The decoy effect is a cognitive bias documented in behavioural economics by which the presence of a third, (partly) inferior choice causes a significant shift in people’s preference for other items. Here, we performed an experiment with human volunteers who played a variant of the repeated prisoner’s dilemma game in which the standard options of “cooperate” and “defect” are supplemented with a new, decoy option, “reward”. We show that although volunteers rarely chose the decoy option, its availability sparks a significant increase in overall cooperativeness and improves the likelihood of success for cooperative individuals in this game. The presence of the decoy increased willingness of volunteers to cooperate in the first step of each game, leading to subsequent propagation of such willingness by (noisy) tit-for-tat. Our study thus points to decoys as a means to elicit voluntary prosocial action across a spectrum of collective endeavours.

Cooperation can be sustained by institutions that punish free-riders. Such institutions, however, tend to be subverted by corruption if they are not closely watched. Monitoring can uphold the enforcement of binding agreements ensuring cooperation, but this usually comes at a price. The temptation to skip monitoring and take the institution’s integrity for granted leads to outbreaks of corruption and the breakdown of cooperation. We model the corresponding mechanism by means of evolutionary game theory, using analytical methods and numerical simulations, and find that it leads to sustained or damped oscillations. The results confirm the view that corruption is endemic and transparency a major factor in reducing it.

(269 words) 17Corruption is one of the most serious obstacles for ecosystem management and biodiversity 18 conservation. In particular, more than half of the loss of forested area in many tropical 19 countries is due to illegal logging, with corruption implicated in a lack of enforcement. Here 20 we study an evolutionary game model to analyze the illegal harvesting of forest trees, coupled 21 with the corruption of rule enforcers. We consider several types of harvesters, who may or 22 may not be committed towards supporting an enforcer service, and who may cooperate (log 23 legally) or defect (log illegally). We also consider two types of rule enforcers, honest and 24 corrupt: while honest enforcers fulfill their function, corrupt enforcers accept bribes from 25 defecting harvesters and refrain from fining them. We report three key findings. First, in the 26 absence of strategy exploration, the harvester-enforcer dynamics are bistable: one continuum 27 of equilibria consists of defecting harvesters and a low fraction of honest enforcers, while 28 another consists of cooperating harvesters and a high fraction of honest enforcers. Both 29 continua attract nearby strategy mixtures. Second, even a small rate of strategy exploration 30 removes this bistability, rendering one of the outcomes globally stable. It is the relative rate of 31 exploration among enforcers that then determines whether most harvesters cooperate or defect 32 and most enforcers are honest or corrupt, respectively. This suggests that the education of 33 enforcers, causing their more frequent trialing of honest conduct, can be a potent means of 34 curbing corruption. Third, if information on corrupt enforcers is available, and players react 35 opportunistically to it, the domain of attraction of cooperative outcomes widens considerably. 36We conclude by discussing policy implications of our results. 37Keywords: line segments of equilibria, bistability, exploration-induced-equilibrium, 38 information 39 40Page 3 of 45 Introduction 41Although the "tragedy of the commons" is ubiquitous (Hardin, 1968), field research on 42 governing the commons, as well as laboratory experiments on public good games, show that, 43 sometimes, cooperation can be maintained and the tragedy avoided (e.g., Ostrom, 1990; 44 Henrich, 2006;Henrich et al., 2006; Rutagi et al., 2010). In particular, research by Elinor 45Ostrom and colleagues has shown that people are frequently able to discuss, establish, and 46 enforce rules defining a system of punishment for rule breakers (Ostrom, 2000). In her view, 47 institutions are tools for providing incentives to promote cooperation (Ostrom and Walker, 48 1997; Ostrom et al., 2004). Ostrom's basic design principles for social settings that allow 49 long-lasting resource use include the successful establishment of a monitoring and 50 sanctioning system (Ostrom, 1990). Such systems provide examples of mechanisms that 51 enforce cooperation by punishing defectors. 52The general theory of sanctioning mechanisms has been stu...

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