When I was a police officer in Baltimore, one sergeant would sometimes motivate his troops in the middle of a shift change by joyfully shouting, "All right, you maggots! Let's lock people up! They don't pay you to stand around. I want production! I want lockups!" He said this while standing in front of a small sign he most likely authored: "Unlike the citizens of the Eastern District, you are required to work for your government check."

In the police world, there are good arrests and better arrests, but there is no such thing as a bad arrest. In recent years, measures of "productivity" have achieved an almost totemic significance. And because they are so easy to count, arrests have come to outweigh more important but harder-to-quantify variables such as crimes prevented, fights mitigated, or public fears assuaged.

There's an argument that putting pressure on rank-and-file officers to make lots of arrests is a good thing. After all, we pay police to arrest criminals. But there's a difference between quantity and quality. Quantity is easy to influence, and the rank and file can easily increase their output of discretionary arrests for minor offenses like loitering, disorderly conduct, and possession of marijuana. They are also influenced by what is known in New York as "collars for dollars": Arrest numbers are influenced by the incentive of overtime pay for finishing up paperwork and appearing in court.

Police would love to arrest only "real" criminals, but that isn't easy. It's difficult to find a good criminal. There's never a felon around when you need one. Fishing for low-level drug arrests is a far easier way to generate overtime.

When I worked in Baltimore, officers would pull up on a drug corner and stop the slowest addict walking away. While conducting a perfectly legal "Terry Frisk"—a cursory search nominally conducted for officer safety—cops would feel some drugs in a pocket. That easy arrest and lockup likely meant two hours of overtime pay.

In some cities, like New York, it's trickier. Overtime for court testimony is harder to get, and the state's highest court has ruled—precisely to prevent the Baltimore-style approach—that feeling drugs during a Terry Frisk does not allow an officer to search that pocket and remove those drugs. The court reasoned that the drugs are not a threat to the officer's safety, and safety is the only justification for these sorts of frisks.

In New York state, small-scale possession of marijuana is virtually decriminalized. It's not even an arrestable offense. But police in need of overtime are nothing if not wily. So a group of officers might approach a man in a high-crime neighborhood and, in no uncertain terms, "ask" him to empty his pockets. Fearful, resigned, or simply taking the path of least resistance, the suspect might do so, and in the process he might reveal a small "dime bag" of weed. While possessing that amount of marijuana is not an arrestable offense, it becomes one as soon as the drug is placed in "public view."

Supporters sometimes say these small-scale drug arrests are part of a "broken windows" approach to preventing crime. This tactic comes from an influential 1982 Atlantic magazine article by George Kelling and James Q. Wilson that combined the 19th century police theories of Robert Peel with the 20th century urban philosophy of Jane Jacobs. The idea is that if you take care of the little things—disorder, quality-of-life issues, and public fear—then the big things like robbery and murder will take care of themselves.

Since Police Commissioner William Bratton implemented a broken windows policing strategy in the early 1990s, homicides in New York dropped more than 80 percent. But the crime didn't drop because police were cracking down on drug users; overall, illegal drug use is as high as ever. When the murder rate was falling fastest in the 1990s, police never arrested more than a few thousand people per year for public-view marijuana. Only after the crime drop slowed did police turn to small-scale drug arrests to meet their "productivity goals." It's as if real criminals became too difficult to find, and the addiction to overtime pay remained strong as ever.

Last year in New York City, 50,300 people—mostly young black and Hispanic men—were arrested solely for misdemeanor "public-view" possession of marijuana. It's true that some may have been up to no good. And some might have been walking down the street proudly smoking a spliff in front of the police. But nobody really believes this accounts for most of those 50,300 lockups. Many were people just going about their business, intending to smoke later, in private, in the very manner the law was intended to decriminalize.

"What is it with the drugs?" a man once asked me while I was policing a 7-11 for coffee, "When there's shootin' or fightin', you don't seem to care! But when there's drugs, you come right away." It's a fair question to ask. Why do we do it? What do we gain? Especially when we know drug arrests are expensive and turn a lot of otherwise law-abiding citizens into cop-hating criminals?

The drug war, because it can't be won, encourages outward signs of police effectiveness at the expense of good old-fashioned policing. Hard-working cops, especially those who ask for little more than a middle-class income in return for the dangerous work they do, turn to drug arrests to make ends meet. The Baltimore sergeant was right: Police officers do need to work for their government check. It's a shame "collars for dollars" has become the easiest way to do it.

Peter Moskos (moskos@gmail.com), a former Baltimore police officer, is an assistant professor of law and political science at John Jay College of Criminal Justice, The City University of New York's Doctoral Program in Sociology, and LaGuardia Community College's Department of Social Science. He is the author of In Defense of Flogging (2011) and Cop in the Hood: My Year Policing Baltimore's Eastern District (2008).