Participants in experiments often cooperate even when faced with a situation like the prisoner's dilemma. This study shows that cooperation is influenced by five main factors: the benefits of cooperation, expectations of others' behavior, aversion to risk and loss, aversion to unfairness, and the interaction of these factors. These factors together explain why people choose to cooperate in the prisoner's dilemma game. Previous attempts to explain choices in this game based on personality alone have not been successful.