It is possible that arrests for serious crimes declined because officers were overwhelmed by the number of cases, and that leads dried up as community cooperation dwindled. But it is also possible, Professor Morgan and Mr. Pally conclude, that months of reduced police activity contributed to the surge in violence and property crimes.

The second study is by Stephen Rushin, a legal scholar, and Griffin Edwards, an economist. In a paper soon to be published, they examine what happens to local crime rates when police departments come to the attention of the federal government for systematically violating civil rights, or, in more extreme cases, become subject to federal oversight and reform. Because such oversight is a form of criticism, it offers a test case of the thesis that increased scrutiny of the police leads to more crime.

Since 1994, when legislation was passed making federal oversight possible, 61 police departments around the country have been investigated (out of 18,000 agencies). Thirty of these investigations didn’t go anywhere. The rest led to settlements or consent decrees in which the departments agreed to change how they did business: how officers used force, how citizen complaints were handled and so on.

The researchers found that in communities whose agencies were only investigated, there was no change in crime rates. By itself, scrutiny of police departments doesn’t seem to move the needle when it comes to crime — contrary to the Ferguson effect thesis. On the other hand, when departments were required to alter their policies, signifying stronger public concern, crime was higher (temporarily) and officers spoke of working in a low-morale environment.

These are only two studies. More are in the works. But it’s not too early to discuss takeaways.

One, there is now some evidence that when all eyes are on police misconduct, crime may edge up. Progressives should acknowledge that this idea isn’t far-fetched.

Two, while it makes for a tidy political narrative to say “Ferguson effect,” researchers have not pinned down the underlying mechanisms. Against Ms. Mac Donald’s theory, the Baltimore study reveals that a decline in broken-windows policing alone does not elevate crime rates. And the study by Professors Rushin and Edwards tells us that crime can rise following the imposition of federal oversight even when stop-and-frisk reform isn’t necessarily the main goal.