U.S. presidents as far back as Harry Truman denounced entry policies based on nationality, calling them discriminatory and un-American.

President Trump’s reported suggestion that the United States needs fewer immigrants from “shithole countries” and more from those like Norway revives an argument made vigorously a century ago—though in less profane terms—only to be discredited in the decades that followed.

Social engineering through immigration policy isn’t simple—and such efforts often produce dramatic, unintended consequences.

Trump adviser Stephen Miller says the new White House plan to amend U.S. immigration law, introduced by Senators Tom Cotton and David Perdue, is “the largest proposed reform to our immigration policy in half a century.”

The White House wants to revisit the 1965 Immigration Act, which opened America’s doors wide to immigrants of color and produced the most sweeping demographic transformation of the country in its history.

Critics of the proposal see it as a thinly veiled effort to constrict the flow of nonwhite groups to the United States. The alt-right leader Richard Spencer, welcoming such a development, told HuffPost the bill “sounds awesome.”

The bill’s proposed changes are certainly significant, but their consequences may not be easily predicted. The key lesson of the 1965 reforms is that social engineering through the adjustment of immigration policy is no simple matter—and almost any such effort will produce dramatic, unintended consequences.