Policing in the United States has changed a lot during the past 50 years. Higher education and training requirements have led to greater police professionalism, and most departments' ranks have benefited from huge increases of personnel, stunning technological advancements, forensics breakthroughs, and affirmative action policies that presumably have led to a more representative workforce sensitive to civil rights. Policing's academic side has also prospered from decades of ample government research grants.

Many observers credit the police because reported crime in the nation has generally been going down for nearly a decade. Reported homicides in New York City and other jurisdictions recently hit their lowest level in more than 40 years.

But discussions of police performance often fail to note another important but overlooked trend, apparently unrelated to the falling crime rate: Federal statistics reveal that the nation's "clearance rate" - the percentage of cases for which police arrest or identify a suspect - has fallen dramatically. And this shift is fraught with implications.

The arrest clearance rate for reported homicides recently dropped to about 60 percent compared with about 90 percent 50 years ago. This means that a murderer today has about a 40 percent chance of avoiding arrest compared with less than 10 percent in 1950. The record for other FBI Index Crimes is even more dismal: The clearance rates have sunk to 42 percent for forcible rape, 26 percent for robbery, and 13 percent for burglary and motor vehicle theft, all way down from earlier eras.

In Boston, the homicide clearance rate plummeted to only 28 percent in 2004 - a shocking development for a city that gained lavish praise for crime reductions in the 1990s.

Judging a police department or the criminal justice system as a whole based simply on arrest statistics wouldn't be wise, for the police can and do fulfill many crucial functions in our society, such as maintaining public order and helping to protect citizens from terrorist attack. But ignoring measures of how the police deal with reported serious crime isn't smart either.

It's not that America's cops haven't been making arrests - in fact, their total annual arrests jumped from 3.3 million in the nation in 1960 to 14 million in 2004, a staggering number that helps to explain why the United States imprisons more of its citizens than any other country in the world.

So, if reported crime has been going down and arrests have gone up, what accounts for the plummeting arrest clearance rates for murder, robbery, rape, burglary, larceny, and motor vehicle theft?

Part of the answer must involve drug law enforcement - victimless offenses that aren't reported to the police or included as FBI Index Crimes. Instead of arresting suspects for burglaries and other serious reported crimes, cops today spend much of their energy going after illegal drugs. Their arrest rate for drug possession (especially marijuana) has shot up more than 500 times from what it was in 1965.

And what are some possible implications of this shift?

For one thing, it may give criminals the impression they can get away with nondrug related crimes.

For another, it may lessen public support for the police. Polls show those who live in "high crime" neighborhoods are generally the most dissatisfied with the police. Maybe this is because they have reported to the police that they have been victimized by robbery and other serious crimes, then witnessed that the police are not arresting anyone for it but are instead aggressively waging a "war on drugs" in the community.

Nevertheless, the matter of falling arrest clearance rates hasn't received much scrutiny from the police or the public.

Asked why the arrest clearance rate has dropped so much, one leading police scholar, Professor David Bayley of the State University of New York at Albany, said, "I haven't a clue. I've been involved in the field for 40 years and best as I can tell, nobody has even raised this stuff. Hearing about it now is like being hit by a bus."

One interpretation might be that the changing statistics actually indicate that today's police are acting more judiciously, for as one former New York Police Department homicide detective, now a private investigator, put it, "Just because cops were more likely to arrest somebody in the old days than they are today doesn't mean they didn't make a lot of mistakes back then, by beating false confessions out of innocent people and such."

Whatever the reasons, this significant trend in the police response to reported crime should prompt some serious discussion about contemporary law enforcement's priorities and effectiveness.

• Scott Christianson, a former New York state criminal justice official, is the author of several books about crime and punishment.