Bryan Caplan has a new post discussing the impact of drug legalization on the overall crime rate.
In the absence of the War on Drugs, many non-drug offenses would never have been committed. Without prohibition, gang-related violence - and related weapons charges (subsumed under "Public-order" at the state level) - would plummet. Habit-related property crimes would probably do the same, albeit to a smaller degree. Theoretically, drug offenders might simply switch into other illegal activities once drugs were legal, but it's hard to believe this effect would be sizable.
My best guess: Five years after the end of the War on Drugs, half of the prison-years handed out for non-robbery violent crime would be gone, along with 20% of the prison-years for robbery and property crimes, and 75% of the prison-years for weapons charges. That's roughly half of all prison-years at the state level, and two-thirds of all prison-years at the federal level.
It's worth noting that when alcohol prohibition was repealed the murder rate in America fell by 40%:
You may want to go to page 30 of the paper to see a larger version of the graph. A few points:
1. The actual decline might appear to be 50%, but the last 10% was an artificially low murder rate during WWII. So 40% is a better estimate.
2. Some of the decline might have been economic recovery after 1933 (when repeal occurred) but in absolute terms the economy was depressed throughout the 1930s, relative to the booming 1920s. And the murder rate was also very high during the 1920s.
3. On the other hand the murder rate did not seem to rise sharply under prohibition.
4. On the other, other hand there was a long-term downtrend in murder as the immigrant wave was "domesticated" between 1910 and 1950. It's reasonable to suppose that the murder rate normally would have declined significantly during the booming 1920s. If you draw a straight line between 1919 and 1950, it looks like prohibition was a big problem.
My best guess is that repeal of prohibition reduced murders by at least 25%. Would that happen today with drug legalization? Hard to say. If it had occurred when the murder rate was quite high in 1990, then the murder rate would have probably fallen sharply. Today it would fall, but perhaps less sharply. On the other hand there would be many, many fewer murder victims in Latin America. And I'm sure that Bryan (as well as liberals, conservative Christians, etc.) would also want us to take their welfare into account when making a decision.