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Reciprocity: Weak Or Strong? What Punishment Experiments Do (And Do Not) Demonstrate

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Reciprocity: Weak Or Strong? What Punishment Experiments Do (And Do Not) Demonstrate BEHAVIORAL AND BRAIN SCIENCES (2012) 35,1–59 doi:10.1017/S0140525X11000069 Reciprocity: Weak or strong? What punishment experiments do (and do not) demonstrate Francesco Guala Department of Economics, University of Milan, 20122 Milan, Italy [email protected] http://users.unimi.it/guala/index.htm Abstract: Economists and biologists have proposed a distinction between two mechanisms – “strong” and “weak” reciprocity – that may explain the evolution of human sociality. Weak reciprocity theorists emphasize the benefits of long-term cooperation and the use of low-cost strategies to deter free-riders. Strong reciprocity theorists, in contrast, claim that cooperation in social dilemma games can be sustained by costly punishment mechanisms, even in one-shot and finitely repeated games. To support this claim, they have generated a large body of evidence concerning the willingness of experimental subjects to punish uncooperative free- riders at a cost to themselves. In this article, I distinguish between a “narrow” and a “wide” reading of the experimental evidence. Under the narrow reading, punishment experiments are just useful devices to measure psychological propensities in controlled laboratory conditions. Under the wide reading, they replicate a mechanism that supports cooperation also in “real-world” situations outside the laboratory. I argue that the wide interpretation must be tested using a combination of laboratory data and evidence about cooperation “in the wild.” In spite of some often-repeated claims, there is no evidence that cooperation in the small egalitarian societies studied by anthropologists is enforced by means of costly punishment. Moreover, studies by economic and social historians show that social dilemmas in the wild are typically solved by institutions that coordinate punishment, reduce its cost, and extend the horizon of cooperation. The lack of field evidence for costly punishment suggests important constraints about what forms of cooperation can or cannot be sustained by means of decentralised policing. Keywords: Cooperation; evolution; experiments; punishment; reciprocity 1. Introduction small fraction of free-riders can drive positive reciproca- tors towards low levels of cooperation. Costly punishment Over the last two decades, research on human cooperation in such circumstances may provide enough policing to pre- has made considerable progress on both the theoretical and serve an environment where cooperation can thrive. To the empirical front. Economists and biologists have support this claim, strong reciprocity theorists have gener- proposed a distinction between two kinds of mechanism ated a large body of evidence concerning the willingness of – “strong” and “weak” reciprocity – that may explain the experimental subjects to punish uncooperative free-riders evolution of human sociality. Reciprocity is, broadly speak- at a cost for themselves. This evidence and its theoretical ing, a tendency to respond “nice” to nice actions and “nasty” implications constitute the main topic of this article. to nasty actions when interacting with other players. Models Although positive reciprocity is at least as important for of weak reciprocity require that reciprocal strategies be the mechanics of cooperation, it deserves a separate analy- profitable for the agents who play them. Strong reciprocity sis and will not be discussed except briefly at the end. models, in contrast, allow players to choose suboptimal strategies, and thus diverge substantially from the models of self-interested behaviour that are typically used by evol- 1 utionary biologists and rational choice theorists. FRANCESCO GUALA is Associate Professor in the The behaviour of strong reciprocators can be less than Department of Economics at the University of Milan optimal in roughly two ways: On the one hand, strong reci- (Italy). He works primarily on the philosophical foun- procators play cooperatively with cooperators, even dations of social science, using experimental and theor- though it would be more advantageous to exploit them etical methods. He is the author of The Methodology of (let us call it positive strong reciprocity). On the other, Experimental Economics (Cambridge University Press, strong reciprocators are willing to punish defectors at a 2005) and co-editor of The Philosophy of Social Science cost to themselves, even though it would be advantageous Reader (Routledge, 2011). In 2002 he was the recipient to simply ignore them (negative strong reciprocity). These of both the International Network of Economic Method Prize and the History of Economic Analysis two types of action constitute the “bright” and the “dark” Award. In 2009 he has been awarded a special “anti- side of reciprocity, so to speak. brain-drain” scholarship by the Italian Ministry of Both sides of reciprocity may be necessary to sustain Higher Education. human cooperation. In a heterogeneous population, a # Cambridge University Press 2012 0140-525X/12 $40.00 1 Guala: Reciprocity I argue that the message of punishment experiments is far from clear. To dispel some confusion, I introduce a few preliminary distinctions between concepts (such as absol- ute and relative costs, symbolic and material, and coordi- nated and uncoordinated punishment) that are often conflated in the writings of reciprocity theorists. It turns out that experimental results can be interpreted in differ- Figure 1. A Prisoner’s Dilemma game. The usual conventions ent ways, and that while some interpretations are empiri- apply: The strategies of Player 1 are represented as rows, and cally warranted, others are just unproven conjectures at those of Player 2 as columns. The first number in each cell is this stage. The first purpose of this article is to clarify the the payoff of Player 1; the second one of Player 2. methods used by economists and biologists and help the resolution of open issues in reciprocity theory. I distinguish between a “narrow” and a “wide” reading In the Prisoner’s Dilemma game two players must of the experimental evidence. Under the narrow reading, choose simultaneously one of two strategies, Cooperate punishment experiments are just useful devices to (C) or Defect (D). It is immediately obvious that mutual measure robust psychological propensities (“social prefer- cooperation (CC) is more efficient than mutual defection ences”) in controlled laboratory conditions. Under the (DD). The payoffs of the game, however, are designed wide reading, they replicate a mechanism that supports in such a way that each player has an incentive to defect, cooperation also in “real-world” situations outside the lab- regardless of what the other player does. If the other oratory. These two interpretations must be kept separate player cooperates, defection delivers three units of because cooperation outside the laboratory may be sus- payoff instead of two; if the other defects, it guarantees tained by mechanisms that have little to do with those one unit instead of nothing. But this reasoning should studied by experimental economists. lead both players to defect: In game-theoretic jargon, I shall argue that the wide interpretation can only be mutual defection (DD) is the only Nash equilibrium in tested using a combination of laboratory data and evidence the one-shot Prisoner’s Dilemma. about cooperation “in the wild.” Field evidence, however, A Nash equilibrium is a set of strategies (one for each brings bad news for strong reciprocity theorists. I will player in a game) such that no one can do better by chan- focus on two points in particular: First, in spite of some ging her strategy unilaterally. Nash equilibria are self- often-repeated claims, there is no evidence that sustaining, or self-policing, in the sense that they are cooperation in the small egalitarian societies studied by robust to individual attempts to gain by deviating from anthropologists is enforced by means of costly punish- the current strategies (because, quite simply, no such ment. Second, studies by economic and social historians gains are possible). It seems highly desirable that social show that social dilemmas in the wild are typically solved institutions should be Nash equilibria, for they would be by institutions that reduce the costs of decentralized pun- robust to exploitation and the constant threat posed by ishment and facilitate the functioning of weak reciprocity individual greed. “Cooperate” in the Prisoner’s Dilemma mechanisms. The second goal of this article, then, is to is a prototypical rule that would enhance social welfare if survey relevant evidence from history and anthropology generally endorsed by the members of the group. It is that economists are usually unfamiliar with, and which is not, however, a stable institution, for it is not a Nash equi- sometimes misrepresented by reciprocity theorists. librium of this simple game. Although mutual cooperation The conclusions to be drawn from this exercise, (CC) is more efficient than mutual defection, it is strictly however, are not entirely negative for strong reciprocity dominated and will not be played by rational selfish indi- theory. I shall argue that costly punishment experiments viduals. If the social contract game were a one-shot Prison- may still be useful as measurement devices, to observe er’s Dilemma, then a population of rational players would motives that would otherwise be difficult to detect never be able to pull themselves out of the war of all outside the laboratory. Negative and positive reciprocity, against all. moreover, may be governed
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