One in every 31 adults, or 7.3 million Americans, is in prison, on parole or probation, at a cost to the states of $47 billion in 2008, according to a new study.

Criminal correction spending is outpacing budget growth in education, transportation and public assistance, based on state and federal data. Only Medicaid spending grew faster than state corrections spending, which quadrupled in the past two decades, according to the report Monday by the Pew Center on the States, the first breakdown of spending in confinement and supervision in the past seven years.

The increases in the number of people in some form of correctional control occurred as crime rates declined by about 25 percent in the past two decades.

As states face huge budget shortfalls, prisons, which hold 1.5 million adults, are driving the spending increases.