Understanding the concept of groups is required to be able to appreciate the insights on group behaviour that I would be posting over the next few days. The insights on group behaviour are important because our behaviour is almost always masked by the perceived group behaviour.
Groups are a set of people with some common characteristics. A group can be as broad as "All Men on earth" or it may be much smaller such as "The Class of 1997, Harvard". Groups may be temporary, e.g "people on Times Square on 31st Dec 1999". Or they may be permanent, "All women born in the year 1985". All of us are part of many groups. We may be part of a group by choice (e.g friend circle, work group, club); by birth (religion, ethnic group); by situation (e.g. At Times Square on 31st Dec 1999); by force (e.g. passengers on board a hijacked flight); or by many other criteria.
Some groups are almost always dominant on us, e.g. family. Other groups are dominant for a brief period, e.g. the diners in a restaurant. At any given time our behaviour is heavily influenced by the groups that are dominant on us at that time. The behaviour of the group, in turn, is the sum total of the actions of each individual in the group. This is a self reinforcing cycle. On one hand it can lead a group to lay down their lives for a cause; and on the other, it can lead to violent mob behaviour that can shame mankind.