Institutions are groups of people held together by a common factor. Examples include schools, prisons, police forces, and terrorist groups. Aggression can occur within or between institutions. A survey found over 84,000 violent incidents against staff in the UK healthcare system in one year. There were also 26,000 prisoner-on-prisoner assaults in US prisons in a given year. There are two main perspectives on why institutional aggression occurs. The importation model argues it is due to individual traits prisoners bring with them, like criminal attitudes. The deprivation model argues the stressful conditions of institutions themselves can cause aggression as a result of situational forces.