a minority within a group wielding influence over the majority. Group dynamics Social psychologists study group-related phenomena such as the behavior of crowds. A group can be defined as two or more individuals who are connected to each other by social relationships. Groups tend to interact, influence each other, and share a common identity. They have a number of emergent qualities that distinguish them from coincidental, temporary gatherings, which are termed social aggregates: Norms: Implicit rules and expectations for group members to follow. Roles: Implicit rules and expectations for specific members within the group. Relations: Patterns of liking within the group, and also differences in prestige or status. The shared social identity of individuals within a group influences intergroup behavior, which denotes the way in which groups behave towards and perceive each other. These perceptions and behaviors in turn define the social identity of individuals within the interacting groups. The tendency to define oneself by membership in a group may lead to intergroup discrimination, which involves favorable perceptions and behaviors directed towards the in-group, but negative perceptions and behaviors directed towards the out-group. Groups often moderate and improve decision making, and are frequently relied upon for these benefits, such as in committees and juries. Groups also affect performance and productivity. Social facilitation, for example, is a tendency to work harder and faster in the presence of others. Another important concept in this area is deindividuation, a reduced state of self-awareness that can be caused by feelings of anonymity. Deindividuation is associated with uninhibited and sometimes dangerous behavior. It is common in crowds and mobs, but it can also be caused by a disguise, a uniform, alcohol, dark environments, or online anonymity. Interpersonal attraction A major area of study of people's relations to each other is interpersonal attraction, which refers to all factors that lead people to like each other, establish relationships, and (in some cases) fall in love. Several general principles of attraction have been discovered by social psychologists. One of the most important factors in interpersonal attraction is how similar two particular people are. The more similar two people are in general attitudes, backgrounds, environments, worldviews, and other traits, the more likely they will be attracted to each other. Physical attractiveness is an important element of romantic relationships, particularly in the early stages characterized by high levels of passion. Later on, similarity and other compatibility factors become more important,