Yes, Making Something Illegal Makes It Less Common

In the wake of the Irish referendum abolishing their constitutional protection of unborn children, some of have attempted to roll out the old: "Oh, don't worry. Banning abortion doesn't reduce abortions, it just makes people go elsewhere to get them."

This "banning something doesn't reduce it" argument is deployed by various people for various causes: Banning abortion doesn't reduce abortion. Banning drugs doesn't reduce drug use. Banning guns doesn't reduce the number of guns available. Banning gambling doesn't reduce gambling.

All of these are false. Making something illegal of course makes that thing less common. Honestly, if we believed that making something illegal had no effect on whether or not people did it, why would we make anything illegal? Why would we ban things like homicide and burglary if we thought that illegality had no effect on whether people do something.

Think of something you might have an attachment to. Would you do that thing less if you knew that, you had to travel out of the jurisdiction or do business with a criminal in order to do that thing?

Often people point to alcohol prohibition in the US to "prove" that making something illegal does not reduce it. The legend goes that alcohol consumption increased during prohibition, and so people gave it up as a bad idea. Of course, it's hard to know exactly how much alcohol was produced and consumed during Prohibition, because it was not being registered and taxed by the government. However, secondary factors would definitely suggest that alcohol consumption went down during Prohibition:

[A]lcohol consumption declined dramatically during Prohibition. Cirrhosis death rates for men were 29.5 per 100,000 in 1911 and 10.7 in 1929. Admissions to state mental hospitals for alcoholic psychosis declined from 10.1 per 100,000 in 1919 to 4.7 in 1928.

Arrests for public drunkenness and disorderly conduct declined 50 percent between 1916 and 1922. For the population as a whole, the best estimates are that consumption of alcohol declined by 30 percent to 50 percent.

Perhaps the speak easy culture made certain kinds of social drinking more visible to certain sectors of society than before, but Prohibition does seem to have cut down on the heavy drinking (and domestic violence that went with it) which was such a social blight in the US.

Legalizing abortion in Ireland will mean that more Irish women get abortions. There is no rational way to doubt that expectation. Similarly, legalizing drugs will mean more people use drugs. Legalizing gambling will mean that more people gamble. And banning guns would mean that fewer people would own guns.

None of these effects is absolute. Many people buy drugs illegally. If guns were banned, many people would refuse to turn theirs in. When abortion is illegal, some doctors provide them anyway. If you believe that the thing being banned is not always and everywhere wrong, you might well formulate arguments around who would abide by the law and who would not, and thus whether it is overall positive. For instance, someone who think drug use is fine might argue that the more destructive drug users already buy drugs in violation of the law, and it's only law abiding recreational users who are inconvenienced. But what you can't do is argue that banning something has no effect on how common it is.