A new study from Saïd Business School, Universidad de Santiago de Chile, and William & Mary suggests that cooperation does not simply fade in a slow, predictable way. Instead, it rises and falls in repeated cycles, driven more by shifting human behaviour than by purely strategic thinking or financial pressures. The findings challenge long-standing assumptions about how and why people choose to work together over time.

“In everyday life, people often face a simple decision: contribute their share or let someone else carry the load,” explains Nicholas Sabin, the study’s first author and an Associate Professor of Behavioral Science at the Universidad de Santiago de Chile. He notes that the research offers new insight into why cooperation is so fragile, even when it benefits everyone involved. More importantly, it may help identify ways to sustain cooperative behaviour over longer periods, particularly in situations where ongoing collaboration is essential.

The research draws on a five-year field study conducted with a microfinance institution in Sierra Leone, tracking more than 47,000 repayment transactions from over 7,000 borrowers. Participants operated within a joint-liability lending system, meaning that each group had to ensure full repayment of its loan each month collectively. If even one member failed to contribute, the entire group risked losing access to future credit. This real-world setting provided a rare opportunity to observe cooperation under meaningful economic and social stakes.

According to co-author David Klinowski, an Assistant Professor of Economics at William & Mary, group lending offers an unusually realistic environment for studying cooperation. Unlike laboratory experiments, where outcomes are often abstract or low-stakes, the consequences here are tangible and immediate. Borrowers who fail to cooperate can face serious financial and social repercussions, making their decisions more reflective of real-life behaviour rather than controlled experimental responses.

The researchers observed a consistent pattern across borrower groups. Cooperation tended to start at high levels, as participants were motivated and attentive to their shared responsibilities. Over time, however, this commitment gradually weakened as individuals became fatigued, less vigilant, or more tempted to rely on others’ efforts. Each time a new loan cycle began, cooperation surged again, boosted by renewed awareness of the consequences of failing to contribute. Yet with each cycle, this rebound became shorter-lived, and cooperation declined more quickly than before. The authors describe this repeating pattern of erosion and recovery as a “punctuated decline.”

These findings run counter to traditional theories that frame declining cooperation as a result of rational learning or strategic adjustment. Instead, the study points to behavioural fatigue and shifting motivation as the primary drivers. Interviews with microfinance clients and staff reinforced this interpretation, revealing how emotions, habits, and social dynamics shape decisions to cooperate or defect. The implications extend far beyond lending groups. Whether in healthcare decisions like vaccination, civic participation such as voting, or international agreements between countries, sustained cooperation depends not only on incentives but on maintaining motivation. The researchers suggest that organisations can support long-term cooperation by introducing periodic resets, fostering intrinsic motivation, and designing systems that make cooperative behaviour easier to sustain over time.

More information: Nicholas Sabin et al, Punctuated decline of human cooperation, Nature. DOI: 10.1038/s41586-026-10380-3

Journal information: Nature Provided by University of Oxford