Peoples’ peers often have a big influence in peoples’ actions, beliefs, and attitudes. You may observe others actions and make decisions based of their actions and consequences. Performance is also often affected when surrounded by others and it can be better or worse depending on the person and situation. Being around peers also can cause people to conform to ideas not of their own because they want to fit in and be accepted.
Attribution Theory: description of the way in which people explain the causes of their own and others' behaviors

Fundamental Attribution Error: tendency to overestimate the extent to which people's behavior is due to internal, dispositional factors and to underestimate the role of situational factors

Self-Serving Bias: explanation for one's success that credit internal, dispositional factors and explanations for one's failure that blame external, situational factors

GroupThink: A mode of thinking that people engage in when they are deeply involved in a cohesive in-group, when the members' striving for unanimity override their motivation to realistically appraise alternate courses of action

Stereotyping: categorizing of others outside the group in ways that see their views as unacceptable
Social Facilitation: The influence of other people on the performer social loafing: The tendency for people in a group to exert less effort when pooling their efforts toward attaining a common goal than when individually accountable.

group polarization: The enhancement of a group's prevailing inclinations through discussion within the group.

aggression : Any physical or verbal behavior intended to hurt or destroy against