That was bad, and worse was to come. Murder, rape, robbery: Year after year, Americans suffered more and more in almost every category of crime. A new generation of "tough on crime" politicians and prosecutors tried everything they could think of, and yet nothing worked. They sent more criminals to prison for longer. They reintroduced the death penalty. They hired more cops, equipped them more lavishly, and trained them better. They loosened gun laws, created the national 911 system, and on and on. Still the crime rates rose, reaching an all-time peak in 1991.

And then ... something happened. Between 1991 and 2015, rates of criminal victimization dropped by half, with the most violent offenses—murder and rape—declining the most precipitously. An American citizen is less likely to be a victim of crime today than at any time since good records began in the 1970s. New York City is almost certainly safer today than at any time in its history. And the weird thing is that nobody quite knows why it all happened. Ever tougher criminal sentencing helped. A single criminal will commit many crimes over his career. The more of that career he spends behind bars, the fewer crimes he'll have opportunity to commit.

More sophisticated policing surely helped. "Broken windows" and community policing got cops out of their cars and onto their beats. CompStat systems directed police resources where they'd do the most good.

Many other technological and social factors played their part too. Some have nominated the aging of the population, or the phasing out of lead paint, or new immigration flows, or improvements in educational levels, or the decline in use of crack cocaine. Some hardcore proponents credit more abortion of unwanted children or more concealed carry of private firearms. Yet the uncomfortable truth is that there's remarkably little clarity about the causes of this hugely important social improvement. Almost nobody predicted it beforehand. Even after it happened, social scientists cannot agree on why.

Stephen Levitt of the University of Chicago in 2004 credits more prisons and more cops for the largest part of the drop in crime. In the opposite corner, the Brennan Center at New York University argues that prisons deserve little credit for the decline in crime in the 1990s and actually became counter-productive in the 2000s. If not prison, then what? The Brennan Center seems baffled. It attributes nearly half the crime drop of the 1990s—and more than half of the crime drop of the 2000s—to mysterious “other factors."

Settling the dispute may ultimately prove unresolvable, for reasons explained by Friedrich Hayek in his 1974 Nobel Prize lecture, “The Pretense of Knowledge”:

Unlike the position that exists in the physical sciences, in economics and other disciplines that deal with essentially complex phenomena, the aspects of the events to be accounted for about which we can get quantitative data are necessarily limited and may not include the important ones. While in the physical sciences it is generally assumed, probably with good reason, that any important factor which determines the observed events will itself be directly observable and measurable, in the study of such complex phenomena as the market, which depend on the actions of many individuals, all the circumstances which will determine the outcome of a process ... will hardly ever be fully known or measurable. And while in the physical sciences the investigator will be able to measure what ... he thinks important, in the social sciences often that is treated as important which happens to be accessible to measurement.

Yet even if we can’t know which factor contributed exactly how much to the drop in crime, what we can say is that a complex of causes taken together has produced a hugely positive result—probably the single most-successful policy outcome in the United States in the past quarter-century. Americans are doing something right, even if they don’t precisely know what that something is. And because they don’t know what that something is, they had better be very, very careful about any changes it’s proposed to make.