When Julian Fisher took his camera to Lowell, Mass., to document the struggles of middle-class Americans, he stopped to photograph a man crossing a bridge. Behind the bridge, visible between the metal girders, stood the city’s historic textile mills, now luxury condos.

In the day, this man could have worked in the mills, said Fisher, a neurology instructor at Harvard Medical School. Today, he now probably could not afford to live in them.

That photo is one that Fisher uses to tell the story of “Trapped in the Middle: The Effect of Income Inequality on the Middle Class in America,” an exhibit on display at the Japan Friends of Harvard Concourse Gallery in the GCIS Building until April 26.

Fisher’s 18-month process to seek out middle-class families and individuals willing to tell their stories began in late fall 2014. He stayed local, mainly exploring the towns surrounding Brookline, where he lives.

“You don’t have to look very far to find the middle class; they’re here,” he said.

Inspired by Walker Evans’ photographs of sharecroppers in the 1930s, the issues raised by the Occupy Wall Street movement in 2011, and the work of economist Joseph Stiglitz, Fisher defined the categories that he wanted to examine. The middle class, he said, is defined not only by how much money a household makes, but also by “practical goals,” including housing, transportation, health care, and education — all of which have gone up in cost, while the buying power of the American dollar declined.

“I wanted to find people who had stories to tell about how they were treading water,” Fisher said. “They have to work harder or give up something to stay where they are.”

Fisher found his subjects through a series of connections: tradespeople with whom he did business, neighbors, and mutual acquaintances. People opened their homes and their lives to him for an hour or two, and he photographed them helping their children with their homework, walking their dog, going to work.