COPS AND JUDGES like to lament the problem of recidivism. A few repeat offenders commit most of the crimes that keep courtrooms hopping and jails jammed - or so the complaint goes.

But a new and still-unreleased study of the Philadelphia prison system shows that first-time offenders, rather than repeaters, are more of a reason for the overcrowding in the city's jails.

And most of those behind bars aren't violent offenders, researcher Paul Heroux writes in the study, "Reincarceration in the Philadelphia Prison System," a copy of which the Daily News obtained.

Former Prisons Commissioner Leon King in September commissioned Heroux, a former military analyst who worked as a special assistant to King, to research prison data with an eye on alleviating crowding. Heroux completed his work under current commissioner Louis Giorla.

Heroux examined data on inmates discharged between 2000 and 2006, and found that reincarceration rates fell to about 35 percent.

Instead of recidivism, increased admissions and longer length of stay are to blame for packing Philadelphia's prisons, Heroux found.

City prisons admit an average of 102 inmates per day but discharge only 99 daily, amounting to an annual jump of almost 1,100 inmates, Heroux discovered.

And inmates' length of stay rose to an average of 91 days, up from about 74 days in past years, he found. Longer stays do not seem to decrease reincarceration, Heroux added.

With more than 9,300 inmates in six city jails built for about 6,400, Heroux charges, the city must rethink how it punishes its law-breakers. (The city released a strategic plan yesterday that pegged the prison population at 9,193 inmates.)

"Philadelphia has more inmates per citizen than any other of the major cities in the United States, but still does not have a lower crime rate," Heroux said. "This suggests that an alternative to incarceration crime strategy is necessary."

Mayor Nutter's administration has vowed, as part of its crime crackdown, to implement alternatives to incarceration for nonviolent offenders such as electronic monitoring, more mental-health and substance-abuse treatment and day-reporting, which is a nonresidential intermediate punishment that combines intensive surveillance and rehabilitative services.

Nutter also appointed Deputy Mayor for Public Safety Everett Gillison to head a new Criminal Justice Advisory Board charged with helping alleviate jail crowding.

Gillison couldn't be reached for comment yesterday.

Among Heroux's findings:

_ Only one-fifth of inmates in city jails are there for violent offenses.

_ About half are jailed on drug offenses, the biggest reason for reincarceration.

_ The city reincarcerates the seriously mentally ill at high rates, and these inmates tend to be held about 50 days longer - and at a higher cost - than their peers who have no mental problems. Heroux contends that alternative sentencing, such as day-reporting, probation and mental-health courts, would better address these inmates' needs.

_ Less than 7 percent of inmates have 10 or more jail admissions. Most inmates have three or fewer admissions, meaning they're not traditional repeat offenders.

One prison advocate expressed guarded hope that the report could bring much-needed change.

"This report offers a key to understanding why the population will continue to go up unless there are serious interventions in terms of who we decide to incarcerate," said attorney David Rudovsky, who filed a class-action lawsuit on behalf of inmates last month to force improvements in the system. "But I remain skeptical, because all of this [suggested improvements] has been on the table before, and nothing has changed."

Although city officials often claim that Philadelphia's recidivism rate stands at 70 percent, Heroux said that figure is inaccurate and reflects a misunderstanding about the reasons for reincarceration, which may include non-recidivism factors such as routine transfers or reincarceration after posting bail and then later being sentenced on the initial charge.