Perhaps the suburbs are luring city folk away. It could be the brutal Boston winters. Or maybe, the US Census Bureau undercounted the city's population.
Between 2000 and July 2004, Boston has lost about 20,000 people, bringing its population down to 569,165, according to figures the Census Bureau released this week.
There is a psychological blow, too. Boston is no longer one of the 20 largest cities in the country. It is 24th, behind growing metropolises like Charlotte, N.C., and Jacksonville, Fla.
A city official sniffed at the numbers and said the census chronically underestimates the population in Boston.
''It is just an estimate," said Alvaro Lima, director of research for the Boston Redevelopment Authority.
But Thomas H. O'Connor, university historian at Boston College, said it is worth trying to find out why people may be leaving. ''If we develop a brain drain where the best and the brightest are going to other cities, I think that can be a discouraging prospect for the future of our economy, maybe our society," he said.
Former mayor Raymond L. Flynn said the figures show that the city is losing middle-class residents, who cannot afford to live in Boston.
''I think that's one of the most troubling demographics that I've . . . seen in a long time," he said.