The U.S. unemployment rate fell to 5.5 percent last month. Good news, but millions of able-bodied, working-age men are still jobless or out of the labor force completely. Why?

Economists have been wrestling with this question for years now. They have many ideas: unemployment benefits are too generous, lazy people are abusing the disability systems, young people don't have the right skills and robots are taking our jobs.

These are good theories. I'm sure all are correct to some extent. Yet the data still show a stubborn remnant who should be working —– who would be working if this recovery were like past ones — but are not.

I have another theory for the list. I think we've unknowingly created a new category that doesn't show up in the official data. I call them "Workers Forbidden to Work." Millions fall in this category. They're able to work, they want to work, but political and economic forces prevent them from working.

The most common way people enter this category is by having a criminal record. According to a recent University of Minnesota study cited in The New York Times, approximately 10 percent of nonincarcerated adult males have a felony record. In 1980, it was only 4 percent.

Why do we have so many more ex-cons now? Mainly because of the war on drugs. Between zero-tolerance laws and mandatory minimum sentencing, we've multiplied the number of people caught up in the judicial system, and given them harsher punishment — rightly so, in most cases. Criminals deserve their punishment.

Not so long ago, however, convicts could serve their sentences and then make a fresh start. This is no longer possible. The reason it is no longer possible is that criminal record checks are now quick, easy and inexpensive.

Large employers today run background checks on all applicants. In a weak economy, where managers can have dozens of applicants for every opening, it is an easy way to narrow the list.

Moreover, ambulance-chasing lawyers have made large companies so nervous about liability that many simply rule out anyone with any kind of criminal record. Likewise, overzealous legislatures have given us licensing laws that prevent those with a past conviction from obtaining necessary credentials.

Nobody coordinated all these trends and decided to eject millions of people (mostly men) from the workforce, but the practical consequence is this new "Workers Forbidden to Work" category.

Obviously, we don't want sex offenders working around children or embezzlers inside banks. Some criminals do go right back to a life of crime when they leave prison, but not all. Many people genuinely want to straighten out. Instead of helping them do so, we've thrown up more barriers.

Employers who indiscriminately rule out blemished applicants are within their rights, but they are also hurting their own future. A company that prefers idiots with clean records to talented people who made a mistake will eventually reap what it sows.

The rest of us reap it as well. Workers Forbidden to Work do not simply disappear. They become a burden on their families and the entire economy.

Here are the choices: we can send these people back to prison (expensive), let them become homeless (cruel), put them on welfare (both expensive and cruel) or give them a chance to work.

The last option makes the most sense for everyone.