Nevertheless, the report bolsters census estimates that the city's population has increased slightly over the last 10 years, said Larry Eichel, project director of the initiative and author of the report.
According to the 2000 census, 1,517,550 people lived in the city a decade ago. The latest population number, released last week in a 2009 census estimate, is 1,547,297 - an increase of around 30,000 people.
The city's population bump has been attributed to a greater number of births than deaths, and an influx of immigrants as well as empty nesters moving from the suburbs, scholars said.
According to the Pew report, the number of people moving into the city has increased steadily, up from 31,837 in 1993 to 42,250 in 2008.
For six decades, however, the story of Philadelphia was one of population loss. The city has been a sieve, with a constant flow of people leaving for the suburbs and beyond.
But the net outflow has fallen from a peak of 20,284 in 1995 to 9,846 in 2008, the report said.
Overall, the number of people moving out of the city is growing less rapidly, increasing slightly from 47,291 in 1993 to 52,096 in 2008.
"I would say the trend is looking as if we may be seeing a reversal of long-term decline in city population," said David Elesh, sociologist and demography expert with the Metropolitan Philadelphia Indicators Project at Temple University.
But Elesh issued a word of caution about the IRS data used by Pew in the report.
"They're not the gold standard of migration data," Elesh said. "I would tend to be somewhat cautious about my interpretation, and hold off judgment on migration until we get the results of the 2010 census."
Elesh said - and Eichel acknowledged - that IRS data tend to undercount people. Many poor people don't make enough money to fill out tax forms, so they wouldn't be included.
Also, because the statistics include individuals or families who have filed federal tax returns under the same Social Security numbers in two consecutive years, certain groups are left out.
They include newly arrived immigrants from other countries, and newly divorced people who normally filed taxes as part of a couple but now are filing as single people under their own Social Security numbers.
The Pew report also said that the net outflow to the Pennsylvania suburbs - Bucks, Chester, Delaware, and Montgomery Counties - declined 42 percent between 1999 and 2008.
The drop in outflow to the New Jersey suburbs - Burlington, Camden, and Gloucester Counties - was even more significant. Between 2002 and 2008, for example, net outflow to South Jersey was down 74 percent.
"That's curious," said Janice Madden, a demographer at the University of Pennsylvania. "Something's going on, keeping people from going to Jersey. I don't know if it's high property taxes."
Regardless, Madden added, "I think Pew should put more focus on in-migration to Philadelphia than on out-migration. That's the more dramatic story."
She said in-migration is caused by Philadelphia becoming a more attractive place to live, especially for "empty-nesters and people without kids of any age who are moving back."
Madden said that the highly criticized Philadelphia school system wouldn't be a detriment to childless couples.
Overall, Madden concluded, "the increases in in-migration are pretty impressive."
Contact staff writer Alfred Lubrano at 215-854-4969 or firstname.lastname@example.org.