US employers who hire undocumented immigrants are already getting a big discount on labor. Some savvy recruiters and supervisors are extracting even more value out of those employees by selling their social security contributions.

While it’s common knowledge that immigrants in the country illegally use fake papers to get a job, in some cases it’s supervisors at the employer who provide the fraudulent documents, a study published earlier this month in the journal Anthropology of Work Review suggests. The practice is so widespread in California farming communities that immigrants subject to it have a name: ghost workers.

Aside from hiding illegal hiring practices, the scheme allows supervisors to, in effect, earn a kickback from the valid holder of the social security number, according to Sarah Horton, the University of Colorado Denver professor who wrote the study. Illegal laborers must use the borrowed identity supervisors give them. But when they get paid, the social security taxes get credited to the real owner of the number. The owner then pays the supervisor for the service of racking up social security credits without having to work.

A worker interviewed by Horton explained how it works: “There are people who give their Social Security cards to the field bosses and say, ‘You know what? Have them work my SSN and I’ll give you so much money. So the field boss puts another [name] down for a worker who doesn’t have a SSN, who doesn’t have papers, and the field boss makes money!”

The ghost status is not exclusive to illegal immigrants, she says. To avoid paying overtime, employers sometimes make legal workers use somebody else’s identity on Sundays, commonly referred to as “the day of the ghost.” Borrowed papers are also used to cover up child labor, she adds.

Although the focus of her research was California, she says there’s evidence that employers in other states and industries also supply their workers with fake papers.

Immigrants face two legal problems. Aside from facing deportation for being in the country illegally, they are exposed to criminal charges and jail time for identity theft. Some states, such as Arizona, have been using identity-theft laws to prosecute undocumented immigrants. In Maricopa County, sheriff Joe Arpaio, self-dubbed “America’s toughest sheriff,” has arrested 700 undocumented immigrants on those grounds.

Horton says prosecutors and judges should carefully consider who the identity thief really is when presented with such cases.

“Very rarely is it actually investigated in court whether the immigrant was the one who provided the documents,” says Horton.