After decades of swelling state correctional populations, Hawai'i prison officials are predicting the rapid growth in the numbers of male and female prison inmates will ease between now and 2013.

What isn't clear is exactly why the growth in the prison population will slow, and that forecast is being questioned by Honolulu Prosecutor Peter Carlisle and some experts who watch criminal justice policy.

The state's long-term prison population projections will be critical as state lawmakers study prison crowding and what to do about it. The projections are the basis for the state Department of Public Safety's plan to dramatically expand Hawai'i's prison and jail capacity by 2013.

If the state is correct and and the growth in the prison population slows, the state corrections master plan estimates that it will still cost $1 billion to expand the correctional system to house all of the state's inmates by 2013.

But if the prison population continues to grow as fast as it did during the 1990s, $1 billion won't be enough money to house all of Hawai'i's inmates.

Carlisle, who as Honolulu prosecutor oversees most state criminal cases in Hawai'i, said he finds the state's prison population projections "curious."

"Well, I can guarantee you that I have no intentions of putting fewer criminals in jail if there are criminals out there to put in jail," Carlisle said.

Carlisle said he can't see how the slow-growth projections could come true unless there is a sudden drop in the state population or in the number of criminals, or there are dramatic changes in state criminal law that result in more offenders being set free.

Carlisle said suddenly relaxing criminal penalties doesn't seem likely because "that doesn't seem to be the environment that we live in. Plus, I don't think it's something that you could sell to the voters very easily."

Factors that caused the rapid growth in the prison population in the 1990s included mandatory sentences for some drug offenses, and tough parole policies that sent more parole violators back to prison.

The future projections were developed by Carter Goble Associates, a consultant the state hired to update the corrections master plan. The company studied the historic growth in the state's prison population and developed projections for the future that it used to calculate how many additional prison beds would be needed in the years to come.

Carter Goble concluded the rapid growth in the men's prison population from 1990 to 2003 will be halved in the next decade, dropping from the historic 11 percent average annual growth to only 4.8 percent growth per year from 2004 to 2013.

The consultant offered an even more upbeat prediction for the women's prison population: The company estimated the growth in the Hawai'i women's prison population will tumble from its average increase of 29 percent a year from 1990 to 2003, to growth of just 7.8 percent a year from 2004 to 2013.

The Carter Goble plan gives no reason for predicting the slowdown in the growth of the men's population. However, for women offenders the consultant predicted Hawai'i will send more women to community-based treatment and other programs instead of prison, which will slow the growth in the prison population.

"I'll be delighted if that turns out to be the case, but the way things are going, I'll also be surprised," said David Johnson, associate professor of sociology at the University of Hawai'i-Manoa. "Recent history suggests there's not much interest in doing that."

For example, last year state lawmakers declined to provide $800,000 to finance a plan for a 30-bed transition program for women who would otherwise be in prison. Instead, lawmakers provided just $100,000, a decision that sunk plans by the nonprofit TJ Mahoney & Associates to operate the 30-bed program at a Honolulu building it was leasing.

TJ Mahoney surrendered the lease on the building, and the state has not yet spent the $100,000 that lawmakers allocated.

Carlisle also said he doesn't see any momentum to embrace alternatives to prison so completely that it would have a major effect on the growth in the prison population.

On the contrary, Carlisle wondered if the state might encounter a phenomenon he called "Ice II" — where some other illegal drug suddenly becomes popular in Hawai'i in the way methamphetamine did years ago, triggering a surge in arrests and convictions. That could lead to unexpected growth in the prison population.

"It seems to me that it's likely that there will be some new horrible drug that's going to raise its ugly face at some point in the future," he said.

Events such as drug epidemics are part of what make it so difficult to predict inmate populations. Other variables that affect prison populations include changes in parole and furlough policies, and changes in sentencing laws that the state Legislature adopts, such as mandatory minimum terms for specific crimes.

In fact, when Carter Goble made prison population projections in 1991 for the state, its predictions were far too low. The consultant estimated that in 2000 the state would have about 3,200 prison and jail inmates. The actual number turned out to be more than 5,000.

To house the additional inmates, the state has been exporting prisoners since 1995 to privately run prisons on the Mainland. The state now holds about 1,850 inmates out of state, which is about half of Hawai'i's total prison population.

Prison officials acknowledge predicting correctional population trends is tricky business, but they believe the consultant's new estimates are accurate.

Frank Lopez, acting director of the state Department of Public Safety, said he trusts the Carter Goble projections because they are close to estimates that state prison officials developed independently.

Michael Gaede, spokesman for the Department of Public Safety, said department statistics show a spike in the men's prison population from 1996 to 1998, and an increase in the women's population from 1997 to 2000, but growth rates have leveled off in recent years.

Reach Kevin Dayton at kdayton@honoluluadvertiser.com.