Of the 13,000 or so people housed in New York City's jails on a given day, roughly 10,000 aren't serving a sentence. They haven't been convicted of anything. They're simply awaiting trial. And most are awaiting trial behind bars because they can't afford not to be. They're there because they can't pay bail.

Dominique Strauss-Kahn, the former head of the International Monetary Fund who is accused of sexual assault, is not among them. He was granted release late last week after posting $1 million in cash.

High-profile criminal cases often involve some public outrage over a bail that seems too low. But bail isn't meant to be a punishment, because the defendant is presumed innocent. Bail is supposed to be set at whatever level will ensure the suspect's return for his or her next court date.

At arraignment, the city's court system evaluates a person's ties to the community—whether he or she is from New York, has family here, holds a job or is in school—which reduce the chance that the defendant will skip court. On the other hand, the court also looks at the severity of the charges and the person's criminal record, since these could increase the incentive to flee.

A very few defendants are deemed too high a flight risk or too dangerous to the community to even entertain the idea of bail, so they are "remanded"—sent to Rikers and held there until trial or at least until a judge changes her mind. Most are released on their own recognizance, without any bail.

But a third of defendants in New York City are given bail. And in 2009 (the last year for which the stellar New York City Criminal Justice Agency has published data), nearly half—some 19,000 people—never made bail.

This includes 41 percent of people with bails of $500 or less.

As City Limits reported in 2007—and as Human Rights Watch recently validated in a major report—bail does become a form of punishment in New York City because even very low bails are not attainable for very poor people. And commercial bailbondsmen rarely handle bails that low because the fee on such a low bail is not worth the overhead.

The inability to afford bail means an arrest, even a mistaken one, can have a damaging impact on a person's life. Work and school are missed, benefits appointments go unattended, child care becomes an issue.

In cases where a low bail can't be met, it means a fairly minor charge is what's keeping a person locked up.

This can lead to perverse outcomes. People will plead guilty to a charge they might otherwise have contested because they will actually do less time serving a sentence than they would waiting for the next court date.

Say you're pinched for jumping a turnstile and arraigned a day later. If you plead guilty and are sentenced to five days in jail, given credit for time served and released early as all inmates are, you can be out in a couple days. If you didn't do it and refuse to say you did but can't afford bail, you might not see a judge for another five days. The math is simple: You take the deal.

The problem is a guilty plea, even to a low-level misdemeanor or violation, can affect employment, housing, education and other activities down the road.

People do skip court, so bail is an attempt to solve that problem. But it's a problematic solution. For Strauss-Kahn, bail was an inconvenience. For low-income defendants, it's real trouble. "Bail is a form of preventive detention for poor people," Bob Gangi, then the head of the Correctional Association of New York State, told me in 2007. "Whatever the theoretical justification for bail, that's what it really is."