In the social sciences a social group has been defined as two or more humans who interact with one another, share similar characteristics and collectively have a sense of unity. Other theorists, however, are a wary of definitions which stress the importance of interdependence or objective similarity. Instead, for researchers in the social identity tradition "a group is defined in terms of those who identify themselves as members of the group". Regardless, social groups come in a myriad of sizes and varieties. For example, a society can be viewed as a large social group.
A social group exhibits some degree of social cohesion and is more than a simple collection or aggregate of individuals, such as people waiting at a bus stop, or people waiting in a line. Characteristics shared by members of a group may include interests, values, representations, ethnic or social background, and kinship ties. Kinship ties being a social bond based on common ancestry, marriage, or adoption. In a similar vein, some researchers consider the defining characteristic of a group as social interaction.
Renowned social psychologist Muzafer Sherif formulated a technical definition with the following elements:
A social unit consisting of a number of individuals interacting with each other with respect to:
- Common motives and goals
- An accepted division of labor, i.e. roles
- Established status (social rank, dominance) relationships
- Accepted norms and values with reference to matters relevant to the group
- Development of accepted sanctions (praise and punishment) if and when norms were respected or violated
Though a group can have any number of members, Sherif held that the optimal size is three persons. A group of three persons can achieve better problem solving abilities than the best three or more individuals can accomplish individually. Group behaviours have a beneficial effect thus in augmenting the group's abilities and at the same time it has a detrimental or negative effect also. This is because members at a wrong stimuli can launch into destructive endeavours, since their wrongdoings will not be found out as individual wrongdoings or of a single person, they have the cover of the group. An example being individuals forming a violent mob burning buses and destroying other public properties.
This definition is long and complex, but it is also precise. It succeeds at providing the researcher with the tools required to answer three important questions:
- "How is a group formed?"
- "How does a group function?"
- "How does one describe those social interactions that occur on the way to forming a group?"
Significance of that definition 
The attention of those who use, participate in, or study groups has focused on functioning groups, on larger organizations, or on the decisions made in these organizations. Much less attention has been paid to the more ubiquitous and universal social behaviors that do not clearly demonstrate one or more of the five necessary elements described by Sherif.
Some of the earliest efforts to understand these social units have been the extensive descriptions of urban street gangs in the 1920s and 1930s, continuing through the 1950s, which understood them to be largely reactions to the established authority. The primary goal of gang members was to defend gang territory, and to define and maintain the dominance structure within the gang. There remains in the popular media and urban law enforcement agencies an avid interest in gangs, reflected in daily headlines which emphasize the criminal aspects of gang behavior. However, these studies and the continued interest have not improved the capacity to influence gang behavior or to reduce gang related violence.
The relevant literature on animal social behaviors, such as work on territory and dominance, has been available since the 1950s. Also, they have been largely neglected by policy makers, sociologists and anthropologists. Indeed, vast literature on organization, property, law enforcement, ownership, religion, warfare, values, conflict resolution, authority, rights, and families have grown and evolved without any reference to any analogous social behaviors in animals. This disconnect may be the result of the belief that social behavior in humankind is radically different from the social behavior in animals because of the human capacity for language use and rationality. Of course, while this is true, it is equally likely that the study of the social (group) behaviors of other animals might shed light on the evolutionary roots of social behavior in people.
Territorial and dominance behaviors in humans are so universal and commonplace that they are simply taken for granted (though sometimes admired, as in home ownership, or deplored, as in violence). But these social behaviors and interactions between human individuals play a special role in the study of groups: they are necessarily prior to the formation of groups. The psychological internalization of territorial and dominance experiences in conscious and unconscious memory are established through the formation of social identity, personal identity, body concept, or self concept. An adequately functioning individual identity is necessary before an individual can function in a division of labor (role), and hence, within a cohesive group. Coming to understand territorial and dominance behaviors may thus help to clarify the development, functioning, and productivity of groups.
Explicitly contrasted against a social cohesion based definition for social groups is the social identity perspective, which draws on insights made in social identity theory. Here, rather than defining a social group based on expressions of cohesive social relationships between individuals, the social identity model assumes that “psychological group membership has primarily a perceptual or cognitive basis”. It posits that the necessary and sufficient condition for individuals to act as group members is “awareness of a common category membership” and that a social group can be "usefully conceptualized as a number of individuals who have internalized the same social category membership as a component of their self concept". Stated otherwise, while the social cohesion approach expects group members to ask ‘who am I attracted to?’, the social identity perspective expects group members to simply ask ‘who am I?’.
Empirical support for the social identity perspective on groups was initially drawn from work using the minimal group paradigm. For example, it has been shown that the mere act of allocating individuals to explicitly random categories is sufficient to lead individuals to act in an ingroup favouring fashion (even where no individual self-interest is possible). Also problematic for the social cohesion account is recent research showing that seemingly meaningless categorization can be an antecedent of perceptions of interdependence with fellow category members.
While the roots of this approach to social groups had its foundations in social identity theory, more concerted exploration of these ideas occurred later in the form of self-categorization theory. Whereas social identity theory was directed initially at the explanation of intergroup conflict in the absence of any conflict of interests, self-categorization theory was developed to explain how individuals come to perceive themselves as members of a group in the first place, and how this self-grouping process underlies and determines all subsequent aspects of group behaviour.
Types of groups 
Primary groups According to Charles Horton Cooley (1864–1929), a primary group is a small social group whose members share personal and lasting relationships. People joined in primary relationships spend a great deal of time together, engage in a wide range of activities, and feel that they know one another well. In short, they show real concern for one another. In every society, the family is the most important primary group. Groups based on lasting friendships are also primary groups.
Secondary groups, in contrast to primary groups, are large groups involving formal and institutional relationships. Secondary relationships involve weak emotional ties and little personal knowledge of one another. Most secondary groups are short term, beginning and ending without particular significance. They may last for years or may disband after a short time. The formation of primary groups happens within secondary groups.
Primary groups can be present in secondary settings. For example, attending a university exemplifies membership of a secondary group, while the friendships that are made there would be considered a primary group that you belong to. Likewise, some businesses care deeply about the well being of one another, while some immediate families have hostile relations within it.
Individuals almost universally have a bond toward what sociologists call reference groups. A reference group is a social group that serves as a point of reference in making evaluations and decisions.
Some examples of types of groups include the following:
- Peer group
- A peer group is a group with members of approximately the same age, social status, and interests. Generally, people are relatively equal in terms of power when they interact with peers.
- A group of people that have many of the same interests & commonly found in a High School/College setting; most of the time they have a name & rules for themselves.
- A club is a group, which usually requires one to apply to become a member. Such clubs may be dedicated to particular activities: sporting clubs, for example.
- All individuals who live in the same home. anglophone culture may include various models of household, including the family, blended families, share housing, and group homes.
- A community is a group of people with a commonality or sometimes a complex net of overlapping commonalities, often–but not always–in proximity with one another with some degree of continuity over time.
- An organization which runs several instances of a business in many locations.
- A gang is usually an urban group that gathers in a particular area. It is a group of people that often hang around each other. They can be like some clubs, but much less formal. They are usually known in many countries to cause social unrest and also have negative influence on the members and may be a target for the law enforcers in case of any social vices
- A mob is usually a group of people that has taken the law into their own hands. Mobs are usually groups which gather temporarily for a particular reason.
- A posse was originally found in English common law. It is generally obsolete, and survives only in America, where it is the law enforcement equivalent of summoning the militia for military purposes. However, it can also refer to a street group.
- This is usually a small group, of around 3 to 15 people, who work as a team to accomplish their goals.
- This is a social group with two members. Social interaction in a dyad is typically more intense than in larger groups because neither member shares the other's attention with anyone else.
- This is a social group with three members, which contains three relationships, each uniting two of the three people. A triad is more stable than a dyad because one member can act as a mediator should the relationship between the other two become strained.
- similar to a squad, though a team may contain many more members. A team works in a similar way to a squad.
- It is a social group toward which a member feels respect and loyalty.
- It is a group that an individual identifies in positive direction. If a person is part of the in-group then they are collectively part of an inner circle of friends. An inner circle may contain sub-groups within the inner circle including the apex (best friends), core (very close friends), outer rim, etc. This group provides a support structure and being exclusive offers protection from anyone in an Out-group (see below.)
- It is a social group toward which a person feels a sense of competition or opposition.
- It is a group that an individual identifies in negative direction.
Groups can also be categorized according to the number of people present within the group. This makes sense if the size of the group has consequences for the way group members relate with each other. In a small group, for example, "each member receives some impression ... of each other member distinct enough so that he or she ... can give some reaction to each of the others as an individual person." This personal interaction is not possible in larger groups.
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In the initial stages of expansion, the groups usually do not accept every applicant. One of the ways to build a reasonably closed group is to accept new members after one or more existing members propose and recommend them. Such group expands along the lines of other existing social networks. Other approach is to use existing members to evaluate the applicant, like in Microsoft interview. Member evaluation can also be delegated to some team that is not part of the group itself (like in High IQ societies). Some groups may choose to easily accept a lot of people but only leave the most efficient new members after probation (discarding others).
Development of a group 
If one brings a small collection of strangers together in a restricted space and environment, provides a common goal and maybe a few ground rules, then a highly probable course of events will follow. Interaction between individuals is the basic requirement. At first, individuals will differentially interact in sets of twos or threes while seeking to interact with those with whom they share something in common: i.e., interests, skills, and cultural background. Relationships will develop some stability in these small sets, in that individuals may temporarily change from one set to another, but will return to the same pairs or trios rather consistently and resist change. Particular twosomes and threesomes will stake out their special spots within the overall space.
Again depending on the common goal, eventually twosomes and threesomes will integrate into larger sets of six or eight, with corresponding revisions of territory, dominance-ranking, and further differentiation of roles. All of this seldom takes place without some conflict or disagreement: for example, fighting over the distribution of resources, the choices of means and different subgoals, the development of what are appropriate norms, rewards and punishments. Some of these conflicts will be territorial in nature: i.e., jealousy over roles, or locations, or favored relationships. But most will be involved with struggles for status, ranging from mild protests to serious verbal conflicts and even dangerous violence.
By analogy to animal behavior, sociologists may term these behaviors territorial behaviors and dominance behaviors. Depending on the pressure of the common goal and on the various skills of individuals, differentiations of leadership, dominance, or authority will develop. Once these relationships solidify, with their defined roles, norms, and sanctions, a productive group will have been established.
Aggression is the mark of unsettled dominance order. Productive group cooperation requires that both dominance order and territorial arrangements (identity, self-concept) be settled with respect to the common goal and with respect to the particular group. Often some individuals will withdraw from interaction or be excluded from the developing group. Depending on the number of individuals in the original collection of strangers, and the number of hangers-on that are tolerated, one or more competing groups of ten or less may form, and the competition for territory and dominance will then also be manifested in the intergroup transactions.
Dispersal and transformation of groups 
Two or more people in interacting situations will over time develop stable territorial relationships. As described above, these may or may not develop into groups. But stable groups can also break up in to several sets of territorial relationships. There are numerous reasons for stable groups to "malfunction" or to disperse, but essentially this is because of loss of compliance with one or more elements of the definition of group provided by Sherif. The two most common causes of a malfunctioning group are the addition of too many individuals, and the failure of the leader to enforce a common purpose, though malfunctions may occur due to a failure of any of the other elements (i.e., confusions status or of norms).
In a society, there is an obvious need for more people to participate in cooperative endeavors than can be accommodated by a few separate groups. The military has been the best example as to how this is done in its hierarchical array of squads, platoons, companies, battalions, regiments, and divisions. Private companies, corporations, government agencies, clubs, and so on have all developed comparable (if less formal and standardized) systems when the number of members or employees exceeds the number that can be accommodated in an effective group. Not all larger social structures require the cohesion that may be found in the small group. Consider the neighborhood, the country club, or the megachurch, which are basically territorial organizations who support large social purposes. Any such large organizations may need only islands of cohesive leadership.
For a functioning group to attempt to add new members in a casual way is a certain prescription for failure, loss of efficiency, or disorganization. The number of functioning members in a group can be reasonably flexible between five and ten, and a long-standing cohesive group may be able to tolerate a few hangers on. The key concept is that the value and success of a group is obtained by each member maintaining a distinct, functioning identity in the minds of each of the members. The cognitive limit to this span of attention in individuals is often set at seven. Rapid shifting of attention can push the limit to about ten. After ten, subgroups will inevitably start to form with the attendant loss of purpose, dominance-order, and individuality, with confusion of roles and rules. The standard classroom with twenty to forty pupils and one teacher offers a rueful example of one supposed leader juggling a number of subgroups.
Weakening of the common purpose once a group is well established can be attributed to: adding new members; unsettled conflicts of identities (i.e., territorial problems in individuals); weakening of a settled dominance-order; and weakening or failure of the leader to tend to the group. The actual loss of a leader is frequently fatal to a group, unless there was lengthy preparation for the transition. The loss of the leader tends to dissolve all dominance relationships, as well as weakening dedication to common purpose, differentiation of roles, and maintenance of norms. The most common symptoms of a troubled group are loss of efficiency, diminished participation, or weakening of purpose, as well as an increase in verbal aggression. Often, if a strong common purpose is still present, a simple reorganization with a new leader and a few new members will be sufficient to re-establish the group, which is somewhat easier than forming an entirely new group. This is the most common factor.
See also 
- "Social Groups." Cliffsnotes.com. Accessed June 2011.
- Turner, J. C. (1982). "Towards a cognitive redefinition of the social group". In Tajfel, H. Social identity and intergroup relations (Cambridge, UK: Cambridge University Press): 15–40.
- Platow, M. J.; Grace, D. M.; Smithson, M. J. (2011). "Examining the Preconditions for Psychological Group Membership: Perceived Social Interdependence as the Outcome of Self-Categorization". Social Psychological and Personality Science 3 (1).
- Reicher, S.D. (1982). The determination of collective behaviour (pp. 41-83). In H. Tajfel (ed.), Social identity and intergroup relations. Cambridge: Cambridge University Press
- John J. Macionis, Linda M. Gerber, "Sociology", Seventh Canadian Edition, Pearson Canada. Missing or empty
- Hare, A. P. (1962). Handbook of small group research. New York: Macmillan Publishers.
- Sherif, Muzafer and Sherif, Carolyn W., An Outline of Social Psychology rev.ed. Harper & Brothers: New York pp. 143–180.
- Simon, Herbert A. Administrative Behavior 3rd rd ed. Free Press 1976 p.123-153
- Sherif, op. cit. p. 149.
- Tajfel, H., & Turner, J. C. (1979). An integrative theory of intergroup conflict. In W. G. Austin & S. Worchel (Eds.), The social psychology of intergroup relations (pp. 33–47). Monterey, CA: Brooks/Cole
- Tajfel, H., Billig, M., Bundy, R. P. & Flament, C. (1971). Social categorization and intergroup behaviour. European Journal of Social Psychology, 2, 149-178,
- Turner, J. C.; Reynolds, K. H. (2001). "The Social Identity Perspective in Intergroup Relations: Theories, Themes, and Controversies". In Brown, S. L.; Gaertner. Blackwell Handbook of Social Psychology 3 (1).
- Turner, J.C. (1987) Rediscovering the Social Group: A Self-Categorization Theory. Oxford: Blackwell. pp. 42-67.
- Macionis, Gerber, John, Linda (2010). Sociology 7th Canadian Ed. Toronto, Ontario: Pearson Canada Inc.. pp. 149.
- Macionis, Gerber, John, Linda (2010). Sociology 7th Canadian Ed. Toronto, Ontario: Pearson Canada Inc.. pp. 152.
- ["Webster's Online Dictionary - with Multilingual Thesaurus Translation." Webster's Online Dictionary - with Multilingual Thesaurus Translation. N.p., n.d. Web. 24 Feb. 2012. <http://www.websters-online-dictionary.org/>. "Webster's Online Dictionary"].
- Macionis, Gerber, John, Linda (2010). Sociology 7th Canadian Ed. Toronto, Ontario: Pearson Canada Inc.. pp. 153.
- Macionis, Gerber, John, Linda (2010). Sociology 7th Canadian Ed. Toronto, Ontario: Pearson Canada Inc.. pp. 154.
- Bales, R. F. (1950). Interaction Process Analysis: A Method for the Study of Small Groups. MA: Addison-Wesley. p. 33
- Reading, Hugo F. (1996). Dictionary Of The Social Sciences. Atlantic Publishers & Distributors. p. 171. ISBN 978-81-7156-605-1. Retrieved 2010-03-06. "recruitment[:] obtaining membership on the part of a group."
- Holy, Ladislav (1996). "Group". In Kuper, Adam; Kuper, Jessica. The social science encyclopedia (2 ed.). London: Routledge. p. 351. ISBN 0-415-10829-2. "Group [...] commonly refers to a plurality of individuals bonded by some principle of recruitment and by the set of membership rights and obligations. Everyone fulfilling the recruitment criteria is a member of the group and occupies a specific status in the group [...]."
- Sherif, op. cit. pp. 181–279
- Scott, John Paul. Animal Behavior, The University of Chicago Press, 1959, 281pp.
- Halloway, Ralph L., Primate Aggression, Territoriality, and Xenophobia, Academic Press: New York, and London 1974. 496 pp.