Conventional wisdom lags reality, and nothing better illustrates that tendency of public opinion than the debate over “mass incarceration” in the United States.

The basic facts are familiar: starting in about 1980, the U.S. prison population began to rise rapidly, until the point where, today, the United States incarcerates more people than any other nation on earth.

According to the conventional wisdom, these statistics represent a moral failing and a policy error. “[W]idespread incarceration at the federal, state, and local levels is both ineffective and unsustainable,” Attorney General Eric Holder declared last August. “It imposes a significant economic burden – totaling $80 billion in 2010 alone – and it comes with human and moral costs that are impossible to calculate.”

The strangest thing about Holder’s protest, and others like it, is that it came at a time when U.S. prison populations are shrinking, and have been for several years. In fact, the inmate population of state and federal prisons decreased by 1.7 percent in 2012, to an estimated 1,571,013 from 1,598,783 in 2011, according to Justice Department statistics. This was the third straight annual decrease.

What’s more, the downward trend is almost certain to continue. The rate of new prison admissions has been declining for the last nine years, and now stands at roughly the level of two decades ago, according to data compiled by Keith Humphreys, a Stanford University psychiatrist who studies criminal justice issues.

The root cause of these welcome trends is another welcome trend: the plunging crime rate of the last two decades. Less crime leads to declining incarceration in two ways. First, and most obvious, there are fewer law-breakers to lock up. Second, safer streets reduce the public’s demand for tough “law and order” policies – like the stiff mandatory minimum sentences that helped drive the U.S. rate of incarceration up in the 80s and 90s.

Yes, declining incarceration also reflects sentencing reforms and other policy changes at the state and local level. But those changes would not be occurring unless lower crime had eased fears about public safety to the point where it was politically possible to discuss them in the first place.

According to data from the General Social Survey compiled by Professor Mark Ramirez of Arizona State University, the percentage of Americans who believe criminals are not punished “harshly enough” declined from 85 percent in 1994 to 62 percent by 2012.

As Holder noted, the federal prison population continues to grow. Yet even federal incarceration may be leveling off; the inmate population grew by 0.7 percent during 2012, compared to an average annual increase of 3.2 percent over the past 10 years. Notably, drug trafficking offenses, which account for half of the 219,000 current federal inmate population, represent a declining percentage of new admissions, according to a 2013 Congressional Research Service report. The big drivers of additional federal incarceration in the last decade were immigration and weapons offenses.

Contrary to much propaganda about a relentless “drug war,” a 2012 Urban Institute study found that the feds shifted enforcement resources from drug trafficking to weapons and immigration offenses between 1998 and 2010, resulting in a prison population that was still growing, but not as much as it would have been if federal law enforcement priorities and practices had remained as drug-focused they were in 1998.

Living as we do now, in a country where the murder rate has, blessedly, receded to levels last seen during the Eisenhower administration, it is understandably difficult to put ourselves in the shoes of those Americans who demanded the draconian laws that contributed to our high prison population — and which so many decry, in hindsight, today.

We forget that the 1960s and 1970s were not only a time of rapidly rising violent crime rates, but also of relatively lenient sentencing, even for murder, rape and armed robbery. It is no accident that the trend toward more incarceration began in 1980, the year when the U.S. murder rate hit an all-time high of 10.2 per 100,000 — more than double what it was in 2012.

The United States incarcerates more people than any other nation. But is this because the United States is uniquely cruel and punitive, or because it is unique in other relevant respects? The U.S. is the only nation in the world that recently met all of the following four criteria: 1) It had a population in excess of 100 million 2) it had a high crime rate, both violent and otherwise 3) it had a functioning criminal justice system, complete with reliable record-keeping,  and 4) it had a highly responsive, decentralized democratic political system.

To ignore such variables is to assume that the incarceration rate in India – 30 per 100,000 inhabitants, according to the Indian Ministry of Home Affairs – is a reliable indicator of that developing country’s order and civility, rather than, say, the inadequacy and corruption of its law enforcement system.

As it happens, while the U.S. incarceration rate has been falling, that of a country typically held up as an enlightened alternative to the U.S. – Britain — was nearly doubling, from 44,000 in 1993 to 86,000 in 2012. It seems that Britain has been turning to tougher sentencing. In France, the number of inmates grew from 44,600 in 2001 to 68,000 in 2013; prison overcrowding is a serious problem there.

Some 53 percent of state inmates in the U.S. are incarcerated for violent crimes. Even if we released all state and federal drug-offense prisoners tomorrow, the U.S. incarceration rate would remain several times that of Canada and most nations of Western Europe. Our rate will probably also be higher that of other nations once the current trend toward shrinking prison populations has run its course. Would that, too, be cause for shame?

The fact is that to some admittedly unknown degree the higher U.S. incarceration rate of recent years contributed to the decline of crime. The social benefits of safer streets – just as “impossible to calculate” in their own way as the costs of incarceration — represent the other side of Holder’s narrative about “coldly efficient” imprisonment efforts.

No one should be pleased that the United States went through a period in which its prison population grew so large for so long. The end of mass incarceration is great news, cause for much more celebration than it’s getting. But it’s crucial that we draw the right lessons from recent experience. Was mass incarceration a punitive overreaction, as some now decree in hindsight, or the foreseeable, regrettable — but not totally unjustified — response to the under-reaction to crime, perceived and actual, that preceded it? Or was it some of both?

The U.S. should not aspire to have the most people in prison, the fewest, or some number in between. Our goal should be whatever rate of imprisonment punishes and prevents the most crime at the least cost.