This paper describes a new multi-player computer game, Colored Trails (CT), which may be played by people, computers and heterogeneous groups. CT was designed to enable investigation of properties of decision-making strategies in multi-agent situations of varying complexity. The paper presents the results of an initial series of experiments of CT games in which agents? choices affected not only their own outcomes but also the outcomes of other agents. It compares the behavior of people with that of computer agents deploying a variety of decision-making strategies. The results align with behavioral economics studies in showing that people cooperate when they play and that factors of social dependency influence their levels of cooperation. Preliminary results indicate that people design agents to play strategies closer to game-theory predictions, yielding lower utility. Additional experiments show that such agents perform worse than agents designed to make choices that resemble human cooperative behavior. The paper describes challenges raised by these results for designers of agents, especially agents that need to operate in heterogeneous groups that include people.