Ramiro Martinez Jr., a professor of criminal justice at Florida International University, has sifted through homicide records in border cities like San Diego and El Paso, both heavily populated by Mexican immigrants, both places where violent crime has fallen significantly in recent years. “Almost without exception,” he told me, “I’ve discovered that the homicide rate for Hispanics was lower than for other groups, even though their poverty rate was very high, if not the highest, in these metropolitan areas.” He found the same thing in the Haitian neighborhoods of Miami. In his book “New York Murder Mystery,” the criminologist Andrew Karmen examined the trend in New York City and likewise found that the “disproportionately youthful, male and poor immigrants” who arrived during the 1980s and 1990s “were surprisingly law-abiding” and that their settlement into once-decaying neighborhoods helped “put a brake on spiraling crime rates.”

The leaders in this area have been Harvard's Robert Sampson and University of Chicago's Stephen Raudenbush, who conducted a large study of over 3,500 residents of Chicago:

“In particular,” Sampson and Raudenbush found, “the proportion of blacks and the proportion of Latinos in a neighborhood were related positively and significantly to perceived disorder.” Once you adjusted for the ethnic, racial and class composition of a community, “much of the variation in levels of disorder that appeared to be explained by what residents saw was spurious.”

What does this mean? Well, if Sampson and others are correct, then recent immigrants – legal or illegal – might actually be reducing crime rates in American cities. This raises important additional questions: Why do we assume illegal immigrants cause crime? Why do they actually decrease crime? How well does America integrate the children of immigrants? And as importantly, why is it that non-immigrant communities are more violent? As the immigration debate continues, these are important questions to answer.