The year the Civil War ended, the U.S. amended the Constitution to prohibit slavery and involuntary servitude. But it purposefully left in one big loophole for people convicted of crimes.

The 13th Amendment, ratified in 1865, says: “Neither slavery nor involuntary servitude, except as a punishment for crime whereof the party shall have been duly convicted, shall exist within the United States, or any place subject to their jurisdiction.” Scholars, activists and prisoners have linked that exception clause to the rise of a prison system that incarcerates Black people at more than five times the rate of white people, and profits off of their unpaid or underpaid labor.

“What we see after the passage of the 13th Amendment is a couple of different things converging,” says Andrea Armstrong, a law professor at Loyola University in New Orleans. “First, the 13th Amendment text allows for involuntary servitude where convicted of a crime.” At the same time, “black codes” in the south created “new types of offenses, especially attitudinal offenses—not showing proper respect, those types of things.”

After the Civil War, new offenses like “malicious mischief” were vague, and could be a felony or misdemeanor depending on the supposed severity of behavior. These laws sent more Black people to prison than ever before, and by the late 19th century the country experienced its first “prison boom,” legal scholar Michelle Alexander writes in her book The New Jim Crow.

“After a brief period of progress during Reconstruction, African Americans found themselves, once again, virtually defenseless,” Alexander writes. “The criminal justice system was strategically employed to force African Americans back into a system of extreme repression and control, a tactic that would continue to prove successful for generations to come.”

States put prisoners to work through a practice called “convict-leasing,” whereby white planters and industrialists “leased” prisoners to work for them. States and private businesses made money doing this, but prisoners didn’t. This meant many Black prisoners found themselves living and working on plantations against their will and for no pay decades after the Civil War.

Was this slavery by another name? Armstrong argues that the 13th Amendment makes an exception for “involuntary servitude,” not “slavery,” and that there are important historical and legal distinctions between the two. However, she says no court has formally dealt with this distinction, and many courts have used the two terms interchangeably. In 1871, the Virginia Supreme Court ruled that a convicted person was “a slave of the State.”

Like chattel slavery before it, convict-leasing was brutal and inhumane. Across the country, “tens of thousands of people, overwhelmingly Black, were leased by the state to plantation owners, privately owned railroad yards, coal mines and road-building chain gangs and made to work under the whip from dawn till dusk—often as punishment for petty crimes such as vagrancy or theft,” reports The Washington Post.

Many prisoners died in these conditions. In July 2018, researcher Reginald Moore announced he’d found the remains of 95 Black prisoners who’d died working in Sugar Land, Texas in the early 20th century. Experts estimate their ages ranged from 14 to 70, meaning some would’ve been born into pre-Civil War slavery, freed, incarcerated and then forced into unpaid labor again. More than 3,500 prisoners died in Texas between 1866 and 1912, the year Texas outlawed convict-leasing because the death toll was so high.

States also benefited and profited off of prison labor by forcing chain gangs to build roads and creating prison farms to grow crops like sugar and snap peas. Today, states and private companies still rely on prisoners performing free or extremely low-paid labor for them. For example, California saves up to $100 million a year, according to state corrections spokesman Bill Sessa, by recruiting incarcerated people as volunteer firefighters.

“[States] would not be able to incarcerate as many people as they do without this, in effect, subsidy of the cost,” Armstrong says. “So it masks the true nature or the true cost of incarceration.”

Decades of prison and civil rights activism have sought to improve conditions and pay for incarcerated workers. In 1971, inmates at New York’s Attica Correctional Facility took control of the prison and issued a list of demands, including the right to join labor unions and earn a minimum wage. More recently, in the summer of 2018, prisoner laborers across the United States went on strike to protest what they called “modern-day slavery.”

In 2020, Congressional Democrats introduced a joint resolution to remove the "punishment" clause from the 13th Amendment. The resolution would need to be passed by a two-thirds majority in both houses of Congress. Then, three-quarters of states would need to approve the change for it to become federal law.