Game theory has repeatedly confirmed the human tendency to help others, even when helping is costly. The Prisoner’s Dilemma is one of the most popular demonstrations of cooperation. Though behavior in the Prisoner’s Dilemma has long been observed, studies in neuroscience continue to elucidate the brain mechanisms underlying the choices that have puzzled some researchers for decades.

In this game, two players choose whether to cooperate or defect. For each player, the highest payout occurs when the player defects and their partner cooperates. Smaller payouts to both players result from mutual cooperation. However, players who cooperate while their partners defect receive no payout.

If the goal is to receive the largest payout, the best strategy in this game is to defect. If your partner cooperates, defecting guarantees you a bigger payout than if you too cooperated. On the other hand, if your partner defects, you get a payout of zero, regardless of your choice. Those familiar with game theory will likely recognize a scenario wherein both players defect as the Nash Equilibrium for this dilemma. However, in practice, players cooperate about 50% of the time in the Prisoner’s Dilemma.

Scientists and economists have described this cooperative behavior as rational when players repeatedly endure the dilemma with the same partner. In a situation where retaliation will prevent players from subsequent payouts, cooperation is the best long-term strategy. Economists and evolutionary biologists have pointed to potential monetary and reproductive benefits as explanations for this ‘direct reciprocity.’

Nonetheless, humans appear to cooperate in the Prisoner’s Dilemma game, even when they know their interaction will occur only once. This ‘indirect reciprocity,’ which does not provide clear benefits to players, is harder to explain. In a study that will be published this month in Proceedings of the National Academy of Sciences, Naoki Masuda and colleagues describe, for the first time, distinct neural mechanisms underlying two forms of indirect reciprocity: ‘reputation based’ and ‘pay-it-forward.’

Reputation-based reciprocity is relatively simple for scientists to tackle, as a good reputation built from cooperative behavior can lead to the same types of benefits reaped from direct reciprocity. Indeed, in a society, the emergence of which may represent the most recent evolutionary shift, success is often inextricably linked to reputation. However, pay-it-forward reciprocity occurs independent of any impact on reputation. Such reciprocity therefore benefits society but has no obvious advantage for the individual provider. So why are humans, the most intelligent species, much more likely than other species to help those to whom they are not related at their own expense?

The choices we make rely heavily on the information that reaches the striatum, the part of the brain that processes value and executes voluntary behaviors. Accordingly, various forms of cooperation have been shown to engage the striatum, including reputation-based reciprocity. It is therefore perhaps not surprising that Masuda and his colleagues found that both forms of indirect reciprocity enhanced the ability of certain brain areas to communicate with the striatum. However, whereas reputation-based reciprocity enhanced communication from areas of the brain involved in cognition, pay-it-forward reciprocity enhanced communication from areas involved in emotion and empathy. Thus, we appear to deem each form of reciprocity as valuable, but for different reasons.

Why may we have evolved to respond emotionally to the needs of those who cannot be of help to us? With a high level of sophistication comes a need for efficiency. Professors David Rand and Martin Nowak, whose combined expertise spans mathematics, biology, economics, and psychology, suggest that this behavior may represent over-generalization of cooperative strategies that are directly beneficial to the cooperator. In other words, as we have learned that certain forms of cooperation are personally beneficial, we may have adapted a general tendency to cooperate, perhaps so as not to use time and biological resources to assess the value of each cooperative action.

Regardless of the reason, we can all be grateful that, in our species, even strangers are likely looking out for us.

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