P, not-P, Q and not-Q, the correct logical solution is to choose the cards that correspond to P and not-Q. Most subjects incorrectly choose P alone or the P and the Q cards. In the social contract version, the cards correspond to people sitting around a table whose drinks or ages are specified (as in column 3). The rule in this case is "If you want to drink alcoholic beverages, you must be over the age of 21 years." Here it is obvious that only the age of the beer drinker (P) and the drink of the 16-year-old (not-Q) need to be checked. Even though most people get the original abstract Wason task wrong, they get the social contract version right.
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