Contemporary philosophers and psychologists seek the roots of ethically sound forms of behavior, including altruism and a sense of fairness, in the basic structure of cooperative action. I argue that recent work on cooperation in both philosophy and psychology has been hampered by what I call “the mutualistic paradigm.” The mutualistic paradigm treats one kind of cooperative situation—what I call a “mutualistic situation”—as paradigmatic of cooperation in general. In mutualistic situations, such as the primeval stag hunt described by Brian Skyrms, every partner in a cooperative action has to do his part in order for the action as a whole to succeed. But many familiar cooperative situations—for example, serving on an academic committee—do not have this structure. Contemporary philosophers and psychologists are right that thinking about cooperation can shed light on how and why ethically sound behavior happens in human beings. But the deep connections between ethics and cooperation only come into view once we have a richer conception of our capacities for cooperation than the mutualistic paradigm provides.