Some of the most pressing contemporary social problems result from the amalgamation of a mass of actions that are not intentionally coordinated. Although these essentially aggregative harms are foreseeable, it is unclear what moral duties individuals have with regards to them. This paper offers a new analysis of these problems and uses a nonideal contractualist approach to argue in favour of two kinds of duties for individuals. Collectivization duties that require individuals to act responsively with a view to ensuring that (...) there are effective governance agents that reliably, fairly, and efficiently prevent these outcomes in the long-term and duties of restraint that require individuals to avoid action of a kind that is likely to come together with other actions to cause serious EAH in the immediate future when restraint with regards to actions of this kind could help prevent the outcome from occurring. (shrink)