John Roman is a senior fellow in the Policy Advisory Group and the Justice Policy Center at the Urban Institute.

In 1970, there were about 4 violent crimes per 1,000 Americans. In 2014, there were 3.75 violent crimes per 1,000 Americans. America is essentially as safe today as when VW Beetles ruled the road and Simon and Garfunkel ruled the radio.

Surprised? If you are under 40 years old, you’ve never lived in a safer country than America is today.

Equally surprising, though, is our reaction to this good news.

Rather than creating a crime dividend, where we save billions by incarcerating fewer people, we have gone in the opposite direction. In 1970, on an average day, the United States incarcerated about 200,000 of its citizens. In 2014, with virtually identical crime rates, the United States incarcerates more than 1.5 million people, seven-and-a-half times the 1970 rate. This is not simply because of population growth. On a given day in 2014, America incarcerated about 5 out of every 1,000 citizens. That’s a five-fold increase from 1970.

Why do we lock up so many people — disrupting so many families and so many communities — to achieve the same crime results we did nearly a half-century ago?

The answer seems pretty straightfoward: A higher percentage of people arrested for felonies are eventually convicted today than back then — probably than ever before. In 1991, about 1 out of 16 people arrested for a felony eligible for a year in prison served that amount of time or more. Today, that number is closer to 1 in 4.

It is fair to ask whether sending more people to prison is a good thing. On one hand, it certainly means fewer guilty criminals are going free. On the other hand, a higher conviction rate almost certainly increases the number of innocent people who are convicted, simply because of the increased volume of prisoners.

But a third point is also worth considering. Does incarcerating a higher proportion of those arrested serve any purpose beyond simply holding more people accountable for their actions?

Both crime theory and economic theory are consistent in saying that policies are most likely to reduce violence when potential criminals believe they will be arrested for an offense, should they commit one. Most scholars argue that it is the certainty of arrest that deters violence. At that one moment when a potential offender contemplates a crime, the only calculus that seems to matter is: Will I get caught?

So, we should ask whether increasing the number of people who are incarcerated after an arrest will change that criminal calculus. The answer is plainly no.

The best measure of how effectively we are increasing the certainty of an arrest is the clearance rate: a simple ratio of the number of arrests per crime. It turns out that the clearance rate today, about 24 percent, is more or less identical to 1970.

Criminals today are no more certain of getting arrested than criminals were in 1970. However, they are more certain of going to prison once arrested. So there is no more deterrence, just more punishment.

Fordham Law professor John Pfaff has written about this question and finds that incarceration rates are up simply because prosecutors are more aggressive today. In other words, just about the same number of people are coming in through the precinct door, but far more of them are ending up in prison. Why?

Pfaff writes that the number of drug offenders is not increasing, as the percentage of drug offenders peaked at 22 percent in 1990 and has declined. Since the prison population has multiplied, that means more drug offenders go to prison in raw numbers, but the percentage of the population of prisoners that are drug offenders is actually declining.

He also notes (and my research confirms) that there is little evidence that technology has made law enforcement substantially better at solving crimes. And, both Pfaff and the Bureau of Justice Statistics find no evidence that American prisoners serve longer sentences today than in the past. There are just more of them.

That means there is no evidence that we are getting better at solving crimes; we are only getting better at prosecuting them.

One theory is that prosecutors may have changed their behavior over time. For example, in Governing Through Crime, Jonathan Simon argues that prosecutors appear to be using tough on crime rhetoric and actions as a ladder to climb to higher office. Alternatively, it could be that expansion of mandatory minimums, three strikes laws and truth in sentencing are simply creating more levers to force plea bargains. That is, because the alternative to a plea is so much more draconian, more people who are arrested are taking a plea and going to prison.

Or, and this is my hypothesis, it could simply be that there has been a huge expansion in prison capacity in the United States. There were about 250,000 jail beds in the early 1980s, and today there are more than 800,000. And, as long as there prison cells to be filled, they will be.

So, the story is straightforward: America has simply created a tremendous capacity to convict and incarcerate its citizens. And, we continue to do so even though violence has declined dramatically. Prosecutors have more beds to fill and they are doing so, and as a result more arrestees find themselves serving prison sentences than ever before. And some of them may be innocent.