There are many good reasons to support comprehensive immigration reform. There are some 11 million undocumented workers in the United States who exist in a legal and economic limbo: Their condition is worse than that of indentured servants. Proposals for emphasizing immigration of skilled workers are also welcome. And an influx of young workers is important to sustaining an aging citizenry’s social-insurance programs.

But large-scale immigration, as has occurred in the United States since 1965—and will probably expand under the new legislation—is not without its pitfalls, especially at a time of high unemployment. Historically, it has depressed the wages of native-born or naturalized workers. The Senate proposal is expected to include measures that will mitigate this effect, but only partially. It will remain an abiding problem.

Immigration depresses wages by increasing the supply of workers relative to demand. Harvard Kennedy School economist George T. Borjas, working alone or with Lawrence Katz and Richard Freeman, has produced a succession of studies over the last two decades showing that immigration hurts the wages of native-born workers. One study, published in the Quarterly Journal of Economics in November 2003, found that between 1980 and 2000 immigrants increased the labor supply of working men by 11 percent and reduced the average wage of native workers by 3.2 percent. Wages fell by 8.9 percent for workers who had not finished high school, 2.9 percent for high school graduates, and 4.9 percent for college graduates.

Business proponents of increased immigration argue that low-wage immigrant workers have taken jobs that native workers don’t want in construction, agriculture, hotels, and meatpacking plants. “Americans Won’t Do Dirty Jobs,” proclaimed a story about immigrant labor in Bloomberg BusinessWeek. That’s sometimes the case, but it’s also often because employers in those industries used immigrant labor to bid down wages so low that native workers no longer want those jobs. Until the 1980s, for instance, workers in meatpacking plants were unionized and made middle-class wages. Over the next two decades, the unions were broken and immigrants—some of whom lacked papers—replaced the native-born workers at one-third the wages.

Conversely, there are “dirty jobs” that native-born Americans have been happy to take because they pay livable wages. Many native-born Americans work as garbage collectors, who are often unionized and earn an average annual salary of $43,000. Oil-rig workers make a decent living, and so do coal miners. If businesses have their way, these jobs might also become anathema to Americans, but it won’t be because Americans are loath to get their hands dirty. It will because the jobs pay too little to support an average family.