Carron and Mark Eys investigated several definitions of groups and discovered five similar characteristics: (1) shared fate—sharing a common outcome with other members; (2) mutual benefit—a pleasurable, gratifying experience linked with group participation; and (3) social structure—a solid system of interactions among members. They also found that all defined groups have one thing in common: a relationship between the members. Group relationships can be positive or negative. Positive groups are those that share a special bond with each other, such as friends or family. Negative groups include enemies, rivals, and bystanders who are not related to each other but still participate in the same activity. Groups can also be defined by their level of integration. Integrated groups are those where members know and like each other, while differentiated groups have only limited contact with outsiders. Integratin is high when members trust and rely on each other, and it decreases if members avoid others outside the group. Groups form whenever people need help from each other for a common purpose. It is important for people to feel included within the group for them to get the benefits of sharing information and resources with others. The history of groups is full of examples where small groups have had a major impact on humanity. The most famous example is probably the six-degrees-of-separation theory proposed by Cornell University professor Duncan Watts.