New Orleans and the various agencies that run its criminal justice system and jail will spend the next six months studying ways to reduce the population at the overcrowded Orleans Parish Prison under a grant from the MacArthur Foundation.

The study will involve both Mayor Mitch Landrieu’s administration and Sheriff Marlin Gusman’s office, which have often been on opposing sides in the fight over the proper size of the jail in a city that consistently locks up twice as large a proportion of its residents as the national average.

The $150,000 grant that will fund the study comes from the John D. and Catherine T. MacArthur Foundation, which selected New Orleans and 19 other jurisdictions from among 190 applicants for an initiative aimed at reducing incarceration rates.

The grant is expected to be formally announced on Wednesday; city officials spoke about the program Tuesday.

“This will make sure that all our policies from arrest through incarceration make sense so that we don’t have a larger (prison) population than we need or a smaller one than is going to keep us safe,” Landrieu said by phone Tuesday.

Two issues underlie the grant and the general push for a reduced inmate population. One is New Orleans’ status as one of the most incarcerated cities in the country, with more than 5 out of every 1,000 residents behind bars at Orleans Parish Prison at any given time. The other is the continued wrangling between the city, which is responsible for paying for the jail and its inmates, and Gusman, the city’s chief jailer, over the size of the new jail.

In theory, the study could help resolve that issue by bringing down the jail population to an average that both sides could live with and that would fit within a federal consent decree governing conditions in the facility.

“Objective data will take you to better public policy decisions,” Landrieu said.

About 42 percent of the inmates at OPP are awaiting trial in the parish on felony charges. Another 41 percent are being held for the state, either serving out their sentence or awaiting a hearing on allegations of probation or parole violations, according to data from the city.

City officials have argued that the jail should reduce the number of state inmates it takes in to help reduce the population.

The rest of the inmates are being held for a mix of other reasons, including awaiting trial on misdemeanor charges, being held until they can be extradited to another jurisdiction or having been picked up on outstanding warrants.

In a written statement provided by city officials, Gusman touted the success of existing programs in reducing the jail’s population from an average of 6,000 a day in 2004 to less than 2,000.

“With this MacArthur Foundation grant, the Sheriff’s Office looks forward to working with our partners on more innovative ways to make meaningful and lasting changes to our justice system,” Gusman said. “I’m pleased to lead the city of New Orleans’ delegation in accepting this support from the MacArthur Foundation.”

Reducing the country’s incarceration rate has been gaining traction among elected officials of both parties, due both to the high cost of keeping residents locked up and to concerns about civil rights and the effect that spending time behind bars can have on individuals and their community.

“New Orleans’ notoriety as the most incarcerated locale in the world is largely founded on inequitable and injurious policies,” Foundation for Louisiana President Flozell Daniels said in a statement provided by the city. “MacArthur’s investment in New Orleans recognizes the city’s growing capacity to engage community advocates and system leaders in making positive policy changes that ensure the criminal justice system works for all our people.”

Among the ideas Landrieu mentioned is studying whether more offenses should be dealt with through summonses to court rather than arrests and whether sentencing policies are in line with best practices.

Other issues could involve clearing bottlenecks in the system to allow inmates to get to trial faster or creating more options for mental health care, which could allow some inmates to be housed at mental health facilities rather than in jail.

The city has already begun work through a subcommittee dedicated to studying jail population issues.

At the end of the MacArthur program, the city will be able to apply for grants worth between $500,000 and $2 million to implement the strategies recommended by the study.

“We’ve been working for years on coming up with a jail reduction strategy, and as I’ve said many times about the new New Orleans way, it’s all about partnership, so I was thrilled to have our private and philanthropic partners with us,” Landrieu said.

Follow Jeff Adelson on Twitter, @jadelson.