Here's a message from the DEA to make Eliot Ness smile:
A word about prohibition: lots of you hear the argument that alcohol prohibition failed—so why are drugs still illegal? Prohibition did work. Alcohol consumption was reduced by almost 60% and incidents of liver cirrhosis and deaths from this disease dropped dramatically (Scientific American, 1996, by David Musto). Today, alcohol consumption is over three times greater than during the Prohibition years. Alcohol use is legal, except for kids under 21, and it causes major problems, especially in drunk driving accidents.
Hmm. This is surely overstated, but it does drive home a point: a law can be politically unsuccessful, yet still have quite impressive positive effects. While these might not be enough to justify the law, it reinforces the point that making something illegal does reduce its incidence--even if it doesn't eliminate it entirely.
The principle is applicable to whole range of issues from drug legalization to abortion rights. It's a good idea to post this point as a square yellow post-it note somewhere in the corner of your mind.