The U.S. has gone through a historically unparalleled expansion in its prison population--from fewer than 400,000 in 1970 to almost 2.1 million in 2000. The expansion continued vigorously even as crime rates fell sharply in recent years. And it has happened at all levels--federal, state and local. For explanations of the causes of the increase, it is helpful to examine the state prisons, which account for 63 percent of all adult prisoners, and the local jails, which account for another 32 percent. (The remainder are held mostly in federal prisons.) Because state laws and policies affect the number of prisoners in local jails, it is proper to consider the two types of institutions together.

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