Like many of the immigrant workers that I represent at the legal-services nonprofit I work for in Los Angeles, Pulido’s legal status in the U.S. was inextricably tied to her employer. This feature of U.S. immigration law gives exploitative employers a powerful tool to control their immigrant workers, whose lack of familiarity with the laws and customs of the United States already render them vulnerable. Aware of this advantage that they hold, some employers believe they can abuse employees with impunity. If workers complain or threaten to seek help, they are told that leaving their employer may very well lead to deportation. For many immigrant workers, who may have borrowed significant amounts of money to come to America, leaving the U.S. early may mean financial ruin at home. U.S. immigration policy, in other words, leaves them no choice but to endure their employers’ abuse.

This power imbalance between immigrant workers and their employers figures prominently in the U.S. immigration system. The State Department has estimated that 14,500 to 17,500 people are trafficked into the United States each year. (That estimate, the most recent reliable one available, is from 2004—data on human trafficking are notoriously difficult to collect.) And evidence suggests a connection between trafficking and certain visa categories. A report by the anti-human-trafficking organization Polaris identified six temporary visas commonly associated with labor exploitation and human trafficking. (Over the 12 months starting in October 2013, Polaris estimated, 500,000 people came into the U.S. on one of these visas.) Notably, all six categories make it difficult, if not impossible, for immigrant workers to leave the employer or agency that sponsored their visa petition. A separate study by the Urban Institute, a think tank, found that over 70 percent of the 122 trafficking victims it analyzed came to the U.S. with legitimate visas, especially with three of the visa categories flagged by Polaris. According to both studies, trafficking victims reported that their employers threatened to report them to immigration authorities to discourage them from leaving or seeking help.

Such studies also make clear that domestic workers like Pulido are not the only targets of such practices. Hotel housekeepers and construction workers on H-2B visas, nurses and teachers on H-1B visas, and others are susceptible as well.

If the U.S. government wanted to end labor trafficking and modern slavery, a good start would be reforming the country’s immigration laws. Although it’d likely be unpalatable to Americans who are opponents of immigration, immigration law would need to be amended so that an immigrant worker’s status in the U.S. is not bound to a particular employer or agency. On top of that, immigration law ideally would ensure that all immigrant workers—not just select visa categories—are eligible to obtain legal permanent residence. As long as immigrant workers are legally bound to their employers, they will remain vulnerable to trafficking and abuse, and will remain reluctant to seek help.