Drug use and the crime rate?

An article in this week’s New York Times Week in Review section about the difficulty in explaining changes in the crime rate contains the following claim:

The idea that illegal drug use drives up crime is not bolstered by statistics that show that the percentage of those arrested in New York City with illegal drugs in their system has remained more or less flat, Mr. Zimring said.

Now, I may be a simple caveman, but it seems to me that illegal drug use could boost the crime rate without changing the proportion of people arrested who have illegal drugs in their system.

Let’s assume for a moment that drugs in fact do cause crime. I’m not sure I agree with the assertion, but let’s take it at face value for the time being. In a high-drug (and thus high-crime) environment, non-drug-users may commit more crimes. One reason might be that the social norms of a high-crime society might encourage even non drug users to participate; another might be because they engage in and profit from the drug trade without being drug users themselves.

An alternative explanation might involve that the only people tested for drugs in the above scenario are arrestees, people who were caught, or who might not even be responsible for the crime of which they’re expected. I’m guessing that a high percentage of people arrested have drugs in their system, and that might be the reason they’re caught, regardless of any other police work. The preference for arresting drug users over others might account for the relatively constant proportion of people arrested with drugs in their system.

I’m not saying that either of these explanations is the answer here; just that they can explain the presented evidence without challenging the notion that drugs and crime are related.