“No man is an island，”wrote the poet John Donne several centuries ago.He was acknowledging one of our most distinctive characteristics：the fact that we are social animals whose behavior and personalities are shaped by the groups to which we belong.
Throughout life，most of our daily activities are performed in the company of others.Whether our purpose is working，playing，raising a family，learning，or simply relaxing，we usually pursue it in groups，even if the group is as small as two or three people.Out need for human contacts is not merely a practical one；it is a deep psychological need as well.If people are deprived of the company of others for prolonged periods，mental breakdown is the usual result.Even the Geneva Convention（日内瓦公约），an international agreement that regulates the treatment of prisoners of war，recognizes this need.It regards solitary（孤独的）imprisonment for more than thirty day as a cruel form of torture（折磨）.
In its strictest sense，a group is a collection of people interacting together in an orderly way on the basis of shared expectations about one another’s behavior.As result of this interaction，members feel a common sense of “belonging.”They distinguish members from nonmembers and expect certain kinds of behavior from outsiders.

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