The importation model argues that prisons are not completely insulated from the happenings of everyday life outside the 'real world'.
Inmates come from the real world, and they bring with them a subculture typical of criminality, which includes a history of aggression and aggressive norms.
Prisoners are wiling to use violence to settle disputes in prisons, which reflects the life they had before they entered prison.
Aggression in prisons is alternatively caused by aggressive characteristics of inmates. It is imported from the outside by prisoners, and is no fault of the environment itself.
Such inmates are predisposed to using violence and would do so in any setting, which is why they do the same in prison. Therefore, this leads to institutional aggression.
The importation model is a dispositional explanation of aggression.