But the forces that drove the Great American Crime Decline remain a mystery. Theories abound among sociologists, economists, and political scientists about the causes, with some hypotheses stronger than others. But there’s no real consensus among scholars about what caused one of the largest social shifts in modern American history.

So, what happened?

ANSWER

I remember the 1990s as a pretty good time for economic growth. Maybe that offered less incentive for crime.

QUESTION

Maybe, but it depends on what metric you use to measure economic growth. Did lower unemployment rates lead to lower crime rates? There’s some research to suggest a connection, but it’s a minor one at best. In its analysis last year on the crime decline’s causes, the Brennan Center for Justice estimated that only about 0 to 5 percent of the decline in the 1990s could be attributed to higher employment.

What about income growth? Some researchers theorize that greater opportunity for legal income reduces the need for illegal sources of it. The Brennan Center’s analysis attributed about 5 to 10 percent of the 1990s decline to it, a relatively modest amount. There’s also early research that suggests more abstract economic factors like inflation and consumer confidence may have played a role.

But the economy’s role also raises a vexing question. If economic growth and criminal activity are linked, why wasn’t there a crime wave during the Great Recession? The national unemployment rate nearly doubled after the crash, peaking at 10 percent in October 2009, and median household income plunged. But crime rates not only failed to spike in response, they actually reached a 40-year low in 2010.

ANSWER

Another national trend must have played a role, then. Maybe the decrease in alcohol consumption over the last few decades had an effect.

QUESTION

How closely related are alcohol and crime? The National Bureau of Economic Research found correlations between its consumption and aggravated assault, rape, and some types of theft. (It also didn’t find one with murder or burglary.)

Since assault is the most common violent crime, it’s logical that increased alcohol use leads to higher crime rates.

Americans only drank slightly less beer, the most common form of alcohol consumption at that time, between 1990 and 2000. But it was enough for the Brennan Center to attribute to it a 7.5 percent drop in crime during the 1990s.

But it’s worth noting that U.S. alcohol consumption isn’t that exceptional on a global scale. World Health Organization data show the average American drinks slightly less alcohol each year compared to the average Canadian or European. Yet violent crime rates in the U.S. are much higher in comparison. Why the disparity?

ANSWER

If we’re measuring against the rest of the world, then maybe mass incarceration played a role. The United States imprisons more people than anywhere else in the world, both in relative and total numbers. It seems logical that fewer criminals are on the streets as a result of tough-on-crime policies from that era.