Reformers on the left and right who worry about mass incarceration have argued lately for kicking private bail bonds out of America’s criminal-justice system. New Jersey virtually eliminated cash bail earlier this year, instructing judges to decide whether to release defendants based on their risk factors. Arizona and Kentucky are doing the same. After a bail bill failed to pass California’s Legislature this term, Gov. Jerry Brown and state Supreme Court Chief Justice Tani Cantil-Sakauye joined hands to advocate reform. On Capitol Hill, Sens. Rand Paul (R., Ky.) and Kamala Harris (D., Calif.) have introduced a bill that would provide federal grants to nudge states in this direction.

The argument is straightforward: A defendant who can’t afford bail languishes in jail, maybe losing his job, simply because he is poor. But anyone who hopes bail reform will lower incarceration rates, making life better for defendants and their families, might want to hold off celebrating. Data from Maryland suggests that remaking the bail system in haste, without careful planning, can actually drive up incarceration rates.

The 24-hour bail-bond business is a peculiarly American institution. If you land in jail, these companies will pay the bond to get you out. The price is a steep nonrefundable commission, typically 10% of the total bond. If you skip town afterward, you’ll be sorry when they send bounty hunters after you.

Bondsmen say they provide a socially useful service. They apply experience and local knowledge to assess the defendant’s flight risk, while working with families to get loved ones out of jail quickly and with minimal hassle. When defendants flee, bounty hunters typically do better than sheriffs at tracking them down.

From a judge’s perspective, the bail system provides a way to handle the wide range of middling cases. Defendants with low flight risk and minor offenses can be released on their own recognizance or a small personal bond. Those with high flight risk and serious charges can be held without bond. What about the people in between? The bail system provides a way to let them out of jail while applying pressure to show up in court.

Squeeze that middle option, and judges will reassign cases up or down—but there can be a bias toward up. If a judge releases a defendant who goes on to commit an atrocious crime, he faces a potentially career-ending furor. The incentive is to err on the side of lockup.

That might explain what has happened in Maryland. Last fall the state’s attorney general, Brian Frosh, issued guidance that suddenly declared past bail methods unlawful, prodding the court system into an unplanned experiment. Judges may not set financial requirements if there is a reason to believe the defendant cannot pay, and unless they hold a suspect without bail, they must impose the “least onerous” conditions.

Now the results are coming in, and they can’t be what Mr. Frosh had in mind. An early report in March by Kelsi Loos in the Frederick News-Post found that since October the share of Maryland defendants held without bail had increased from 10% to 14%. The Washington Post later reported that from September 2016 to May the figure had jumped from 7% to 15%.

Meanwhile, fewer released defendants are showing up for trial. The Post, confirming anecdotal reports, writes that the “failure to appear” rate in January was 14.5%, up five points from October. Failing to show up for court sets up a defendant for more-severe consequences down the road, which can include being held without bail.

This result has vindicated politicians who opposed the bail changes. Democratic state delegate C.T. Wilson, a criminal defense attorney and member of the Legislative Black Caucus, told the Post that the intent of reform “was to not have as many African-American males in jail until their trial date.” He continued: “What has been done has had a more detrimental impact on African-Americans in the system.”

If bail is taken away, judges need other tools to do the same job. Decades ago, when Congress steered the federal criminal-justice system away from bail bonds, lawmakers provided practical replacements, including systematic help in assessing a defendant’s risk of flight or re-offense, options for pretrial supervision, and methods of home and electronic detention. Several states have done the same. New Jersey now uses a mathematical algorithm to assess a person’s risk of fleeing or committing another crime. But the Maryland legislature, deeply split over Mr. Frosh’s destabilizing changes, has failed to set up such alternatives.

Maryland’s example doesn’t refute the idea of bail reform. But it does suggest state leaders should work to build consensus for comprehensive changes, instead of charging ahead with moralizing experiments.