A&Q is a special series that inverts the classic Q&A, taking some of the most frequently posed solutions to pressing matters of policy and exploring their complexity.

In the early 1990s, U.S. crime rates had been on a steep upward climb since the Lyndon B. Johnson presidency. The crack-cocaine epidemic in the mid-1980s added fuel to the fire, and handgun-related homicides more than doubled between 1985 and 1990. That year, murders peaked in New York City with 2,245 killings. Politicians embraced tough-on-crime platforms and enacted harshly punitive policies. Experts warned the worst could be yet to come.

Then crime rates went down. And then they kept going down.

By decade’s end, the homicide rate plunged 42 percent nationwide. Violent crime decreased by one-third. What turned into a precipitous decline started later in some areas and took longer in others. But it happened everywhere: in each region of the country, in cities large and small, in rural and urban areas alike. In the Northeast, which reaped the largest benefits, the homicide rate was halved. Murders plummeted by 75 percent in New York City alone as the city entered the new millennium.

The trend kept ticking downward from there, more slowly and with some fluctuations, to the present day. By virtually any metric, Americans now live in one of the least violent times in the nation’s history.

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.

QUESTION

Reasonable as it might sound, the research turns out to be far less conclusive. A panel from the National Academy of Sciences looked at the existing research for its landmark 2012 report on the American prison system. They concluded that “on balance,” higher incarceration rates had a “modest” effect on the decline. But they also cautioned that a lack of clear evidence means any benefits were “unlikely to have been large.” What caused the rest of the decline?

The panel also found little indication of a deterrent effect. Most offenders reach a point when they age out of criminal behavior, limiting the utility of mandatory minimum sentencing. For this reason, the academy’s report concluded lengthy prison sentences are “ineffective as a crime-control measure” in virtually all circumstances. Some research even suggests harsh prison conditions could make inmates more likely to reoffend.

These meager gains also shouldn’t be invoked without reckoning with the losses incurred. Mass incarceration came with ruinous consequences for communities of color and American society as a whole.

ANSWER

I get where you’re coming from on mass incarceration’s sins. But surely law enforcement and better policing played some kind of role in the decline.

QUESTION

Let’s break that down further by policy. One of the most common responses to crime in the 1980s and early ’90s was to hire more police officers, for example. But how much of an impact did it have?

In its analysis last year on the crime decline’s causes, the Brennan Center found a “modest, downward effect on crime in the 1990s, likely 0 to 10 percent” from increased hiring of police officers. There’s also some scholarly consensus on the role CompStat, a crime-statistics tracking tool, played in improving police responses.

But other policing tactics appeared to have little impact, even in New York City, which led the innovative wave in the early  ’90s. Mayor Rudy Giuliani and NYPD Commissioner Bill Bratton credit their broken-windows-style reforms for crime’s sharp decline after taking office in 1994. But scholars are quick to note the trend actually began in 1990. Crime also continued to decline after the NYPD largely abandoned its controversial stop-and-frisk policy in recent years, for example. Economist Steven Levitt attributed New York City’s successes to the city’s higher rate of abortion, part of his broader argument linking Roe v. Wade to the crime decline.

ANSWER

I’m a little skeptical a Supreme Court opinion could reduce crime rates two decades later.

QUESTION

In the original paper outlining the theory, Levitt and fellow economist John Donohoe argued the 1973 ruling reduced the number of children born in unwanted circumstances, thereby reducing the number of children predisposed to violent crime later in life. Overall, they estimated this 20-year lag effect might account for as much as half of the crime decline in the ’90s.

How well does this hold up? One complication is the prevalence of illegal abortion before ’73. The Guttmacher Institute estimates between 700,000 and 800,000 women terminated their pregnancies each year in the decades preceding Roe. If large numbers of women prevented unwanted births prior to the ruling, the sudden availability of legal abortion might not have radically changed the overall number.

Another thought: How does the theory scale across the globe? Crime rates rose and fell in other industrialized countries in tandem with the United States over the past 50 years. Did they see similar declines when they liberalized their abortion laws? Demonstrating its effect in lowering foreign crime rates would strengthen the argument for its role in the U.S. decline, too.

ANSWER

Wait, if other countries also had similar rises and declines in crime, that suggests the decline’s cause might be a factor that crosses borders. I think I read a Mother Jones article about something like this a few years ago…

QUESTION

Are you thinking of lead? The neurotoxic element stunts intellectual growth in children and causes behavioral problems when they become adults, but it wasn’t seen as a possible culprit for a nationwide crime wave until recently. In her 2007 paper on the relationship, economist Jessica Reyes attributed a 56 percent drop in violent crime in the 1990s to the removal of lead from gasoline after the Clean Air Act of 1970.

With children born after the early 1970s less affected by lead’s toxic effects, the logic goes, they would be less likely to commit crimes once they reached their 20s in the early 1990s. Mother Jones reporter Kevin Drum helped popularize the theory in his 2013 cover story. “In states where consumption of leaded gasoline declined slowly, crime declined slowly,” he wrote. “Where it declined quickly, crime declined quickly.” And, perhaps most intriguingly, the correlation held in other countries, too.

But as convincing as all this might sound, there are gray areas for researchers to explore further. One of them is the data itself. Reyes’s original study relied on the Uniform Crime Reports, the FBI’s annual compilation of crimes documented by police departments nationwide. But a recent study found that using another major crime data set—the National Crime Victimization Survey, conducted by the federal Bureau of Justice Statistics—significantly reduced the correlation between lead exposure and violent crime. (Drum examined the implications at length in January.) Which data set is right?

***

These aren’t the only theories about the Great American Crime Decline out there. Indeed, the Brennan Center’s estimate only accounted for roughly one-third of the overall decline in crime during the 1990s. Nor are these the only questions the decline raises about our understanding of crime, justice, and public policy.

What’s the correlation, for example, between economic inequality and crime? How does gentrification affect crime rates in major cities?

What other factors may have influenced the decline? Can it be attributed to antidepressants or the proliferation of cell phones? What about the aging of the baby boomer generation, or higher rates of gun ownership?

Why do so many Americans think crime hasn’t gone down at all?

And—however non-sequitur it might sound—will climate change reverse it?

Maybe there’s an answer we haven’t considered yet. Drop your thoughts into an email to hello@theatlantic.com.