The Bureau of Justice Statistics has released its annual report on prison populations. As usual, the report emphasizes the number of people in prison and what they call the "incarceration rate," i.e., the number in prison relative to the total population. The latter invariably prompts much hand-wringing every time it is announced, even though by itself it is a nearly useless number that tells us almost nothing about policy.
This "incarceration rate" is actually made up of at least two factors which must be determined and considered separately to see anything meaningful. The number of prisoners per capita is the product of prisoners per criminal times criminals per capita. For the algebraically inclined, Pr/Pop = Pr/Cr * Cr/Pop. The first factor is the percentage of criminals society chooses to lock up; the second is the percentage of people who choose to commit crimes. Because the two factors represent different choices by different people, it makes little sense to lump them together, and the undifferentiated product of the two tells us very little. A high "incarceration rate" could mean a society has strict sentencing policies, or it could mean the society is plagued with a high crime rate, or it could be a combination of the two.
The first factor could actually be broken down further as prisoners per criminal we catch times the proportion of criminals we catch. That separation would further separate our ability and determination to catch criminals from our determination to punish the ones we catch.
The situation is further complicated by interrelation of the factors. The probabilities of being caught and of being punished if caught are factors that go into a rational actor's decision to commit a crime. Were the low sentencing rates of the 60s and 70s a cause of the high crime rates of the 80s and 90s and the subsequent high sentencing rates? Quite possibly. The increase in prison population in California from the Three Strikes Law was much less than projected. That may be in part because the law contributed to California's rapidly declining crime rate. A dated but possibly still interesting article on these topics by yours truly and Michael Rushford is available here.
Stand by for a raft of simplistic denunciations that ignore these issues and cite the "incarceration rate" as proof that America is a cruel and heartless society. Bonus points to any reader who finds a single mention of how many people have not been robbed, raped, or murdered because we toughened up sentencing in the 80s and 90s.