One of the most continually vexing problems in society is the variability with which citizens support endeavors that are designed to help a great number of people. In this article, we examine the twin roles of cooperative and antagonistic behavior in this variability. We find that each plays an important role, though their contributions are, understandably, at odds. It is this opposition that produces seeming unpredictability in citizen response to collective need. In fact, we suggest that careful consideration of the research allows one to often predict when efforts to provide a collectively beneficial good will succeed and when they will fail. To understand the dynamics of participation in response to collective need, it is necessary to distinguish between the primary types of need situations. A public good is an entity that relies in whole or in part on contributions to be provided. Examples of public goods are charities and public broadcasting. Public goods require that citizens experience a short-term loss (of their contribution) in order to realize a long-term gain (of the good). However, because everyone can use the good once it is provided, there is also an incentive to not contribute, let others give, and then take advantage of their efforts. This state of affairs introduces a conflict between doing what is best for oneself and what is best for the group. In a public goods situation, cooperation and antagonism impact how one resolves this conflict. The other major type of need situation is a common-pool resource problem. Here, a good is fully provided at the outset, and citizens may sample from it. The resource is usually, but not necessarily, partially replenished. Examples of replenished resources are drinking water and trees; examples of resources that are functionally not replenished are oil and minerals. Common-pool resources allow citizens to experience a short-term gain (by getting what they want in the early life of the resource) but also present the possibility of a long-term loss (if the resource dries up). As with public goods, there is thus a conflict between, on the one hand, acting in one's best interest and taking as much as one wants all the time and, on the other, acting for the good of the group, which requires taking a lesser amount so that the replenishment rate can keep up with the rate of use. As with public goods, both cooperation and antagonism affect this decision. With these situations in mind, we can now dig deeply into the dynamics of both cooperation and antagonism. Cooperation is one of the most heavily studied aspects of human behavior, yet despite this attention, there is much that is not understood about it, including its fundamental base. There are a number of different perspectives on the base. Interdependence theory argues that cooperation is driven by how one interprets the subjective value of the outcomes that will result from various combinations of behaviors. A person who sees a potential result of "50 to you, 50 to me" as "We both would do well" is more likely to ...