An Evolutionary Approach to Norms Author(s): Robert Axelrod Source: The American Political Science Review, Vol. 80, No. 4 (Dec., 1986), pp. 1095-1111 Published by: American Political Science Association Stable URL: http://www.jstor.org/stable/1960858 Accessed: 14/10/2009 09:23

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http://www.jstor.org AN EVOLUTIONARY APPROACH TO NORMS ROBERT AXELROD Universityof Michigan

Iorms providea powerfulmechanism for regulatingconflict in groups, even when there are more than two people and no central authority. This paper investigatesthe emergenceand stability of behavioralnorms in the context of a game played by people of limitedrationality. The dynamicsof this new normsgame are analyzedwith a computersimulation based upon the evolutionaryprinciple that strate- gies shown to be relatively effective will be used more in the future than less effective strategies. The results show the conditions under which norms can evolve and prove stable. One interestingpossibility is the employmentof metanorms,the willingness to punishsomeone who did not enforcea norm. Many historicalexamples of domesticand internationalnorms are used to illustrate the wide variety of mechanisms that can support norms, including metanorms, dominance, internalization,deterrence, social proof, membershipin groups, law, and reputation.

An established support of dueling caused Hamilton to norm can have tremendouspower. This is risk, and ultimately to lose, his life-a illustratedby a historical instance of the powerful norm indeed, and yet one that norm of dueling.In 1804 Aaron Burrchal- has all but disappearedtoday after cen- lenged Alexander Hamilton to a duel. turies of power over life and death. Hamilton sat down the night before the Today, norms still govern much of our duel was to take place and wrote down his political and social lives. In politics, civil thoughts. He gave five reasons against rights and civil libertiesare as much pro- accepting the duel: his principles were tected by informal norms for what is against sheddingblood in a private com- acceptable as they are by the powers of bat forbiddenby law; he had a wife and the formal legal system. Leadership is children; he felt a sense of obligation itself subject to the power of norms, as towardhis creditors;he bore no ill against Nixon learned when he violated political Colonel Burr;and he would hazardmuch norms in trying to cover up Watergate. and could gain little. Moreover, he was The operation of Congress is shaped by reluctantto set a bad exampleby accept- many norms, including those governing ing a duel. Yet he did accept, because"the reciprocity(Matthews, 1960) and appren- ability to be useful, whether in resisting ticeship (Krehbiel, 1985). Across many mischiefor effectinggood, in those crises nations, tolerance of opposition is a of our public affairswhich seem likely to fragile norm that has great impact on happen, would probably be inseparable whether a democracy can survive in a from a conformity with public prejudice given country (Almond and Verba, 1963; in this particular"(Truman, 1884, pp. Dahl, 1966). In international political 345-48). In other words, the prospect of economy, norms are essential for the sanctionsimposed by the generalpublic in understandingof the operations of many

AMERICAN POLITICAL SCIENCE REVIEW VOL. 80 NO. 4 DECEMBER, 1986 American Political Science Review Vol. 80 functionaldomains such as banking, oil, example,after many centuriesof colonial- and foreign aid (Axelrod and Keohane, ism, the intolerance of colonial depen- 1985; Keohane, 1984; Krasner, 1983). dence took hold in the relatively short Even in the domain of power politics, period of just two decades after World norms have virtually wiped out colonial- War II. Beforeand after such a transition, ism, inhibited the use of chemical war- the state of affairs seems very stable and fare, and retarded the spread of nuclear perhapseven permanent.For this reason, weapons. awarenessof a given norm is most intense Not only are norms important for precisely when it is being challenged. many central issues in political science, Examples of norms being challenged but they are vital to the other social today includethe right to smoke in public sciencesas well. Sociology seeks to under- without asking permission, the use of stand how different societies work, and gender-ladenlanguage, and the prohibi- clearlynorms are importantin these proc- tion against the use of chemicalwarfare. esses (e.g., Opp, 1979, 1983). Anthropol- Some of these challenges will succeed in ogy frequently deals with the unique establishing new norms, and some will featuresof various peoples by describing fail altogether. Thus, what is needed is in great detail their practicesand values, a theory that accounts not only for as in the case of feuding (e.g., Black- the norms existing at any point in time, Michaud, 1975). Psychologists are con- but also for how norms change over time. cerned with how people influence each To clarify these processes, one must first other and the manner in which an be clear about exactly what is being individualbecomes socializedinto a com- discussed. munity (e.g., Darley and Batson, 1973; In this next section the evolutionary Sherif, 1936). Economists are becoming approach to be used in this paper is interestedin the origin and operation of explained. Following this, the results of norms as they have come to realize that computersimulations of the evolution of markets involve a great deal of behavior norms are presented.The computersimu- based on standardsthat no one individual lations are then extended to include a can determinealone (e.g., Furubotnand specificmechanism for the enforcementof Pejovich, 1974; Schotter, 1981). norms, called metanorms. After these for- Largenumbers of individualsand even mal models are investigated,a wide vari- nations often display a great degree of ety of processesthat might help to sustain coordinatedbehavior that serves to regu- norms are discussed, along with sugges- late conflict. When this coordinated tions about how they too can be modeled. behavior takes place without the inter- The question of the origin and content of vention of a centralauthority to police the norms is considered, and finally, a sum- behavior, we tend to attribute the coor- mary and conclusionpresents the findings dinatedbehavior and the resultingregula- of this paperin the broad context of social tion of conflict to the existence of norms. and political change. To make this appeal to norms a useful explanation, we need a good theory of norms. Such a theory should help explain The EvolutionaryApproach threethings: how norms arise, how norms are maintained, and how one norm dis- Norms have been defined in various places another. ways in the differentliteratures and even One of the most importantfeatures of within the same literature.The threemost norms is that the standing of a norm can common types of definitions are based change in a surprisinglyshort time. For upon expectations,values, and behavior.

1096 1986 Evolution of Norms

That these different definitions are used problems of defining credible threatsand for the same conceptreflects how expecta- of showing the consequencesof requiring tions, values, and behavior are often actors' expectations about each other to closely linked. Definitions based upon be consistentwith the experiencethat will expectations or values are favored by be generated by the resulting actions those who study norms as they exist in a (Abreu, Pearce, and Stacchetti, 1985; given social setting. Such definitions are Friedman,1971; Krepsand Wilson, 1982; convenient because interviews can elicit Selten, 1975). the beliefs and values of the participants, While deductions about what fully whereas systematically observing their rational actors will do are valuable for actual behavior is more difficult. Because their own sake, empirical examples of for many purposes the most important changing norms suggest that real people thing is actual behavior, a behavioral are more likely to use trial and error definitionwill be used in this study. behavior than detailed calculationsbased on accurate beliefs about the future. DEFINITION.A norm exists in a given Therefore,I have chosen not to study the social setting to the extent that indi- dynamics of norms using an approach viduals usuallyact in a certainway and that depends on the assumption of are often punishedwhen seen not to be rationality. acting in this way. Instead, I use an evolutionary ap- proach. This approach is based on the This definitionmakes the existenceof a principle that what works well for a norm a matter of degree, rather than an player is more likely to be used again all or nothing proposition, which allows while what turnsout poorly is more likely one to speak of the growth or decay of a to be discarded (Axelrod, 1984). As in norm. According to this definition, the game theory, the players use their strate- extent to which a given type of action is gies with each other to achieve a payoff a norm depends on just how often the based upon their own choice and the action is taken and just how often some- choices of others. In an evolutionary one is punishedfor not taking it. approach, however, there is no need to To investigatethe growth and decay of assume a rational calculation to identify norms, I have formulateda norms game the best strategy. Instead, the analysis of in which playerscan choose to defect and what is chosen at any specific time is to punish those they have seen defecting. based upon an operationalizationof the The goal of the investigation is to see idea that effective strategies are more when cooperation based upon emerging likely to be retained than ineffective norms will develop. Ultimately, the pur- strategies. Moreover, the evolutionary pose is to learn what conditions favor the approach allows the introductionof new development of norms so that coopera- strategiesas occasionalrandom mutations tion can be promoted where it might not of old strategies. otherwiseexist or be secure. The evolutionaryprinciple itself can be To see what rational actors would do thought of as the consequenceof any one in a particular setting, a game theory of three differentmechanisms. It could be approach can be used. Game theory that the more effective individuals are assumesthe players are fully rationaland more likely to survive and reproduce. choose the strategy that gives the highest This is true in biological systems and in expected utility over time, given their some economic and political systems. A expectationsabout what the other players second interpretationis that the players will do. Recent work by economists has learn by trial and error, keepingeffective shown greatsophistication in dealingwith strategiesand altering ones that turn out

1097 American Political Science Review Vol. 80

Figure 1. Nouns Game process can be analyzed to determineits overall implications.

j punishes s gets P.9 ' gets -2

S duet Jots. Jodoeswtoandmishi The Norms Game .4.-go'~ S

1098 1986 Evolution of Norms

Table 1. Example of Payoffs in the Norms Game Attained by a Player With Boldness Equal to 2/7 and Vengefulness Equal to 4/7

Payoff per Numberof Event Event Events Payoff Defection T= 3 1 3 Punishment P =-9 1 -9 Hurt by others H = -1 36 -36 Enforcementcost E = -2 9 -18 Score -60

Simulation of the Norms Game Unfortunatelyfor Lee, one of the other players observed the defection and chose The simulation of the norms game to punish it, leading to a loss for Lee of P determines how the players' strategies = -9 points. In addition the other evolve over time. The two dimensionsof players defected a total of 36 times, each a strategy,boldness and vengefulness,are hurting Lee H = -1 point. Finally, Lee each allowed to take one of eight levels, observed who was responsiblefor about from 0/7 to 7/7. Becausethe representa- half of these defections and chose to tion of eight levels requiresthree binary punish each of them with a probability bits, the representation of a player's determined by his vengefulness of 4/7. strategy requiresa total of six bits, three This lead to a punishment of 9 of the for boldness and three for vengefulness. defectionsat an enforcementcost of E = The simulation itself proceeds in five -2 each, for a furtherloss of 18 points. steps, as follows: The net resultof these four types of events (1) The strategiesfor the initial popula- was a total score of -60 for Lee. tion of 20 players are chosen at random (3) When the scores of all the players from the set of all possible strategies. are determined,individuals whose strate- (2) The score of each player is deter- gies were relativelysuccessful are selected mined from the player'sown choices and to have more offspring.'The method is to the choices of the other players. Each give an average individual one offspring individual gets four opportunities to and to give two offspringto an individual defect. For each of these opportunities, who is one standard deviation more the chanceof being seen, St is drawnfrom effective than the average. An individual a uniform distributionbetween 0 and 1. who is one standarddeviation below the To see how the scores are attained, let us population average will not have his or focus on an arbitraryplayer in the initial her strategy reproducedat all. For con- populationof one of the runs, who will be venience, the number of offspring is called Lee. Lee has a boldnesslevel of 2/7 adjusted to maintain a constant popula- and vengefulnesslevel of 4/7. The total tion of 20. A final step is the introduction payoff Lee achievedwas the resultof four of some mutation so that new strategies different kinds of events, as shown in can arise and be tested. This is done by Table 1. Lee defected only once because allowing a 1% chance that each bit of an only one of the four opportunitieshad a individual'snew strategy will be altered. chance of being seen that was less than This mutationrate gives a little more than Lee'sboldness of 2/7. This defectiongave one mutation per generationin the entire a temptation payoff of T = 3 points. population.

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Figure2. Norms Game Dynamics vengefulness are then grouped together, and their average boldness and vengeful- 7/71 ness one generationlater is measured.The Vengefulness * po average 400 generations after a random start results are indicated by the arrows in 6/7 aC--chang in pot6average in one generation(see teat) Figure2. 0 Now the various outcomes begin to fit 5/7. into a common pattern. All five of the runs begin near the middle of the field, with average boldness and vengefulness levels near one-half. The first thing to 3/744/ happen is a dramaticfall in the boldness 4/7 . e .- level. The reason for the decline is that when there is enough vengefulnessin the population, it is very costly to be bold. Once the boldness level 2/7 '*9 9 1 7 falls, the main trend is a lowering of vengefulness. The reason for this is that to be vengeful and punish an observed defection requires 0 0 1/7 2?7 3/7 4)7 5/7 6/7 7/7 paying an enforcementcost without any Boldness direct return to the individual. Finally, t* / once the vengefulness level has fallen (4) Steps 2 and 3 are repeatedfor 100 nearly to zero, the can be bold the players generationsto determinehow popula- with impunity. This results in an increase tion evolves. in boldness, destroyingwhatever restraint (5) Steps 1 to 4 are repeatedto give five was established in the first stage of the complete runs of the simulation. process-a sad but stable state in this The resultsof the five runsare shown in norms the game. Figure 2. The five circles indicate This result raises the of of a question just average boldness and vengefulness what it takes to get a norm Three established. population after 100 generations. Because the problem is that no one has different outcomes completely appear any incentive to a the In of the there was a punish defection, possible. one runs, next section explores one of the mecha- moderatelevel of vengefulnessand almost nisms that an the provides incentive to be no boldness, indicating partial estab- vengeful. lishmentof a norm against defection. On two other runs there was little boldness and little vengefulness, and on the Metanorms remaining two runs, there was a great deal of boldness and almost no vengeful- A little-lamentednorm of once great ness-the very opposite of a norm against strength was the practice of lynching to defection. What could be happening? enforcewhite rule in the South. A particu- The way the strategiesactually evolve larly illuminating episode took place in over time is revealed by the change that Texas in 1930 after a black man was takes place in a single generation in a arrested for attacking a white woman. population'saverage boldness and venge- The mob was impatient, so they burned fulness. To calculate this, the data are down the courthouse to kill the prisoner used from all 100 generationsof all five within. A witness said, runs, giving 500 populations.The popula- I heard a man right behind me remark of the fire, tions with similar average boldness and "Now ain't that a shame?" No sooner had the

1100 1986 Evolution of Norms

Figure3. MetanormsGame

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in-peon PD. bask -m Wm.

Key: ij,k individuals S probability of a defection's being seen by any given individual Bi boldness of i V. vengefulness of j T temptation to defect H hurt suffered by others P cost of being punished E punisher's enforcement cost P' cost of being punished for not punishing a defection E' cost of punishing someone for not punishing a defection

words left his mouth than someone knocked him (Bronfenbrenner,1970; Meyers and Brad- down with a pop bottle. He was hit in the mouth bury, 1968). and had several teeth broken. (Cantril, 1941, p. 101) As another example, when the Soviet Union supportedthe suppressionnof the This is one way to enforcea norm:punish Solidarity movement in Poland, the those who do not support it. In other United States asked its allies to stop sup- words, be vengeful, not only against the plying components to the Soviet Union violators of the norm, but also against for its new gas pipeline. The allies, not anyone who refuses to punish the defec- wanting to pay the enforcementcost of tors. This amounts to establishinga norm this punishment, refused. The United that one must punish those who do not States government then undertook the punish a defection.This is what I will call metapunishmentof imposingsanctions on a metanorm. foreign companies that defied the sales Metanormsare widely used in the sys- ban (New York Times, January 5 and June tems of denunciationin communistsocie- 19, 1982). ties. When the authoritiesaccuse someone The formulationof a metanormsgame of doing something wrong, others are can help in the exploration of the effec- called upon to denouncethe accused.Not tiveness of this mechanism. Figure 3 to join in this form of punishmentis itself shows how the metanormsgame is based taken as a defection against the group upon an extension of the norms game. If

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Figure4. MetanormsGame Dynamics be bold. Thus the entire system is self- policing, and the norm becomes well established. to7 A&* _ . . _- This result is dependent, however, on 6/7- \ the population'sstarting with a sufficient- ly high level of vengefulness. Otherwise 5/4 the norm still collapses. Thus, while the Vengefulness norms game collapsesno matterwhat the 4/7? initial conditions are, the metanorms game can prevent defectionsif the initial 3/7- conditions are favorable enough.

2/7 - 0 pawwoos '00 meeioo after a randomstart Mechanismsto Support Norms in am /7 .P eagein an* e9hofiton (intet) The simulationsof the normsgame and game have allowed the Ii I I 1 I the metanorms 6/7 Ot /7 2/7 3/7 4/7 W/ 7/7 exploration of some of the important Boldness processesin the dynamics of norms. The simulationof the norms game shows that someone defects, and Lee sees but does relying on individuals to punish defec- not punish that defection, then the other tions may not be enough to maintain a players have a chance to see and punish norm. Therefore,the question to be con- Lee. The model makesthe criticalassump- sidered now is, What mechanisms can tion that a player's vengefulnessagainst serve to support a norm that is only par- nonpunishmentis the same as the player's tially established? The evolutionary vengefulness against an original defec- approach helps to develop a list of such tion.2The validity of this assumptionwill processes, and in some cases, suggests be addressedlater, but first let us see what specificmethods for modelingthe process affect it has on the evolution of the by which a norm can be supported. process. A set of five runs was conductedwith Metanorms the metanormsgame, each done as before with a population of 20 players and a As the computersimulations show, the duration of 100 generations.The results existenceof a metanormcan be an effec- are shown -in Figure 4. They are un- tive way to get a norm startedand to pro- ambiguous.In all five runsa norm against tect it once it is established. By linking defection was established.The dynamics vengefulness against nonpunishers with are clear. The amount of vengefulness vengefulnessagainst defectors, the meta- quickly increasedto very high levels, and norm providesa mechanismby which the this in turn drove down boldness. The norm against defection becomes self- logic is also clear. At first there was a policing. The trick, of course, is to link moderate amount of vengefulnessin the the two kinds of vengefulness. Without population. This meant that a player had this link, the system could unravel. An a strongincentive to be vengeful, namely, individual might reduce the metaven- to escape punishmentfor not punishing geance level while still being vengefuland an observed defection. Moreover, when then laterstop being vengefulwhen others each of the players is vengeful out of self- stopped being metavengeful. protection, it does not pay for anyone to The examplescited earliersuggest that

1102 1986 Evolution of Norms people may well punish those who do not The two advantages of the whites are help to enforcea valued norm. The model modeled separately. Their greater eco- suggestsnorms can be supportedif people nomic or political power is reflected in tend to have correlateddegrees of venge- their lessenedcost of being punishedby a fulness or anger against someone who black. This was done by letting P = -3 violates a particularnorm and someone for whites while retaining P = -9 for who toleratessuch a violation. What the blacks. The greaternumbers of whites are evolutionary approach has done is raise reflecteddirectly in the relativesize of the the possibility that metanorms are a two populations, giving the whites a mechanismthat can help support norms, greater chance to observe and punish a thus suggesting the interestingempirical black defection than vice versa. This was question of whether the two types of done by letting the population be 20 vengefulness are indeed correlated. My whites and 10 blacks. guess is that there is such a correlation. Analysis of runs based upon these con- The types of defectionwe are most angry ditions shows that resistance to punish- about are likely to be the ones whose ment and increasedsize can help a group, toleration also makes us angry. As of but only if thereare metanorms.Without now, however, the possibility of meta- metanorms,even membersof the stronger norms remainsspeculative. group tend to be free riders, with no private incentive to bear enforcement Dominance costs. This in turn leads to low vengeful- Another mechanism for supporting a ness and high boldness in both groups. norm is the dominanceof one group over When metanormsare added, it becomes another. Forexample, it is no coincidence relatively easier for the strong group to that in the South, whites lynched blacks, keep the weak group from being bold, but blacks did not lynch whites. The while it is not so easy for the weak group whites had two basic advantages:greater to keep the strong one from defecting. economicand politicalpower, and greater Another form of potential strength is numbers. illustratedby the case of a major power Simulationof the effects of power and interacting with many smaller nations. numberscan be readily done with slight For example, the U.S. may not only be in extensionsof the basic model to allow for a favorable position on a given bilateral the existenceof two differentgroups. The interactionbut also may have many more competition between two groups can be bilateral interactionsthan others. Thus, modeled by assumingthat the defections its behavior has a greaterimpact on the of a player only hurt the membersof the development of norms than would the other group and are thereforeonly pun- behavior of a minor power. When Libya ished by members of the other group. wanted to modify the internationalnorm Similarly,in the metanormsversion of the of the twelve-mile limit of territorial model, punishmentsfor not punishing a waters to includethe entireGulf of Sidra, defectorwould only occurwithin a group, the United States fleet deliberatelysailed as illustratedby the pop bottle used by into the Gulf and shot down two Libyan one white against anotherin the lynching planes sent up to try to change the norm. example discussed above. Moreover, in Clearly the U.S. was not only stronger determiningstrategies for the next genera- but had incentivesto enforcethe old norm tion, the strategiesof two groups would based upon its naval interests in other be allowed to adapt separately so that parts of the world. whites learn from whites and blacks learn While the process of frequent inter- from blacks. actions by a single strong player has not

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yet been simulated, it is plausible that many people have to internalizea norm in such a process would help to establish order for it to remainstable? a norm against defection because the The logic of the norms game suggests central player would have a greateruni- that lowering the temptation to defect lateral incentive to be vengeful against might not be enough. After all, even if defections. most people did not defect, if no one had Norms can also be promoted by the an incentive to punish the remaining interestsof a few majoractors, such as the defectors, the norm could still collapse. U.S. and the Soviet Union'sboth working This point suggeststhat we look for inter- to retard the proliferation of nuclear nalization, not only in the reducedincen- weapons. Their actions need not be coor- tive to defect, but also in an increased dinated in detail as long as together they incentive to punish someone else who are importantenough to others to enforce does defect. a norm of the major actors' choice. The An increased incentive to punish, logic is somewhat analogous to Olson's through internalizationor by some other "privilegedgroup" in a collective action means, would lead some people to feel a problem (Olson, 1965, pp. 48-50). gain from punishinga defector.For them, the payoff from enforcement, E, would Internalization actuallybe positive. Suchpeople are often known as self-righteousbusy bodies and Norms frequentlybecome internalized often are not very well liked by those who (Scott, 1971). This means that violating enjoy a defection now and then. Given an established norm is psychologically enough people who enjoy enforcing the painfuleven if the directmaterial benefits norm, the question of its maintenance observed are positive. This is frequently then becomes whether the chance is where high in laboratoryexperiments subjects or low that the defectionwill be seen. are more equitable than they have to be and explain their behavior by saying Deterrence things like "you have to live with your- self." In terms of the norms game, this In the norms game and the metanorms type of internalization means that the game the players do not look ahead. temptationto defect, T, is negativerather Insteadthey try a particularstrategy, see than positive. If everyone internalizesa how it does, compare their payoff with given norm this strongly, there is no the payoff of others, and switch strategies incentive to defect and the norm remains if they are doing relativelypoorly. While stable. Obviously families and societies trial and erroris a sensibleway of model- work very hard to internalize a wide ing players of very limited rationality, it variety of norms, especiallyin the impres- does not capturethe idea that playersmay sionable young. They do so with greater have a great enough understandingof the or lesser success dependingon many fac- situationto do some forward-lookingcal- tors, including the degree to which the culations as well as backward-looking individual identifies with the group and comparisonswith others. In particular,a the degree to which the norm and its person may realizethat even if punishing sponsors are seen as legitimate.3 a defection is costly now, it might have Clearly, it is rare for everyone in a long-term gains by discouraging other group to have a norm so strongly inter- defectionslater. nalized that for each the temptation to A good example is the strong U.S. defect is actually negative. An interesting responseto New Zealand'srefusal in Feb- question for future modeling is, How ruary 1985 to allow a U.S. destroyerinto

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Aukland harbor without assurancesthat of those around us, we fulfill a psycho- it did not carry nuclear weapons. The logical need to be part of a group. U.S. governmentpresumably did not care Our propensity to act on the principle very much about nuclear access to New of social proof is a major mechanismin Zealandports, but it did care a greatdeal the support of norms. The currentmodel about deterringthe spreadof a new norm of norms alreadyhas a form of this mech- of "nuclearallergy" among its many allies anism built in: when a relatively unsuc- in other parts of the world (Arkin and cessful individual seeks a new strategy, Fieldhouse,1985). that strategy is selected from those being used by the rest of the population.This is Social Proof a form of social proof, refinedby giving weight to the more successful strategies An important principle from social being employed in the population. psychology is "social proof," which ap- In cases where other people differ in plies especially to what people decide is important ways, the principle of social correct behavior. As Cialdini (1984, p. proof tends to apply to those who are 117) explains, most like us. This too is easy to build into we view a behavior as more correctin a given simulationswith more than one group. In situation to the degree that we see others per- the simulation of blacks and whites, the formingit. Whetherthe questionis what to do blacks look only to other blacks when with an empty popcornbox in a movie theater, selecting a new strategy, and the whites how fast to driveon a certainstretch of highway, look only to other whites. This makes or how to eat chicken at a dinner party, the actions of those aroundus will be importantin good sense becausea strategythat is very definingthe answer. successfulfor a white might be disastrous The actions of those around us serve if employed by a black. several functions. First, they provide Membership informationabout the boldness levels of others, and indirectlyabout the vengeful- Another mechanismfor the support of ness of the population. Hence, we can norms is voluntary membership in a infer somethingabout whetherit pays for group working together for a common us to be bold or not. Second, the actions end.4 Contracts, treaties, alliances, and of others might contain clues about what membershipsin social groups all carry is the best course of action even if thereis with them some power to impose obliga- no vengefulness. For example, people tions upon individuals.The power of the may be drivingslowly on a certainstretch membershipworks in threeways. First,it of highway, not because there is a speed directly affects the individual's utility trap there, but becausethe road is poorly function, making a defection less attrac- paved just ahead. Eitherway, the actions tive because to defect against a voluntar- of others can provide informationabout ily accepted commitmentwould tend to how the population has been adaptingto lower one's self-esteem. Second, group a particularenvironment. If we are new to membershipallows like-mindedpeople to that environment, this is valuable infor- interact with each other, and this self- mation about what our own behavior selection tends to make it much easierfor should be (Asch, 1951, 1956; Sherif, the membersto enforcethe norm implicit 1936).The actions of othersprovide infor- in the agreementto form or join a group. mation about what is properfor us, even Finally, the very agreement to form a if we do not know the reasons. Finally,in group helps definewhat is expectedof the many cases, by conformingto the actions participants, thereby clarifying when a

1105 American Political Science Review Vol. 80 defection occurs and when a punishment used to give legitimacy to a very real set is called for. of laws and institutions. One might supposeit would be easy for a bold individualto join and then exploit Law a group that had gatheredtogether in the expectation of mutual compliance. Ac- Norms often precedelaws but are then tually, this does not usually happen, in supported, maintained,and extended by part becausethe factorsjust outlined tend laws. For example, social norms about to isolate a defectorand make it relatively smoking in public are now changing. As easy for the others to be vengeful- more and more people turn vengeful especially with the help of metanorms. against someone who lights up in a con- Anotherfactor is that, accordingto recent fined space, fewer and fewer smokersare experimental evidence, cooperators are so bold as to do so without askingpermis- more likely to stay in a group than are sion. As this norm becomes firmer, there defectors (Orbell, Schwarz-Shea, and is growingsupport to formalizeit through Simmons, 1984). This happens because the promulgationof laws defining where cooperators have a stronger ethical or smoking is and is not permitted.6 group-regardingimpulse than defectors,a A law supportsa norm in severalways. factor that led them to cooperate in the The most obvious is that it supplements first place. private enforcementmechanisms with the The metanormsgame can be expanded strengthof the state. Becauseenforcement to include the choice of whether to join a can be expensive for the individual, this group or not.5 In general, the value to a can be a tremendous asset. In effect, person of joining a group would depend under the law the collective goods prob- on how many others joined. Each player lem of enforcement is avoided because would make this choice at the start of the selective incentives are given to special- game. Then the interactions concerning ized individuals (inspectors, police, defections and punishmentswould occur judges,etc.) to find and punishviolations. as before, with the interactionslimited to The law also has a substantialpower of those who had actually joined. As an its own, quite apart from whetherit is or example, an alliance for collective secur- can be enforced. Many people are likely ity would include a group of nations that to take seriously the idea that a specific had joined for this common purpose. act is mandatedby the law, whetherit is a Once a nation had joined, a defection requirementto use seat belts or an income would consist of not supportingthe alli- tax on capital gains. However, we all ance in some collective security task. A know this respect for the law has its defection would hurt the other members limits, and we suspect that many people of the alliance, and some of them might do not pay all the tax they should. Even choose to punish the defector;they might when enforcement is possible and is also choose to punish someone who did attempted, the strength of the law is not punish the defector. Typically, the limited. In most cases, the law can only larger the number of nations joining the work as a supplement(and not a replace- group, the greaterthe benefitsof coopera- ment) for informal enforcement of the tion would be for its members. norm. The failure of Prohibition is a In the political sphere,voluntary mem- classicexample of an attemptto enforcea bership taking the form of a social con- norm without sufficientsocial support. tract has been a powerful image for the In additionto enforcementand respect, support of democratic forms of govern- a third advantage of the law is clarity. ance. In effect, a mythical agreementis The law tends to define obligationsmuch

1106 1986 Evolution of Norms more clearly than does an informalnorm. others might make some quite general A social norm might say that a landlord inferencesabout you. should provide safe housing for tenants, The importance of dressing formally but a housingcode is more likely to define when the occasionrequires is not just that safety in terms of fire escapes. Over the others will punish you for violating the domain covered by the law, the norm norm (say, by giving you a disapproving might become quite clear. However, this look) but also that they will infer things clarity is gained at the expenseof suggest- about you and then act in ways you wish ing that conformity with the law is the they wouldn't. This is an example of the limit of one's social obligations. signalingprinciple: a violation of a norm Modeling the power and operation of is not only a bit of behaviorhaving a pay- the law is beyond the scope of this project. off for the defector and for others; it is However, it should still be emphasized also a signal that contains information that often law is the formalizationof what about the future behavior of the defector has alreadyattained strength as a social or in a wide variety of situations.7 political norm. An importantexample is There are several important implica- civil liberties, the very foundation of a tions of the signaling principle for the democratic system. There are laws and origin and durabilityof a norm. A norm constitutional provisions in support of is likely to originatein a type of behavior civil liberties such as freedom of speech, that signals things about individualsthat but the legal system can only protect free will lead others to reward them. For speechif thereis substantialsupport for it example, if a certain accent signals good among a population willing to tolerate breeding, then others may give better dissent and willing to protect those who treatmentto those who speak that way. exercise it. Once this happens,more people are likely In short, social norms and laws are to try to speak that way. Eventually,peo- often mutually supporting. This is true ple might be punished(e.g., despised)for because social norms can become formal- not having the right accent. Thus, what ized into laws and because laws provide starts out as a signal about one person's external validation of norms. They are backgroundcan become a norm for all.6 also mutually supporting because they The signaling principle helps explain have complementarystrengths and weak- how an "is"becomes an "ought."As more nesses. Social norms are often best at pre- and more people use the signal to gain venting numeroussmall defectionswhere informationabout others, more and more the cost of enforcementis low. Laws, on people will adopt the behavior that leads the other hand, often function best to to being treatedwell. Graduallythe signal prevent rare but large defectionsbecause will change from indicatinga rare person substantial resources are available for to indicatinga common person. On the enforcement. other hand, the absence of the signal, which originally carried little informa- tion, will come to carry substantialinfor- mation when the signal becomes com- Reputation mon. When almost everyone behaves in An important, and often dominant, conformitywith a signal, those who don't reason to respecta norm is that violating stand out. These people can now be it would provide a signal about the type regarded as violators of a norm-and of person you are. Forexample, if thereis dealt with accordingly. a norm dictatingthat people should dress Note that there is an importantdistinc- formally for dinner, and you don't, then tion between a convention, which has no

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directpayoffs one way or the other (such forms disguised as equalitarian or in as wearing a tie for men), and a coopera- forms that are blatantly hierarchical.An tive act, the violation of which leads to apparentlyequalitarian norm is that the injury to others (e.g., queuing for serv- rich and the poor are equally prohibited ice). A type of behavior with no direct from sleeping under bridges at night. A payoffs can become a norm once it blatantly hierarchical norm is that develops some signaling value, as is the soldiers shall obey their officers. Both case when fashion leaders adopt a new forms are "normsof partiality,"to use the style (Veblen,1899). Once this happens,a term of Ullman-Margalit(1977). violator of this style will be looked down To say that the powerful can start a upon. Thus the style will become a norm; norm suggests a great deal about the individualswill usually follow the style, potential substanceof such norms. Once and those who do not will likely be started, the strong support the norms punished. because the norms support the strong. Dominance is not the only mechanism The Originand Contentof Norms capable of starting a norm. Reputation can do so as well. Consider,for example, Eightmechanisms have now been iden- the idea of keeping one's promise. In a tified that can serve to support a norm hypotheticalsociety in which few people that is already at least partially estab- kept their promises,you would be happy lished. What, however, are the charac- to deal with someone who did. You teristics of the behaviors that arise and would find it in your narrow self-interest then become more and more established to continue dealing with such a person, as norms?Or to put it anotherway, just and this in turnwould be rewardingto the what is the contentof behaviorthat might promise-keeper.Conversely, you would later turn into a norm? try to avoid deals with those you knew The answer depends on what types of did not keep their promises. You would, behaviorcan appearand spreadin a pop- in effect, be vengeful against defectors ulation even when only a few people without having to pay an enforcement initially exhibit the behavior. This, in cost. Indeed, your enforcement would turn, dependson what kind of behavioris simplybe the resultof your actingin your likely to be rewardedand punishedfor its own interests,based upon the reputations own sake, independentlyof whether or of othersand your calculationabout what not it is common behavior. was good for yourself. Two of the supporting mechanisms Internationalregimes depend on just alreadyconsidered can serve in this initial such reputational mechanisms to get role: dominance and reputation. Domi- norms started (Keohane, 1984). In such nancecan work becauseif only a few very cases, countries can be very deliberate powerfulactors want to promotea certain about what promises they make and pattern of behavior, their punishments which ones they want to keep when the alone can often be sufficient to establish stakes are high (Axelrod, 1979). Reputa- it, even if the others are not vengeful tional effects can also be based upon the against defections. The implications for limitedrationality of trialand errorlearn- the substanceof norms are obvious: it is ing. If a person associates another's easier to get a norm startedif it serves the responseto a particularact (say a refusal interestsof the powerful few. to continue dealing as a reaction to the In fact, many norms obeyed and even breakingof a promise), then the violator enforced by almost everyone actually can learn not to break promises. serve the powerful. This can happen in This learning approach suggests the

1108 1986 Evolution of Norms importance of being able to link the enforcement cost of punishing a defec- behavior with the response. Behaviors tion. Once vengeance became rare, the will be easier to establishas norms if the averagelevel of boldness rose again, and optimal responseof others is prompt and the norm completely collapsed. More- rewarding. Failing a prompt response, over, the collapse was a stable outcome. learningcan also take place if the delayed This result led to a search for mech- punishment is explicitly cited as a anisms that could sustain a partially response to the earlierdefection. establishednorm. One possibility is the metanorm: the treatment of nonpunish- Summaryand Conclusion ment as if it were anotherform of defec- tion; that is, a player will be vengeful To study the development of norms, against someone who observed a defec- the strategic situation has been modeled tion but did not punish it. Simulationof as an n-persongame. In the basic norms the evolution of strategies in this meta- game, everyone has two types of choice: norms game demonstrated that players the choice to cooperate or defect, which had a strong incentive to increase their affects everyone, and the choice of vengefulness lest they be punished by whetheror not to punish a specificperson others, and this in turn led to a decline of seen defecting. A player's strategy is boldness. Thus, metanormscan promote describedin terms of how these choices and sustain cooperationin a population. will be made. A strategy consists of two Other mechanismsfor the support of parameters:boldness (the largest chance norms are also important. These include of being seen that will lead to a choice of dominance, internalization, deterrence, defection)and vengefulness(the probabil- social proof, membership,law, and repu- ity of punishingsomeone observeddefect- tation. In some cases, the resultingnorms ing). To the extent that playersare venge- are hierarchicalrather than equalitarian, ful, but not very bold, a norm can be said and the cooperationexhibited is coerced to have been established. rather than freely offered. A good exam- To study the dynamics of the process, ple is the norm of black deferencein the an evolutionaryapproach was employed. old South. In this approach, the initial strategiesare Dominance processes have been simu- chosen at random, and the populationof lated by subdividingthe population and players is given opportunities to defect letting one segmentbe relativelyresistant and to punishthe defectionsthey observe. to the effects of punishmentby members The evolutionary approach dictates that of the other segment. Internalizationcan strategies proving relatively effective be investigatedby studying the effects of are more likely to be employed in the making defection costly rather than future while less effective strategies are rewardingfor some of the defectorsand dropped. Moreover, strategies undergo by making punishmenta pleasurerather some random mutation so that new ones than a cost for some of the observersof a are always being introduced into the defection. A more drastic change in the population. modelingprocedures would be necessary The computersimulation of this proc- to study some of the other mechanismsin ess revealedan interestingdynamic in the question. norms game. At first, boldness levels fell Norms are important in society and, dramaticallydue to the vengefulness in not surprisingly,have been given a great the population. Then, gradually, the deal of attention in the social sciences, amount of vengefulnessalso fell because includingsociology, anthropology,politi- there was no direct incentive to pay the cal science, psychology, and economics.

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While descriptions of actual norms algorithm of computer scientist John Holland (1975, abound, investigationsof the reasons for 1980). norm 2. For convenience, it is also assumed that the people to obey or violate a given chance of being seen not punishing is the same have been much less common. Even as the chance of the original defection being seen. among the strategicapproaches to norms, The payoff for metapunishment is P' = -9, and the relativelylittle attentionhas been devoted metaenforcement cost is E' = -2. to the dynamicsof norms: 3. Marx goes as far as to say that social norms are understanding merely reflections of the interests of the ruling class, how they can get started, how a partial and the other classes are socialized into accepting norm can be sustained and become well these norms under "false consciousness." established, and how one norm can dis- 4. I thank David Yoon and Lynn Sanders for place another. An evolutionaryapproach pointing this out to me. dynamics 5. I thank David Yoon for formulating this vari- is helpful in studying these ant of the metanorms game and the application to because it can help show how strategies alliances that follows. change over time as a function of their 6. The same process of formalizing norms applies relative success in an ever-changing to private laws and regulations, as in the case of a of other players who are business that issues an internal rule about who is environment responsible for making coffee. also changing their own strategies with 7. For the theory of signaling, see Spence (1974). experience. For a theory of how customs can be sustained by A major goal of investigating how reputations, see Akerlof (1980). cooperative norms in societal settings 8. Signals can also help to differentiate groups and thereby maintain group boundaries and have been established is a better under- cohesiveness. standing of how to promote cooperative normsin internationalsettings. This is not as utopian as it might seem becauseinter- References national norms against slavery and Abreu, Dilip, David Pearce, and Ennio Stacchetti. colonialismare alreadystrong while inter- 1985. Optimal Cartel Equilibriawith Imperfect nationalnorms are partlyeffective against Monitoring. Minneapolis: University of Min- racial discrimination,chemical warfare, nesota Institute for Mathematicsand its Ap- plications. and the proliferationof nuclearweapons. Akerlof, GeorgeA. 1980. A Theory of Social Cus- Becausenorms sometimesbecome estab- tom, of Which UnemploymentMay Be One lished surprisinglyquickly, there may be Consequence.Quarterly Journal of Economics, some useful cooperativenorms that could 94:749-75. hurried along with relatively modest Almond, Gabriel, and Sidney Verba. 1963. The be Civic Culture. Princeton:Princeton University interventions. Press. Arkin, William, and RichardW. Fieldhouse.1985. Focus on the NuclearInfrastructure. Bulletin of Notes the Atomic Scientists,41:11-15. Asch, Solomon E. 1951. Effectsof Group Pressure I owe a great deal to Stephanie Forrest, my upon the Modificationand Distortion of Judg- researchassistant, and to thosewho helpedme think ment. In HaroldGuetzkow, ed., Groups,Leader- about norms: Michael Cohen, Jeffrey Coleman, ship and Men. Pittsburgh:Carnegie Press. John Ferejohn, Morris Fiorina, Robert Gilpin, Asch, Solomon E. 1956. Studies of Independence Donald Herzog, John Holland, Melanie Manion, and Conformity:I. A Minorityof One Againsta Ann McGuire, Robert Keohane, Robert McCalla, Unanimous Majority. Psychological Mono- Amy Saldinger, Lynn Sanders, Kim Scheppele, graphs, 41:258-90. Andrew Sobel, Charles Stein, Laura Stoker, and Axelrod,Robert. 1979. The RationalTiming of Sur- David Yoon. I am also pleasedto thank those who prise. WorldPolitics, 31:228-46. helped support various aspects of this work: the Axelrod, Robert. 1984. The Evolutionof Coopera- HarryFrank Guggenheim Foundation, the National tion. New York:Basic Books. ScienceFoundation, the Sloan Foundation,and the Axelrod, Robert, and Robert 0. Keohane. 1985. MichiganMemorial Phoenix Project. Achieving CooperationUnder Anarchy: Strate- 1. The procedureused is inspiredby the genetic gies and Institutions.World Politics, 38:226-54.

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Black-Michaud, Jacob. 1975. Cohesive Force: Feud SequentialEquilibria. Econometrica, 50:863-94. in the Mediterranean and the Middle East. Matthews, Donald R. 1960. U.S. Senators and Oxford: Basil Blackwell. Their World. Chapel Hill: Universityof North Bronfenbrenner, Urie. 1970. Two Worlds of Child- CarolinaPress. hood: U.S. and U.S.S.R. New York: Russell Meyers, Samuel M., and William C. Bradbury. Sage. 1968. The Political Behavior of Korean and Cantril, Hadley. 1941. The Psychology of Social ChinesePrisoners of Warin the KoreanConflict: Movements. New York: Wiley. A HistoricalAnalysis. In SamuelM. Meyersand Cialdini, Robert H. 1984. Influence-How and Albert D. Briderman,eds., Mass Behavior in Why People Agree to Things. New York: Battleand Captivity, The CommunistSoldier in Morrow. the KoreanWar. Chicago: University of Chicago Cyert, Richard M., and James G. March. 1963. A Press. Behavioral Theory of the Firm. Englewood Olson, Mancur. 1965. The Logic of Collective Cliffs, NJ: Prentice-Hall. Action. Cambridge,MA: Harvard University Dahl, Robert A., ed. 1966. Political Oppositions in Press. Western Democracies. New Haven: Yale Univer- Opp, Karl-Dieter.1979. Emergenceand Effects of sity Press. Social Norms-Confrontation of Some Hypoth- Darley, John M., and C. Daniel Batson. 1973. "From eses of Sociology and Economics. Kylos, 32: Jerusalem to Jerico": A Study of Situational and 775-801. Dispositional Variables in Helping Behavior. app, Karl-Dieter.1983. EvolutionaryEmergence of Journal of Personality and Social Psychology, Norms. BritishJournal of SocialPsychology, 21: 27:100-108. 139-49. Friedman, James W. 1971. A Non-cooperative Orbell, John M., Peregrine Schwartz-Shea,and Equilibrium for Supergames. Review of Eco- RandallT. Simmons.1984. Do CooperatorsExit nomic Studies, 38:1-12. MoreReadily than Defectors? American Political Furubotn, Eirik G., and Svetozar Pejovich, eds. ScienceReview, 78:163-78. 1974. The Economics of Property Rights. Cam- Schelling,Thomas. 1978. Micromotivesand Macro- bridge, MA: Ballinger. behavior.New York:W. W. Norton. Hardin, Garrett. 1968. The Tragedy of the Com- Schotter,Andrew. 1981. EconomicTheory of Social mons. Science, 162:1243-48. Institutions.Cambridge: Cambridge University Hardin, Russell. 1982. Collective Action. Baltimore: Press. Johns Hopkins University Press. Scott, John F. 1971. Internalizationof Norms. Holland, John H. 1975. Adaptation in Natural and EnglewoodCliffs, NJ: Prentice-Hall. Artificial Systems. Ann Arbor: University of Selten, R. 1975. Reexaminationof the Perfectness Michigan Press. Concept for EquilibriumPoints in Extensive Holland, John H. 1980. Adaptive Algorithms for Games. InternationalJournal of Game Theory, Discovering and Using General Patterns in 4:25-55. Growing Knowledge Bases. International Journal Sherif, Muzafer. 1936. The Psychology of Social of Policy Analysis and Information Systems, 4: Norms. New York: Harperand Brothers. 245-68. Spence,A. Michael.1974. MarketSignalling. Cam- Keohane, Robert 0. 1984. After Hegemony: Coop- bridge,MA: HarvardUniversity Press. eration and Discord in the World Political Econ- Truman,Ben C. 1884. Fieldof Honor:A Complete omy. Princeton: Princeton University Press. and ComprehensiveHistory of Dueling in All Krasner, Stephen D., ed. 1983. International Countries. New York: Fords, Howard and Regimes. Ithaca: Cornell University Press. Hilbert. Krehbiel, Keith. 1985. Unanimous Consent Agree- Ullmann-Margalit,Edna. 1977. The Emergenceof ments: Going Along in the Senate. Working Norms. Oxford:Oxford University Press. paper no. 568. California Institute of Technol- Veblen, Thorstein. 1899. The Theory of the ogy, Social Science Department. LeisureClass. New York:Macmillan. Kreps, David M., and Robert Wilson. 1982.

RobertAxelrod is Professorof Political Science,University of Michigan,Ann Arbor, MI 48109.

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