Recent reports of an increase in crime in St. Louis bring back memories of a time, not so long ago, when crime rates were at an all-time high. Historical perspective may be cold comfort to today's crime victims, but it helps to know that crime rates do not always rise; they also fall, sometimes dramatically.

Knowing why crime declined in the past can help us to figure out why it is increasing now and how to avoid a return to a far worse period in recent history.

After dropping during the early 1980s, homicide rates in St. Louis city climbed to a record high over the next decade, peaking in 1993 at 267 killings, or about 70 homicides for every 100,000 city residents. The St. Louis Post-Dispatch even ran a daily box score of the body count. And then homicides fell continuously for the next five years. By the end of the 20th century, St. Louis homicide rates had returned to levels not seen since the 1980s. The same is true for other serious crimes, such as robbery and burglary.

St. Louis wasn't exporting its problems to other places in the region. Crime dropped throughout the metro area, including East St. Louis where homicide rates were cut in half during the 1990s. In fact, crime was plummeting across the nation.

St. Louis homicide rates from 1980 to 2006 are compared with those for the nation in the accompanying chart (compiled using Uniform Crime Reports). Although the St. Louis rates are much higher, the time trends are very similar. The close correspondence between the local and national trends suggests that whatever drives St. Louis homicides up and down is unlikely to be of purely local origin.

We do not know all of the reasons for the nation's crime drop during the 1990s, but the evidence suggests three factors were particularly important.

• First, rates of firearm violence fell with the ebbing of the crack cocaine epidemic in the early 1990s.
• Second, skyrocketing levels of imprisonment took some of the most violent offenders off the street.
• Third, the record economic expansion during the decade increased the rewards of lawful behavior, especially for inner-city youth, whose unemployment rates hit historic lows.

This triple-header was no less important in reducing criminal violence in St. Louis than elsewhere in the country.

The crime drop ended at the turn of the century. The benefits of shrinking crack markets had long since abated, growth in imprisonment slowed, and the economy soured. Crime rates have fluctuated since 2000, largely reflecting the volatile economic trends of the new century.

St. Louis homicide rates rose for a few years and then fell in 2002. In 2003, the city recorded 74 homicides, the fewest since 1962, and a remarkable 72 percent drop since the 1993 peak. No one knows why the number fell, but innovative police department programs in place at the time likely contributed to the downturn.

Now we face another upsurge in lethal violence. Sixty-three homicides occurred in the city during the first five months of 2008, a 37 percent rise over the same period last year. Homicide is increasing in cities across the country, so local conditions can explain only part of the recent increase. But effective law enforcement can help stem the tide, and we need only look back a few years to find out how much.

If we don't understand all of the reasons that crime rates fall, the consequences are clear. Cities thrive when crime declines. St. Louis has undergone significant revitalization during the past 15 years. Neighborhoods across the city -- from Old North St. Louis to South Grand -- came back to life. Downtown attracted new development and residents. The city reversed a half century of population loss. Progress has been uneven but undeniably real. And it can end as abruptly as it began.

The right question to ask about the current crime increase is not whether we will return to the mayhem of 1993; that is very unlikely. The right question is how to get back to 2003. Local officials cannot reverse the business cycle, but they can figure out what worked five years ago and do it again - before crime undoes the progress on which the city's future depends.