The two leading Democratic presidential candidates are pandering to their liberal base on private prisons and mass incarceration.

Over the past year, Bernie Sanders and Hillary Clinton vowed to do away with private prisons: Sanders released a plan to ban them, and Clinton reportedly told Black Lives Matter activists that she wanted to do the same.

This is smart politics: Among liberals, there's a widely held belief that private prisons created a for-profit incentive to lock up as many people as possible, and, as a result, private prison companies have pushed for policies that led to mass incarceration to keep their big profits flowing.

This plays into the more antagonistic worldview liberals have toward corporations, money, and politics — that the wealthy are playing the political system to their own means. So under this view, if you get rid of private prisons, you can begin to undo mass incarceration.

But this focus on privatization overestimates the effect of private prison companies, which make up a tiny portion of America's vast prison system. There's also a much simpler explanation for why mass incarceration began: It was a response — however wrongheaded — to real crises in America back in the 1970s and '80s.

Private prisons are a small portion of the prison system

For-profit companies are responsible for confining about 6 percent of state prisoners — which make up more than 86 percent of the prison system — and 16 percent of federal prisoners, according to the American Civil Liberties Union. So private companies really hold very little sway in America's enormous prison system.

Private prisons are also a response to mass incarceration, not a cause of it.

In 2011, for example, Ohio became the first state to sell a state prison to a private company. The state pursued this as a cost-cutting measure — it figured that the expense of running the Lake Erie Correctional Institution was so high that it would be better if a private company bore the costs. But this cost-cutting measure only became necessary for Ohio as mass incarceration consumed a big portion of its budget, forcing the state to look for new ways to save money.

A similar story has played out in other states, from California to Virginia, as private prisons have been touted as a way to reduce spending as incarceration continued to grow. The Sentencing Project, which advocates for reduced incarceration, explained this in 2004, noting that a new era of privatization came after mass incarceration began:

The 1980s, though, ushered in a new era of prison privatization. With a burgeoning prison population resulting from the "war on drugs" and increased use of incarceration, prison overcrowding and rising costs became increasingly problematic for local, state, and federal governments. In response to this expanding criminal justice system, private business interests saw an opportunity for expansion, and consequently, private-sector involvement in prisons moved from the simple contracting of services to contracting for the complete management and operation of entire prisons.

None of this is to say private prisons aren't fraught with problems. States are typically required to pay for a minimum number of inmates even if they can't fill a prison, so they have an incentive to fill up private prisons to an extent to get bang for their buck. And more inmates mean more money for prison companies, while stronger rehabilitation programs and security may be seen as too costly.

But as criminal justice expert John Pfaff pointed out, public prisons have some of these problems, too: States have to pay for keeping public prisons open even if they don't have enough inmates for full occupation, and that may create an incentive to fill up public prisons, as well.

2B. The public sector union keeps prisons open to keep guards paid. How is that different than paying CCA for unused beds? — John Pfaff (@JohnFPfaff) February 4, 2016

Moreover, there's no sign private prisons have even perpetuated mass incarceration.

Consider the push for criminal justice reform: If private prisons really hold a lot of sway over prison policies, why have bipartisan coalitions in most states — from California to Georgia — pushed criminal justice reform and reduced their incarcerated populations over the past several years? And why are bipartisan groups in the US Senate and House proposing reform that would cut down on the federal prison population?

The reality is that criminal justice policies are driven by much broader concerns: the crime rate, and how lawmakers think they should work to reduce it without straining budgets.

High crime rates and drug use led to mass incarceration

In the 1970s and '80s, crime rates and drug use were historically high. This created a political crisis in America, as the public, media, and politicians bought into the idea that punitive measures were necessary to combat the breakdown of society's moral fabric.

Take, for instance, the murder rate over the past several decades, which was unusually high from the 1970s through the early '90s:

Drug use was similarly high during the late 1970s and early '80s, based on Monitoring the Future data for high school seniors:

The media went into a frenzy over these types of numbers, widely covering gang and gun violence, drug use among children, and the crack cocaine epidemic. This drove the public to demand that lawmakers do something about these problems. And policymakers responded with mass incarceration.

The idea was that more arrests, more prosecutions, and longer prison sentences would deter crime and drug use. Facing a crisis, US lawmakers pursued these policies with little concern for the costs — not just the financial burden, but the racial disparities and erosion of civil liberties they produced as well.

Mass incarceration was an ineffective way to fight crime and drug use

It's now easy to look back at these ideas with scorn. Criminal justice experts point out, for example, that incarceration reached the point of diminishing returns by the 1990s — there are only so many serious criminals out there, and by then the people getting put in prison weren't people who'd be committing crime after crime on the street. So mass incarceration was an ineffective way to fight crime and drug use.

But these policies were desperate responses to real crises at the time. Policymakers didn't need private prison companies lobbying them to pursue mass incarceration. They just had to look at the country's problems with crime and drugs — and the media and public's demands that something be done — to realize they had to act.

All of this is crucial to understanding why mass incarceration is now widely regarded by people of both political parties as unnecessary: Just as historically high crime rates and drug use encouraged America to increase its prison population, plummeting crime rates and reduced drug use suggest punitive measures are no longer needed.

But it will take criminal justice reform far beyond banning private prisons to end mass incarceration.