How group size affects cooperation: Insights from brain science

Humans are inherently social creatures and heavily rely on teamwork to succeed. This ability to collaborate is deeply rooted in how our brains function. Notably, the size of an animal’s brain often correlates with the size of its social groups. Humans, with our relatively large brains, can form larger and more complex groups than can other animals. Traditionally, however, scientists believed that as groups grow in size, cooperation becomes more difficult. In large groups, losing one connection may not seem significant, and interacting with everyone sufficiently to build trust becomes challenging. Earlier studies reinforced this belief, suggesting that cooperation tends to diminish in larger groups.

A recent study led by Rei Akaishi at RIKEN CBS has turned this assumption on its head, demonstrating that larger groups can actually foster greater cooperation. The researchers conducted an experiment with 83 participants who played the “prisoner’s dilemma” game — a scenario in which players choose between cooperating for mutual benefit or acting selfishly. Group sizes ranged from two to six people, and participants were allowed to leave groups they found undesirable or remove uncooperative members. Throughout the game, participant brain activity was monitored using functional magnetic resonance imaging.

The results were unexpected: people in larger groups cooperated more frequently, with 57% of all decisions being cooperative. The tendency to cooperate increased as group size grew. While group size itself did not directly promote cooperative behavior, it influenced how people managed memory and made decisions during social interactions. Even when participants struggled to clearly recall past interactions, they often defaulted to prosocial behaviors, relying on their general inclinations to trust or cooperate. This suggests that when memory becomes unclear, people prioritize cooperation over caution, enabling smoother group dynamics.

Brain scans offered further insights into the decision-making process. Specific brain regions, such as the fusiform gyrus and precuneus, processed memories of past interactions, while the nucleus accumbens connected these memories to feelings of reward. The prefrontal cortex played a critical role in weighing these memories against personal tendencies, guiding decisions about whether a participant should cooperate with someone who might have previously betrayed them. When memories were less reliable, the brain appeared to naturally encourage cooperation, perhaps as a way to maintain group harmony.

These findings offer a fresh perspective on how humans build trust and interact within groups. Rather than relying solely on stable, long-term relationships. The study highlights the benefits of flexible and fluid social connections for fostering cooperation. This insight is particularly relevant in today’s world, where digital platforms and online communities thrive on dynamic, ever-evolving relationships.

“In practical terms, our findings could help improve teamwork in schools, workplaces, and online environments,” says Akaishi. Allowing people to freely form and adjust connections, rather than adhering to rigid group structures, could lead to better collaboration. For organizations, embracing this natural flexibility in system design may enhance overall group dynamics.

The research also sheds light on how humans evolved to cooperate in large societies. By leveraging memory and adaptability, humans have developed the capacity to work together effectively, even under uncertain circumstances. “We plan to further explore these findings in real-world settings, such as schools or companies, to better understand how factors like culture, leadership, and individual personalities influence group behavior,” says Akaishi. “Our work aims to provide practical strategies for creating more harmonious and productive communities.”