Large Number of Unenployed Young Men= High Crime?

I once reaad that the biggest determinant of the violent crime rate (in any country) was the relative number of young men (ages 16-30) who are unemployed.
The theory goes that young males account for most violent crime, and having large numbers of them (with nothing to do) leads many of them to become criminals.
Is this true? Are idle minds the Devil’s workshop?