"Our findings, from the fifth-largest city in the United States, raise questions regarding the fundamental fairness of the criminal justice system and whether it provides equal justice under the law," the paper reads.
The study says several institutional factors contribute to the disparity, including a flat-fee payment for court-appointed lawyers, now being challenged in court.
Lawyers get a flat fee to prepare a case for trial: $1,333 if the case is resolved before trial and $2,000 if the case goes to trial. After the first day of trial, lawyers get $200 for three hours or less of daily court time and $400 for more than three hours.
Pennsylvania is the only state to leave the responsibility for funding defenses for indigent murder defendants to the counties - a special hardship for cash-strapped larger cities such as Philadelphia, where impoverished defendants account for 95 percent of murder cases.
Philadelphia pays appointed lawyers in death-penalty cases less than any other Pennsylvania county, according to a petition challenging the flat-fee system filed by Marc Bookman, executive director of the Philadelphia-based nonprofit Atlantic Center for Capital Representation.
The RAND study's results, Bookman said, "are exactly what you would expect."
"We have allowed a very small group of underpaid and under-resourced lawyers to handle an outrageously high number of our city's most serious cases."
Philadelphia Common Pleas Court Judge Benjamin Lerner agreed that "the conclusions should not be a surprise to anybody who has been close to the system for any period of time."
In September, the state Supreme Court appointed Lerner to do fact-finding on the Atlantic Center challenge. One hearing was enough to convince the city's ranking judges that the fee system should be adjusted, and talks are ongoing.
The RAND findings dovetailed with an Inquirer report earlier this year that more than 125 capital-murder trials in Pennsylvania - 69 from Philadelphia - have been reversed or remanded by appeals courts because serious mistakes by defense lawyers deprived the accused of a fair trial.
That number amounts to almost a third of 391 capital convictions in Pennsylvania since the modern death penalty took effect in 1978.
Lawyers quoted in the story repeatedly cited low pay for court-appointed lawyers as a major factor.
Since April 1993, one in five defendants has been randomly assigned a public defender by lottery; the four others get a lawyer from a pool of criminal-defense lawyers willing to accept such appointments.
The Atlantic Center petition maintains that the situation may violate an individual's Sixth Amendment right to effective legal representation.
"The solution is obvious," Bookman said. "We have to expand the pool of lawyers handling these cases, provide them with proper resources, and regulate their caseload. That's what every other city does."
The RAND authors - James M. Anderson and Paul Heaton - have been at work since 2009, when they received a grant from the National Institute of Justice, a research arm of the Justice Department.
Anderson, a RAND behavioral/social scientist, had firsthand experience. From 1996 to 2004, he was a lawyer in the Philadelphia Defender Association's Capital Habeas Unit, which handles federal-level appeals for Pennsylvanians convicted of first-degree murder and sentenced to death.
Heaton is a RAND economist.
Philadelphia was selected, the researchers wrote, because the process of randomly assigning 20 percent of indigent murder cases to the defender's office made it possible to compare outcomes resulting from the two types of appointed lawyers.
The defender's homicide unit represents its clients with a group of 10 lawyers well-versed in Philadelphia's criminal court system.
Each case is staffed by two salaried lawyers, one who typically handles the trial's guilt phase while the other prepares for the possible death-penalty hearing. The lawyers have their own staff of investigators and mitigation specialists to research the defendant's background and family history to present mitigating evidence in a death-penalty hearing.
By contrast, the RAND researchers found that Philadelphia's compensation for court-appointed lawyers in murder cases is effectively $2 an hour, much less than minimum wage.
The compensation has certainly dissuaded lawyers from accepting capital appointments: Fewer than 30 of the city's 13,000 lawyers are willing to accept court appointments and meet minimum state standards to do so.
The RAND researchers said that court-appointed lawyers also tend to be isolated and outgunned when it comes to legal resources.
Most are sole practitioners, and some earn virtually all their income from death-penalty appointments.
In 2003 - a year before the end of the RAND study data period - the disparity between the better-staffed defender's capital unit and court-appointed counsel led the then-president judge of Common Pleas Court, Frederica Massiah-Jackson, to require appointment of two lawyers in death-penalty cases: one for the guilt phase and one for the penalty.
Some who accept appointments in capital cases say the focus on funding is misplaced, that they accept death-penalty cases out of a sense of mission to poor people in dire situations.
Still, there is evidence that the lack of resources for capital lawyers has a price.
The RAND researchers estimated that Philadelphians wrongly convicted and serving life terms or longer terms than needed add up to 6,400 years of excess prison costs: a taxpayer bill of more than $200 million.
Contact staff writer Joseph A. Slobodzian at 215-854-2985, firstname.lastname@example.org, or @joeslobo on Twitter.