The shift to tougher penal policies three decades ago was originally credited with helping people in poor neighborhoods by reducing crime. But now that America’s incarceration rate has risen to be the world’s highest, many social scientists find the social benefits to be far outweighed by the costs to those communities.

“Prison has become the new poverty trap,” said Bruce Western, a Harvard sociologist. “It has become a routine event for poor African-American men and their families, creating an enduring disadvantage at the very bottom of American society.”

Among African-Americans who have grown up during the era of mass incarceration, one in four has had a parent locked up at some point during childhood. For black men in their 20s and early 30s without a high school diploma, the incarceration rate is so high — nearly 40 percent nationwide — that they’re more likely to be behind bars than to have a job.

No one denies that some people belong in prison. Mr. Harris, now 47, and his wife, 45, agree that in his early 20s he deserved to be there. But they don’t see what good was accomplished by keeping him there for two decades, and neither do most of the researchers who have been analyzing the prison boom.

The number of Americans in state and federal prisons hasquintupled since 1980, and a major reason is that prisoners serve longer terms than before. They remain inmates into middle age and old age, well beyond the peak age for crime, which is in the late teenage years — just when Mr. Harris first got into trouble.