Why do people cooperate? Why does not every individuals in their groups choose to be a free-rider all the time? Based on two series of experiments, the author finds that reciprocity and punishment contribute to cooperation. When a group allows positive reciprocity where people can choose to cooperate, positive reciprocity facilitates cooperation. When reciprocity between group members turns to negative, however, group members make little contribution to the group and start to retaliate against their partners. Once punishment is incorporated into the experiment, people¡¦s behavior of cooperation varies. In the high positive reciprocity environment where punishment is used, people, as expected, continue to give more of their resources to their group. But it is worth noting that negative reciprocity did not get worse if punishment is used in group dynamics.