The guy with his name on the big building downtown decided to run for president after years of flirting with the possibility, bringing a celebrity businessman and reality-show star's penchant for bluster in a contest where discretion is at a premium. And he immediately landed in it:

"When Mexico sends its people, they're not sending their best," he said. "They're sending people that have lots of problems, and they're bringing those problems with us. They're bringing drugs. They're bringing crime. They're rapists."

This is deeply flawed, and should be worrisome in a region that's increasingly dependent on immigration. (Trump is scheduled to speak about immigration in Phoenix on Saturday; given that he'll be joined by notorious Maricopa County sheriff Joe Arpaio, the odds are not good that he'll change his tune much.) Robert Sampson, the former University of Chicago sociologist—now at Harvard—who wrote one of the most in-depth studies of the city, Great American City: Chicago and the Enduring Neighborhood Effect, just published a reminder of how wrong it is.

First, "when Mexico sends its people, they're not sending their best." I guess it depends on your definition of best, but:

Immigration may affect crime rates, first of all, because of who chooses to immigrate—a factor that social scientists refer to as “selection bias.” Although there are exceptions, it is widely recognized that most immigrants, Mexicans in particular, selectively migrate to the United States based on characteristics that predispose them to low crime, such as motivation to work and ambition. First-generation immigrants (those born outside the United States) may also be more law-abiding because of their interest in not being deported.

An influx of people with a motivation to work—not least because many send money back home—tends to lead to dense neighborhoods with considerable economic activity. As the Metropolitan Policy Council's Marisa Novara pointed out to me, a neighborhood that is poor but dense can support businesses, such as Harlem in New York and Little Village in Chicago. Novara has written about how ports-of-call for immigrants in Chicago are moving west from Pilsen and Little Village to Brighton Park, Gage Park, Clearing, and other far-southwest side areas, which are the only neighborhoods that have shown population growth outside of downtown. And, Novara argues, that's crucial for Chicago; it simply needs more people to thrive.

And Chicago doesn't even compare to Detroit's dire struggles and population-loss vortex, which is why two authors, Stanford political scientist David Laitin and former NYC Housing Development Corporation president Marc Jahr, called for America to resettle thousands of Syrian refugees in the city, building on the trend of immigrants being overrepresented, compared to their part of the population, in the city's entrepreneurial class.

Michigan's governor, Rick Snyder, wants to bring in 50,000 high-skilled immigrants to the city. But more broad immigration patterns bring benefits. We might not be getting the "best" immigrants—then again, there's nothing on the Statue of Liberty reading "give us your energetic, your upper-middle-class, your academics yearning to do research"—but maybe they're better than us.

Notably, we found a significantly lower rate of violence among Mexican-Americans compared to blacks and whites. A major reason is that more than a quarter of those of Mexican descent were born abroad and more than half lived in neighborhoods where the majority of residents were also Mexican. In particular, first-generation immigrants (those born outside the United States) were 45 percent less likely to commit violence than third-generation Americans, adjusting for individual, family, and neighborhood background.

Also misleading: "They're sending people that have lots of problems, and they're bringing those problems with us." Here's Sampson, writing with co-author Corina Graif:

Building on Chicago-School notions of social control at the family and community level, segmented assimilation theory (Portes and Zhou 1993; Zhou 1997) suggests that when children of immigrants learn American customs and English at a faster pace than their parents' (dissonant acculturation), assimilation into a highly disadvantaged neighborhood may lead to adoption of inner-city oppositional subcultures (Portes and Rumbaut 2001) and indirectly may lead to higher odds of involvement in gangs or crime. Morenoff and Astor's (2006) analysis of self-reported offending by Chicago youth finds that linguistic acculturation predicts higher involvement in crime. Moreover, disadvantaged neighborhoods are associated with higher levels of violence by the third generation youth. These patterns suggest that neighborhoods with high concentrations of first generation immigrants may be more successful in overcoming structural conditions that boost crime than neighborhoods dominated by equally disadvantaged native-born co-ethnics.

Sampson isn't alone in finding this. In short, violence is learned here more than it is imported.

There's a problem there, but also promise. Rather than stigmatizing immigrants, as has been the case for most of modern American history, the country could seek to lock in the social and economic gains it gets from immigration—and perhaps learn more about why the enemy, far from being outside or borders, is so often us.