We all know that the social context shapes our own behaviour, as well as our expectations of other people’s actions. When the vibe in a room is positive we feel relaxed, are more likely to smile and expect other people to be friendly. Conversely, when the atmosphere is tense we feel stressed, look stern and fear others might become hostile or aggressive.

But what creates the vibe and social context in the room? Other people’s attitudes towards us are an important factor. Will they be cooperative and open to mutually advantageous endeavours like collaborating on a project or watching a movie we both like? Or will they act competitively, trying to outshine us in front of others or dictate the terms during a negotiation?

People and animals routinely alternate between competitive and cooperative behaviours and researchers have long tried to understand how we choose between these two social orientations for access to resources, territories, and status.

A classic game used by economists and psychologists to probe how humans decide whether or not to cooperate with each other is the Prisoner’s Dilemma. The name of the game, devised by mathematicians Merrill Flood and Melvin Dresher in 1950, originated by the story first used to describe it, created by their colleague Albert Tucker. It goes like this: two prisoners are interrogated by the police in separate rooms about a crime they have committed together but for which the investigators have no evidence. So, they might get away as long as they both cooperate and refuse to confess. Meanwhile, if one of the prisoners betrays their accomplice, confessing the crime instead of narrating the agreed alibi, the one who chose to cooperate by denying their involvement will be punished, while the “defector” will walk away freely. Defecting can hence pay off. However, there is a risk: if both accuse each other, investigators will send both to prison. This story can be formalised into a game in which two players can cooperate for a modest gain, whilst tempted to betray their partner for a higher gain, but when both betray each other simultaneously they get punished.

Researchers have shown that humans (and monkeys) typically cooperate in the Prisoner’s Dilemma, even in single shot games when they know they’ll never meet the other player again. When playing multiple times with the same person players tend to retaliate occasional betrayals, which in the long term also reduces the chances of further betrayals. The instinct for cooperation and the ability to punish “free riders” are seen as key foundations for the evolution of trust, which underlines any form of cooperation. This website allows you to play the game, and see for yourself how different attitudes towards cooperation can evolve over time in a population depending on the initial conditions (proportion of “cooperative people”) and different rules in the allocation of payoffs (for instance, increasing the temptation to betray).

A general limitation of these studies is that predispositions to cooperate are measured with binary choices, where people can either cooperate or not. Yet, real world behaviour is nuanced. Many people are neither “fully cooperative” nor “competitive” but somewhere in between. What makes people decide how cooperative to be? And how do they adjust it over time?

In a recent study that I co-authored, 54 volunteers played a continuous version of the Prisoner’s Dilemma that allowed them to choose how much they wanted to cooperate. The more they cooperated the more they risked if the partner betrayed them. Yet, as in the Prisoner’s Dilemma, if both cooperated, the reward was on average higher for both than if both betrayed each other. As participants chose how much to cooperate by positioning themselves with respect to a dominant position, we called this game Space Dilemma.

We also added three twists to the original game. Rewards were probabilistic instead of being certain, which added excitement. Furthermore, in any trial we declared a winner based on their respective positions, providing an incentive to compete for the dominant position. Finally we defined a cooperative, a neutral and a competitive condition by gradually increasing the temptation to betray and conversely, the risk of being betrayed.

When rewards were distributed fairly and the temptation to betray was low (cooperative condition) people were consistently very cooperative, as we expected. However, as we biassed rewards towards the winners (competitive condition) they become consistently more competitive. Interestingly in the neutral condition we saw that people became increasingly more competitive over time, little by little. Looking at how they interacted we could see that they retaliated to small increases in their partner’s competitiveness in the next trial, inching towards the dominant position. This effect was similar to a well-known economic principle described by the Hotelling law whereby shops selling the same goods end up irrationally close to each other in the best location on a high street.

An important aspect of this study was that one of the two participants played the game during functional magnetic resonance imaging (fMRI).  This brain-imaging technique showed us how different parts of the brain encoded the player’s behaviour as well as that of their co-player. We found that the latter was encoded in the right Temporal-Parietal Junction (TPJ), a region known for encoding Theory-of-Mind, which signalled if the co-player had been more or less cooperative than expected and how much so. This evidence suggests that people closely monitor each other’s behaviour and register even minor changes in the social attitude of the people they interact with. We also found that different parts of the anterior cingulate cortex (ACC), a part of the brain known to respond to rewards for both the self and other people, were sensitive to players being in the cooperative or competitive condition, suggesting that the activity of this areas is shaped by the social context in which the behaviour takes place.

Understanding which social contexts facilitate cooperative behaviours is of paramount importance for societies, both to increase people’s well-being and to reduce conflicts. Conversely, understanding how to control and constrain competitive behaviours may improve the performance of a group and its benefit to the wider society. This new paradigm can hopefully contribute to probing further hypotheses on how different factors and social contexts can increase cooperation and how they are encoded by the human brain.