Without necessarily disagreeing with these explanations, so far as they go, I want to emphasize the public-good aspect of the problem. The alliance we call society, normally not in danger of collapse, is threatened in time of disaster. In these circumstances alliance-supportive activities, cooperativeness and self-sacrifice, become an important public good. But, I also want to argue, a public good in large part describable in terms of the Weakest-link social composition function.