The immigration measures announced Wednesday by President Donald Trump fall in the latter category. The consequences will mostly be more or less the opposite of what he and his supporters imagine.
His promised wall is supposed to stop the flow of unauthorized immigrants and reduce the number of undocumented foreigners living here. But it's not likely to do either.
Stop the flow? Even if you assume smugglers won't find ways to breach the wall or tunnel below it, it will continue. On a typical day, more than a half-million people stream over the border from Mexico with the required documents. Some 40 percent of undocumented foreigners living in the United States came legally and overstayed their visas.
Putting up a wall won't keep out people we knowingly admit -- and it won't help find those who decline to leave. It will merely encourage more people to drive or fly in on a tourist visa rather than swim the Rio Grande.
If past measures to fortify the border shut some unauthorized foreigners out, they also kept millions of others in. Princeton sociologist Douglas Massey notes that in 1986, nearly half of the Mexicans here without permission eventually went back to Mexico, knowing that they could always change their minds.
But when enforcement was stepped up, they learned a lesson: Once you're here, you had better stay. The number choosing permanent residence rose. How's that for a solution?
One complaint about people sneaking over the border comes from ranchers whose lands they cross and befoul. But the migrants are there because of tight enforcement.
In the old days, they sneaked across in border cities. "It used to be that you could literally sit at a bar in Tijuana, Mexico, look across the border into San Diego, wait for the Border Patrol to drive in the other direction and make a run for it,'' Steve Atkiss, a former chief of staff of Customs and Border Protection, told The Washington Post in 2015.
When security improved at major ports of entry, it pushed illicit migrants into areas with more rattlesnakes than people, which are harder to police.
That phenomenon would persist if the 653 miles of fencing now in place were extended, because filling in the other 1,300 miles would take years. In the meantime, landowners who have rarely, if ever, seen migrants before may play host to a steady procession of furtive skulkers.
Trump also wants to punish sanctuary cities -- whose policies bar police from making arrests for immigration violations or asking people about their immigration status. He decries these accommodations as a threat to public safety. In fact, they enhance it -- by encouraging the 11 million undocumented foreigners living here to cooperate with cops.
The Major County Sheriffs' Association warned in 2015 that cutting off funds to these cities would "prevent law enforcement from effectively protecting their communities and themselves." Chuck Wexler, executive director of the Police Executive Research Forum, told USA Today, "If people are afraid to come to the police, that domestic violence incident today will be a homicide tomorrow, and that's in no one's interest."
Most of what Trump says on this topic is lacking evidence. During his campaign, he accused undocumented immigrants of "taking our jobs." But if he expects tougher enforcement to create jobs and raise wages for American workers, he's in for a crushing disappointment.
A report last year commissioned by the National Academy of Sciences detected "little evidence that immigration significantly affects the overall employment levels of native-born workers" and found it has only a minimal impact on wages.
A study by economists Gihoon Hong of Indiana University South Bend and John McLaren of the University of Virginia concluded that by raising demand for goods and services in the communities where they take up residence, new immigrants serve to create 1.2 new jobs each and boost the pay of Americans. Cutting down on illegal immigration wouldn't save jobs, on net; it would eliminate them.
Right now, Trump is happy to brag about his crackdown. Will he still own it when it backfires?