Watch “Real Money with Ali Velshi” Monday, May 19, through Wednesday, May 21, for our three-part series on Philadelphia’s middle class.

Philadelphia is a city beloved for reasons as diverse as those who live here. It’s celebrated for its rich history, cheesesteaks and many, many a “Rocky” movie.

But over the last 40 years, the city has lost a quarter of its population — almost 400,000 residents. And 43 percent of that population loss has been from the middle class. Nationwide, the middle class has shrunk since the 1970s, from 60 percent to 51 percent. But Philly’s middle class was reduced even more dramatically, to just 42 percent.

The strength of the suburbs is one reason for this. “Chester County, Montgomery, Bucks, Delaware County — those economies have grown significantly over the past 40 years, and their success has come at the expense of the city of Philadelphia,” said Ryan Sweet, an economist at Moody’s Analytics.

But it’s also due to the decline in manufacturing there. In 1970 the middle class worked primarily in manufacturing and midlevel jobs. No longer. Manufacturing jobs have plummeted to just 10 percent of the workforce.

Still, there are signs of hope. “Even though there’s been this huge decline in the middle class over the last 40 years, if you look back from 2000 on, it seems to have stabilized,” said Larry Eichel, project director for the Philadelphia Research Initiative at Pew Charitable Trusts.