It's simply that prisoners in jail, no matter what kind of jail it is, are living in very close proximity. Unless you have them all in solitary confinement, there's no way you can prevent them from socializing--talking, sharing stories. It's through that story sharing, that information sharing, that young people who are less sophisticated in the ways of crime will learn better ways to commit crime from the boasts of the older prisoners. That's a truism and a long-standing belief.
On November 20th, 2006. See this statement in context.