Intergroup Conflict Drives Commitment Signalling: A Cross-Cultural Experimental Evidence

Human evolutionary history has been shaped by recurrent intergroup competition, where cooperation within groups is critical and defection carries severe consequences. These pressures may have favoured mechanisms that reliably signal group commitment. Here, we test whether sacrificing strategic resources to signal commitment may be one such mechanism. Using a Public Goods Game with a cross-cultural sample (2,666 participants from eight countries), we manipulated intergroup competition by allowing groups to compete over collective resources and enabling individuals to defect to rival groups. Participants could signal commitment by sacrificing part of their endowment. We find that intergroup competition (relative to a no-competition condition) increases signalling both when it is costly and when it is more symbolic, suggesting that individuals exploit available signalling opportunities regardless of cost. Moreover, signallers contribute more to the public good despite reduced endowments and are less likely to defect. Simulations further show that signalling groups are nearly four times more likely to win competitions than non-signalling groups. These findings suggest that intergroup competition may have been a key selective pressure in the evolution of commitment signalling, with implications for how societies build trust, sustain cooperation, and deter defection in competitive contexts.