My aim in this chapter is to investigate what justice requires of agents who find themselves in situations that have the following general form. There are many agents who by acting together can avert some anticipated harm. Together, then, they share responsibility for avoiding that harm, but furthermore this collective responsibility can be divided fairly between them so that each knows what he or she must do to discharge that fair share. Despite this, however, some comply but others do not; the ones who do not comply could have chosen to do so, but they have shirked their share of the responsibility and in doing so have acted unfairly. The question then arises: How should we understand the position of those who have already complied or those who have yet to decide what to do? What are their responsibilities now, in the face of partial compliance? Must they take up the slack, by doing more than they were required to do according to the original fair division of responsibility? What does justice demand in this situation?

Situations of this general kind are not merely hypothetical: one can readily think of a number of real-world circumstances that either already do or easily could take the form just described. For instance, we might think of rescue cases where there are numbers of people awaiting rescue and several potential rescuers and it is reasonably clear how much fairness requires each to contribute to the operation. Or we might think of world poverty and the position of all those who could contribute to poverty relief by voluntary contributions to aid agencies. Or again if we switch focus from individual agents to collective agents such as nation-states, we could think of measures to conserve natural resources, such as agreements to reduce the catch of ocean fish, where each country is given a target quota such that if all countries comply, fish stocks will not be further depleted. Finally, we could consider the case of global warming and think of an agreement to cut greenhouse gas emissions with each nation being given a target to aim at, targets being set to reflect some underlying principle of fairness.