WASHINGTON — A new study says a handful of counties across the country — including New Orleans and East Baton Rouge and Jefferson parishes — are responsible for a disproportionate share of prison terms of life without parole for people younger than 18.

The report issued Tuesday by the Phillips Black Project found that such sentences have become increasingly rare in recent years. The drop in sentences comes amid a long nationwide decline in violent crime and Supreme Court decisions that have limited imposition of life sentences with no chance of release for people who were younger than 18 when they committed their crimes.

But prosecutors in Los Angeles, Miami, New Orleans and Philadelphia continue to seek the harshest terms for young offenders who are convicted of murder, the report said.

Those places are among the top seven jurisdictions that account for more than one-quarter of all life sentences for juveniles since 2011. The others are Tulare County, in California, and the East Baton Rouge and Jefferson parishes in Louisiana.

The life terms are “isolated not just in states, but they are isolated in counties, indicating that the role of prosecutorial discretion is having a large impact on the way juvenile life without parole sentences are imposed,” said John Mills, a lawyer who is one of the report’s authors. The Phillips Black Project backs the abolition of life terms for young offenders.

The geographic pattern of the life terms is similar to that for death sentences.

Even within a state that has capital punishment, “the imposition of the death penalty heavily depends on the county in which a defendant is tried,” Justice Stephen Breyer said in his dissenting opinion in June in a case that upheld Oklahoma’s use of a controversial sedative in its lethal injection executions.

The report also identified racial disparity in the way sentences are imposed. A young, black convicted criminal has been twice as likely to receive the life term than his white counterpart, the report said.

The Supreme Court has said in a series of decisions that children are less responsible for their crimes and more capable of rehabilitation than adults. Young killers may no longer be executed and life terms cannot be automatically imposed.

Nine states abolished the sentence after the Supreme Court ruled three years ago that such terms cannot be issued automatically. There are now 15 states that prohibit a life term for those under 18.

“Until the court weighs in, there will be the possibility of outlier jurisdictions imposing penalties that the vast majority of the country has abandoned and finds anathema to our values,” Mills said.

The report was issued in advance of high court argument next month over whether the 2012 decision on life terms should be applied to people whose cases were decided long ago.