Bystander effect

Biological Parallel

In mixed-species bird flocks, individual species rely on others to raise alarm calls—each assumes another will signal, creating collective inaction when all species defer. This diffusion of vigilance evolved because monitoring is costly; free-riding on others' watchfulness conserves energy. The bystander effect is the tragedy of distributed responsibility: when sentinel duty distributes across a group, each individual's threshold for action rises as group size increases, paradoxically reducing per-capita responsiveness despite more total eyes.