A riot is a form of civil disorder characterized often by what is thought of as disorganized groups lashing out in a sudden and intense rash of violence against authority, property or people. While individuals may attempt to lead or control a riot, riots are thought to be typically chaotic and exhibit herd behaviour, and usually generated by civil unrest. However, there is a growing body of evidence to suggest that riots are not irrational, herd-like behaviour, but follow inverted social norms. Riots often occur in reaction to a perceived grievance or out of dissent. Historically, riots have occurred due to poor working or living conditions, governmental oppression, taxation or conscription, conflicts between ethnic groups, food supply or religions (see race riot, sectarian violence and pogrom), the outcome of a sporting event (see football hooliganism) or frustration with legal channels through which to air grievances.