Bail is meant to make sure defendants show up for trial. It has ancient roots in English common law, which relied on sworn promises and on pledges of land or property from the defendants or their relatives to make sure they did not flee.

America’s open frontier and entrepreneurial spirit injected an innovation into the process: by the early 1800s, private businesses were allowed to post bail in exchange for payments from the defendants and the promise that they would hunt down the defendants and return them if they failed to appear.

Commercial bail bond companies dominate the pretrial release systems of only two nations, the United States and the Philippines.

The flaw in the system most often cited by critics is that defendants who have not been convicted of a crime and who turn up for every court appearance are nonetheless required to pay a nonrefundable fee to a private business, assuming they do not want to remain in jail.

“Life is not fair, and I probably would feel the same way if I were a defendant,” said Bill Kreins, a spokesman for the Professional Bail Agents of the United States, a trade group. “But the system is the best in world.”

The system costs taxpayers nothing, Mr. Kreins said, and it is exceptionally effective at ensuring that defendants appear for court.

Mr. Spath’s experience confirms that.

If Mr. Spath considers a potential client a good risk, he will post bail in exchange for a nonrefundable 10 percent fee. In a 35-month period ending in November, his records show, Mr. Spath posted about $37 million in bonds  7,934 of them. That would suggest revenues of about $1.3 million a year, given his fee.