Peer pressure (or social pressure) is direct influence on people by peers, or an individual who gets encouraged to follow their peers by changing their attitudes, values, or behaviors to conform to those of the influencing group or individual. This type of pressure differs from general social pressure because it causes an individual to change in response to a feeling of being pressured or influenced from a peer or peer group. Social groups affected include both membership groups, in which individuals are "formally" members (such as political parties and trade unions), and cliques in which membership is not clearly defined. However, a person does not need to be a member or be seeking membership of a group to be affected by peer pressure.
There has been considerable study regarding peer pressure's effects on children and adolescents, and in popular discourse the term is mostly used in the contexts of those age groups. For children, the common themes for study regard their abilities for independent decision making; for adolescents, peer pressure's relationship with sexual intercourse and substance abuse have been significantly researched. Nevertheless, peer pressure can affect individuals of all ethnicities, genders, and ages. Peer pressure serves as an important leadership tool and a powerful motivator for charitable giving and voting. Regarding two of history's recent genocides, the Holocaust and the Rwandan genocide, there has been strong debate over the role peer pressure played in the atrocities.
Imitation plays a large role in childrens' lives; in order to pick up skills and techniques that they use in their own life, children are always searching for behaviors and attitudes around them that they can co-opt. Children are aware of their position in the social hierarchy from a young age: their instinct is to defer to adults' judgements and majority opinions. Similar to the Asch conformity experiments, a study done on groups of preschool children showed that they were influenced by groups of their peers to change their opinion to a demonstrably wrong one. Each child was handed a book with two sets of images on each page, with a groups of differently sized animals on the left hand page and one animal on the right hand, and each child was asked to indicate the size of the lone animal. All the books appeared the same, but the last child would sometimes get a book that was different. The children reported their size judgements in turn, and the child being tested was asked last. Before him or her, however, were a group of children working in conjunction with the researchers. Sometimes, the children who answered before the test subject all gave an answer that was incorrect. When asked in the presence of the other children, the last child's response was often the same as his or her peers. However, when allowed to privately share their responses with a researcher the children proved much more resistant to their peers' pressure, illustrating the importance of the physical presence of their peers in shaping their opinions.