Despite decreasing crime rates, the number of people incarcerated in U.S. prisons and jails has been rising steadily over the past two decades. Since 1985, the incarceration rate has more than doubled, according to a Justice Department study released yesterday.

• The total number of Americans locked up in jails and prisons reached 1,725,842 last June -- equal to 645 per 100,000 population.

• As of 1985, the incarceration rate was 313 per 100,000.

• The number of those confined to jails jumped 9.4 percent last year -- almost double the average annual increase of 4.9 percent since 1990.

• Meanwhile, the number of state and federal prisoners rose only 4.7 percent -- less than the annual average of 7.7 percent since 1990.

Jails generally hold those awaiting trials or serving terms of less than a year, while prisons house convicts serving longer sentences.

More than half the growth in prisoners was accounted for by just four states and the federal prison system. The four were California, Texas, Missouri and Illinois.

While the rate for violent crimes and property crimes has been falling since 1991, these figures do not include the sale and possession of drugs -- which are responsible for the largest increase in incarceration. Drug offenses have accounted for more than one-third of the increase in incarcerations since the early 1970s. Since 1980, incarcerations for drug arrests have increased 1,000 percent.

Noted Princeton criminologist John J. DiIulio Jr. has found that 25 percent of new inmates entering prison in New York state are "drug-only" offenders, with no record of other types of crimes. If that estimate is borne out by further research, he says, the criminal justice system is doing "a worse and worse job of diverting drug-only offenders" into alternative programs that would be less expensive and where drug users might be more likely to get treatment.