A crowd is a temporary gathering of people who share a common focus of attention and whose members influence one another. Crowds can vary in size, purpose, and emotional intensity. They can form for various reasons, such as attending a sporting event, a concert, a protest, or in response to an emergency. The behaviour of individuals within a crowd can be influenced by factors such as anonymity, social contagion (the spread of emotions and behaviours through a crowd), and the perceived unity of the group. Sociologists study crowds to understand the dynamics of collective behaviour and how individuals act in group settings, which can sometimes differ significantly from their behaviour when alone. Theories of crowd behaviour have evolved over time, from early ideas about the "mob mentality" to more nuanced understandings of the social processes and interactions that occur within crowds.