Just five counties are responsible for a quarter of all the juvenile life-without-parole sentences nationwide, according to a new report. The leader is Philadelphia, with 9 percent of all juvenile lifers. The report arrives just weeks before the Supreme Court hears oral arguments in Montgomery v. Louisiana, a case that could decide the fate of juvenile lifers around the country.

“Prior reports showed that a handful of states were responsible for many of the life without parole sentences. We were able to show this was driven by a handful of counties within those states,” says John Mills, one of the authors.

The report, from a nonprofit law firm, Phillips Black, is the first to look at county-level data, tallying how many people are serving juvenile life-without-parole sentences from each of the nation’s 3,000-some counties. Mills and his colleagues queried state and federal corrections departments for data, then verified when possible using independent studies, court documents, and news reports. Phillips Black represents clients in death penalty cases and, more recently, life-without-parole cases. Its explicit aim with this report was to portray “a growing consensus against the practice”—a conclusion that could have significant implications at the Supreme Court.

But most everything about these sentences remains a matter of debate, right down to how many people are actually serving such sentences.

The report counted 2,341 people serving juvenile life-without-parole sentences nationwide. The counties responsible for imposing a quarter of all such sentences are: Philadelphia; Los Angeles; Orleans, in Louisiana; Cook, in Illinois, and St. Louis. Michigan, which has one of the country’s biggest populations of juvenile lifers, declined to provide information. (Detroit’s Wayne County would likely have ranked alongside Philadelphia had it been included.)

The concentration of juvenile life-without-parole sentences follows a pattern familiar to one found in studies of the death penalty. A 2013 study by the Death Penalty Information Center found that just two percent of counties impose the majority of death sentences.

And like the death penalty, Mills says, “the discretion is in the hands of prosecutors.”

In the 1990s, states passed a raft of legislation aimed at “superpredators,” a supposedly new brand of teenage criminal: “radically impulsive, brutally remorseless youngsters,” wrote the theory’s biggest proponent, the Princeton political scientist John DiIulio. From 1992 to 1999, almost every state amended its laws to make it easier for juveniles to be tried in adult court and face adult penalties. More than half of states made transfer to adult court mandatory for crimes like murder, removing the judge’s discretion entirely, sometimes even for defendants as young as 10. And in many states, adults convicted of murder face life without parole automatically.

So for a teenager who kills, or is involved in a crime that ends with the victim’s death, there are often only two decision points: what crime to charge them with, which rests with the prosecutor; and whether they are guilty, which is decided by the jury (or, more often, by plea bargain—a process which also puts much of the power in the hands of the prosecutor). Once the prosecutor decides to charge a juvenile with murder, the defendant in many states is automatically tried as an adult and, if convicted, automatically sentenced to life.

In Philadelphia, in particular, “there have been over the years a lot of aggressive prosecutors, which has led to a lot of sentences being resolved by life-without-parole sentences,” says Brad Bridge, a lawyer with the Philadelphia public defender’s office. “We’ll seek death against you, or you can opt for life imprisonment.” (Lynne Abraham, Philadelphia’s previous District Attorney, routinely made the list of “America’s Deadliest Prosecutors,” for the number of death sentences she obtained. The death penalty was outlawed for juveniles in 2005.)

In Philadelphia County, Mills found 214 people serving life without parole for crimes they committed as teenagers.

But Bridge, the public defender, disputes that as too low. “If that’s his number, he needs to go back and do a better job,” Bridge says. He and the state corrections department have been working together to come up with a tally of those serving juvenile life-without-parole sentences from Philadelphia; his number, he says, is 291. (A spokeswoman for the Department of Corrections said she could not confirm this.)

Prisons typically track the inmate’s age from the start of the prison term, not at the time of the crime. Given the years that can pass between crime, arrest, conviction, and sentencing, tallying the number of people who were under 18 when they arrived at prison inevitably undercounts the number who were under 18 when they committed their crime.

The count is more than just academic. In the coming term the Supreme Court, in Montgomery v. Louisiana, will decide whether its 2012 rulingoutlawing mandatory life without parole for juveniles must apply retroactively to those already serving the sentence. In their Eighth Amendment rulings, the justices are often guided by “evolving standards of decency that mark the progress of a maturing society.” Whether a particular punishment is rare or widespread is one of their yardsticks for measuring contemporary standards.

Those standards are changing quickly in the wake of of the earlier case, Miller v. Alabama. Mills found that, since 2012, nine states have abolished juvenile life without parole entirely. Seventeen have chosen to apply Miller retroactively. All these changes have led to resentencing hearings and parole eligibility for as many as a thousand people previously sentenced to life without parole. (Whether they’re sentenced to life without parole a second time at their resentencing hearings is an open question. Only mandatory sentences were outlawed; the sentence is allowed if the judge has discretion to choose.) Accounting for these changes, “the true number of persons subject to [life without parole] is likely closer to 1,300,” Mills writes. Just as John DiIulio recanted his superpredator theory when ranks of homicidal teenagers failed to materialize, so a generation later the law may be catching up.

An earlier version of this story incorrectly described John DiIulio as a sociologist. He is a political scientist.