What accounts for the gradual onset of domestic tranquility? Locking up more criminals probably did some good, but that trend has run its course. Last year, more inmates were released than admitted. There's been no recent boom in police hiring.
Abortion is another explanation for the decline of the 1990s -- a view popularized in the book "Freakonomics," by University of Chicago economist Steven Levitt and journalist Stephen Dubner. They surmised that by legalizing abortion nationwide in 1973, the Supreme Court prevented many births to women who were poor, young, unwed, or all three.
Those births, they argued, would have produced a high number of unwanted, abused and neglected children who would be prone to criminality. Eliminating them prevented a lot of felonious mischief that would have occurred a couple of decades later, when the kids reached adulthood.
In fact, as University of California at Berkeley law professor Franklin Zimring notes in his book, "The Great American Crime Decline," births to unwed teens didn't fall after the abortion decision -- they rose. "There were no visible signs of changes in the demography of births to match the theories," he writes.
Nor does abortion seem to account for the decline that has taken place lately. Abortion rates peaked in 1981, fell 12 percent by 1993 and have kept dropping. If higher abortion rates lead to lower crime, as "Freakonomics" suggests, shouldn't lower abortion rates lead to higher crime?
The truth, Carnegie Mellon University criminologist Alfred Blumstein informed me, is that "no one has a definite explanation." Lots of factors may have played a role, and simple lessons are hard to find.
Call it a Christmas miracle. We don't know how we reached the promised land. But we might as well enjoy it.