After six years of dramatically falling U.S. crime rates, criminologists are coming to the conclusion that there are several factors behind the trend.

Papers commissioned by The Journal of Criminal Law and Criminology at Northwestern University School of Law cite many of the usual explanations -- along with some novel ones.

• Among the latest reasons suggested for the drop in violence are a drop in alcohol consumption, and a return of greater social stability after the upheavals in politics, the economy and the family in the 1960s.

• The usual explanations center on improved police tactics, more criminals behind bars, a better economy and a revulsion by young people in the nation's inner-cities against the culture of drugs and guns that spawned much of the violence of the 1980s.

• There are enormous variances in crime rates from city to city and even block to block, some researchers point out, and the reasons crime has declined in New York may be very different from Los Angeles.

There have been two different crime trends -- a long-term decline in murder by adults, and a sudden increase followed by a drop in crimes by juveniles. Different factors must be at work in each case, according to Alfred Blumstein of Carnegie-Mellon University.

For adults, he said, the crucial reasons are a decline in murder by spouses, a reduction in traditional barroom brawls as the neighborhood tavern has disappeared and more people in prison.

For young people, the causes appear to be a change in drug markets as older, more established dealers have taken control -- and the impact of new police tactics that focus on taking guns away from young people.