How could US politicians possibly think it was a good idea to incarcerate millions of Americans starting in the 1980s, creating the system of mass incarceration we have today?

It's a question that gets tossed around a lot nowadays, with varied answers — from claims it was an attempt to control the population to arguments that private prisons created a profit motive for locking up millions of Americans.

But there's a much simpler explanation: The public wanted mass incarceration.

It's easy to forget now, but the politics of crime were huge in the 1990s. According to data from Gallup, never before or after the nineties have so many Americans said that crime is the most important problem facing the country today.

Americans had a very good reason for these concerns. From the late 1960s to the early 1990s, crime was unusually high. The country was still coming off what was perceived as a crack cocaine epidemic, in which the drug ran rampant across urban streets and fueled deadly gang violence. So Americans, by and large, demanded their lawmakers do something — and politicians reacted with mass incarceration and other tough-on-crime policies.

It's very easy in hindsight to consider this an overreaction — now that we know crime began its decades-long decline in the early 1990s, and now that research has shown that mass incarceration only partly contributed to this decline.

But people didn't know that at the time. They didn't know crime was about to begin its long-term drop, and the research on mass incarceration was far from conclusive.

Politicians thought crime would get worse, not better

In fact, there were warnings at the time that things were on the verge of getting worse. One prominent concern in the 1990s — based on what turned out to be very bad social science research — suggested that there was an incoming epidemic of superpredators, violent youth who would rob and kill people. This great video, from the New York Times, captures the era well:

In this context, it was expected that all politicians — liberal and conservative — take a tough stance on crime. That's partly why liberals like Hillary Clinton, Joe Biden, and Bernie Sanders supported the 1994 crime law that contributed to mass incarceration. It's why dueling candidates for governor in the liberal state of New York campaigned on who could be tougher on crime. And it's why practically every state passed tough-on-crime policies throughout the 1980s and 1990s.

More than two decades later, criminal justice reform is all the rage. It's an expectation for Democratic presidential candidates to have a progressive criminal justice platform. So the same politicians who caused this problem are being asked to undo what they did in the past. And they face a common question: How can they be expected to solve a problem that they helped cause?

Popular demand for tough-on-crime laws in the past doesn't in any way excuse the devastation lawmakers inflicted on millions of people through mass incarceration and other policies. But based on voters' concerns in the 1990s, if a politician didn't contribute to the problem back then, he or she may not be prominent enough to run for president today. That's how America ended up with mass incarceration — and the seemingly contradictory Democratic presidential candidates for 2016.