Crowds are large groups of adolescents defined by their shared image and reputation. Crowd membership is externally imposed and not a direct consequence of interaction with other members of the crowd.

Crowds are large groups of adolescents socially connected by a shared image and reputation, especially within the setting of a single school. It is possible for a single person to belong to more than one crowd if their image matches the crowds’ criteria. Because membership in a crowd depends on peers' perceptions, crowds in any given peer group will correspond to the local preconceived "types" of adolescents. Specific stereotypes vary from place to place, but many remain consistent, based on peer status, socioeconomic status, residential area, activities, social characteristics, or a combination of attributes (jocks, nerds, populars, and druggies are among the most commonly observed). Crowds are very different from cliques: while cliques are relatively small, close-knit groups based on frequent interaction and collectively determined membership, members of a crowd may not even know each other. Crowd membership reflects external assessments and expectations, providing a social context for identity exploration and self-definition as adolescents internalize or reject their crowd identities.

Because crowd membership is initially outwardly imposed, it is possible for an adolescent's peers to classify them as belonging to a crowd they may not consider themselves part of. Members of some crowds are more aware of and comfortable with their crowd designation than others; members of stigmatized or low-status groups, in particular, may resist or deny their undesirable categorization. Usually, however, adolescents embrace their crowd affiliation, using it to define themselves and advertise where they fit in their peer group's social structure.

Crowds serve an essential purpose in adolescent identity development, shaping individual values, behavior, and personal and peer expectations. "[One's group] is often tantamount to one's own provisional identity;" the individual defines herself by the crowd she sees herself fitting into. Different crowds expose the individual to different norms. These norms encourage adolescents to interact with some people while avoiding others and reward certain behaviors while discouraging others, a process of normative social influence. For example, a member of a "preppy" crowd might be rewarded for dressing in a fashion for which a member of an "emo" crowd would be teased, and vice versa.

Crowd effects on norms of interaction:

Often crowds reinforce the behaviors that originally caused an individual to be labeled part of that crowd, which can positively or negatively influence the individual (toward academic achievement or drug use, for example). These pressures are often linked to the stereotypes members of crowds hold about themselves and members of other crowds: unity by denigration of the outgroup (see social identity theory).

Adolescents' perception of crowd differences may depend on how closely related the adolescent observer is to a particular crowd. The basic, recurring crowd divisions (jocks, geeks, partiers) have been most often studied in predominantly white high schools, but they also exist for minority students. In multiracial schools, students seem to divide along ethnic lines first, then into these archetypical crowds within their own ethnicity. However, one ethnic group may not notice the further divisions in other ethnic groups after the first, race-based split. For instance, black students see themselves as divided into jocks, geeks, emos, stoners, popular kids, and so on, but white students may see them as just one crowd defined solely by ethnicity, "the black kids." Sometimes crowd membership transcends race, however, and adolescents are classified as "jocks" or "geeks" regardless of race. This seems to vary and depend heavily on the context of the individual school.

While crowds are structured around prototypical caricatures of their members, real adolescents rarely match these extremes. Furthermore, not all adolescents agree on the characteristics typical of a stereotype. In other words, regular manifestation of just a few central characteristics of a crowd is a sufficient basis for classification as a member of that crowd. Thus, not all "jocks" neglect their schoolwork, though that is part of the typical jock stereotype, and a person interested in fashion could still be considered a "geek."