IMMIGRATION is in the headlines again, with President Obama’s decision last week to stop deporting young illegal immigrants who came to the United States as children, and the Supreme Court’s approaching decision on the constitutionality of Arizona’s crackdown on undocumented migrants.

But too much of the public debate has focused on the legality of immigration without considering a more fundamental question: What effects has mass immigration had on American society?

As a result of the 1965 immigration act, which opened the door widely to non-European immigrants, 40 million foreign-born immigrants now live in the United States. They make up 13 percent of the population, the largest such proportion since the 1920s. More than half of these migrants are from Latin America and the Caribbean, although a study released Tuesday by the Pew Research Center found that Asians overtook Hispanics in 2009 as the fastest-growing group of immigrants.

For the May issue of the Annals of the American Academy of Political and Social Science, we commissioned some of the most meticulous research done to date about the effects of immigration on a cross section of American communities — urban, suburban and rural.

The scholars who participated were in remarkable agreement: while new immigrants are poorer than the general population and face considerable hardship, there is no evidence that they have reshaped the social fabric in harmful ways.

America is neither less safe because of immigration nor is it worse off economically. In fact, in the regions where immigrants have settled in the past two decades, crime has gone down, cities have grown, poor urban neighborhoods have been rebuilt, and small towns that were once on life support are springing back.

Scholars can’t say for sure that immigration caused these positive developments, but we know enough to debunk the notion that immigrants worsen social ills.

For example, in rural counties that experienced an influx of immigrants in the 1980s and ’90s, crime rates dropped by more than they did in rural counties that did not see high immigrant growth. Higher immigration was associated with reductions in homicide rates for white, black and Latino victims. In both Hazleton, Pa., which has a recent history of hostility toward immigration, and St. James, Minn., a much more welcoming community, migrants have also bolstered dwindling populations and helped to reverse economic decline.

In large gateway cities, immigration has been associated not only with a decrease in crime but also with economic revitalization and reductions in concentrated poverty. Data from the 2005 American Community Survey showed, for example, that the income of blacks in the New York City borough of Queens surpassed that of whites for the first time, a development driven largely by immigration from the West Indies.

Scholars found that immigrant youths in Los Angeles were involved in less crime and violence than their native-born peers in similar economic circumstances. Research also has shown that an increase in immigration in cities like San Antonio and Miami did not produce an increase in the homicide rate. Furthermore, social scientists found that people in immigrant communities in New York were less cynical about the law than were people in less diverse communities; they were also more likely to indicate that they would cooperate with the police.

If migration has had such beneficial effects, why, then, has there been such a persistent backlash?

Part of the answer surely lies in the social changes — language, political attitudes, religious mores — that immigrants bring, in addition to the effects of the recession. The leveling-off of migration, especially from Mexico, may bring a sense of relief to opponents of these social changes, but if the new research is any guide, the consequences of the slowdown may be the opposite of what the critics intend.

Comprehensive immigration reform — last attempted during the second term of President George W. Bush — should be a priority for whoever wins in November. Mr. Obama’s decision to exempt undocumented children who were brought to the United States by their parents from harsh deportation rules is an overdue, but welcome, first step.

Establishing a clear path to citizenship for undocumented adults, creating a more permissive guest-worker program, reducing unwarranted police stops of immigrants and preserving families rather than separating them through deportation are controversial ideas, but they deserve a hearing.