CHICAGO — HOW can we reduce the enormous populations of our country’s local jails?

Last month, Mayor Bill de Blasio of New York unveiled a plan to decrease the population of the Rikers Island jail complex by reducing the backlog of cases in state courts. About 85 percent of those at Rikers haven’t been convicted of any offense; they’re just awaiting trial, sometimes for as long as hundreds of days.

Mayor de Blasio’s plan is a positive step. Yet it ignores a deeper question: Why are so many people — particularly poor people of color — in jail awaiting trial in the first place?

Usually, it is because they cannot afford bail. According to a 2011 report by the city’s Independent Budget Office, 79 percent of pretrial detainees were sent to Rikers because they couldn’t post bail right away.

This is a national problem. Across the United States, most of the people incarcerated in local jails have not been convicted of a crime but are awaiting trial. And most of those are waiting in jail not because of any specific risk they have been deemed to pose, but because they can’t pay their bail.