The long-running debate over what's driving the country's staggering (and ongoing) drop in crime just got more complicated. With a major new report, the Brennan Center for Justice at NYU has effectively squashed one popular theory: throwing massive amounts of people in prison did not bring down crime.

"At today's high incarceration rates, continuing to incarcerate more people has almost no effect on reducing crime," the researchers wrote. The study found that the mass incarceration has had on the crime rate has been limited since about 1990, and essentially nonexistent since the turn of the century.

Indeed, the researchers found that crime kept falling dropping even after states cut their prison populations:

For many reasons, including straitened budgets and a desire to diminish prison populations, many states have started to cut back on imprisonment. What happened? Interestingly, and encouragingly, crime did not explode. In fact, it dropped. In the last decade, 14 states saw declines in both incarceration and crime. New York reduced imprisonment by 26 percent, while seeing a 28 percent reduction in crime. Imprisonment and crime both decreased by more than 15 percent in California, Maryland, New Jersey, New York, and Texas. Eight states—Connecticut, Delaware, Massachusetts, Michigan, Nevada, North Carolina, South Carolina, and Utah—lowered their imprisonment rates by 2 to 15 percent while seeing more than a 15 percent decrease in crime.

So if the Brennan Center is right, and mass incarceration is off the table, what's left? Last fall, the Marshall Project's Dana Goldstein rounded up a bunch of "not crazy" possible explanations for the crime drop. There's the end of the Crack Era, new technology (better car security system, fewer people carrying cash), and the aging of the historically huge cohort of men in the boomer generation. These seem to sit comfortably in the "not crazy" pile for most sociologists. Others are more controversial: the legalization of abortion (which, in theory, reduced the number of poorly parented children) and the removal of lead, which has been linked with poor impulse control, from pipes and paint.

If you're having trouble keeping track, you're not alone. Franklin E. Zimring, a criminal justice expert from University of California, Berkeley,told the New York Times last month that the search for an answer to the crime drop quandary is like "criminological astrology."

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