Harvard political scientist Robert Putnam, author of Bowling Alone, is very nervous about releasing his new research, and understandably so. His five-year study shows that immigration and ethnic diversity have a devastating short- and medium-term influence on the social capital, fabric of associations, trust, and neighborliness that create and sustain communities. He fears that his work on the surprisingly negative effects of diversity will become part of the immigration debate, even though he finds that in the long run, people do forge new communities and new ties.

Putnam’s study reveals that immigration and diversity not only reduce social capital between ethnic groups, but also within the groups themselves. Trust, even for members of one’s own race, is lower, altruism and community cooperation rarer, friendships fewer. The problem isn’t ethnic conflict or troubled racial relations, but withdrawal and isolation. Putnam writes: “In colloquial language, people living in ethnically diverse settings appear to ‘hunker down’—that is, to pull in like a turtle.”

In the 41 sites Putnam studied in the U.S., he found that the more diverse the neighborhood, the less residents trust neighbors. This proved true in communities large and small, from big cities like Los Angeles, Chicago, Houston, and Boston to tiny Yakima, Washington, rural South Dakota, and the mountains of West Virginia. In diverse San Francisco and Los Angeles, about 30 percent of people say that they trust neighbors a lot. In ethnically homogeneous communities in the Dakotas, the figure is 70 percent to 80 percent.

Diversity does not produce “bad race relations,” Putnam says. Rather, people in diverse communities tend “to withdraw even from close friends, to expect the worst from their community and its leaders, to volunteer less, give less to charity and work on community projects less often, to register to vote less, to agitate for social reform more, but have less faith that they can actually make a difference, and to huddle unhappily in front of the television.” Putnam adds a crushing footnote: his findings “may underestimate the real effect of diversity on social withdrawal.”

Neither age nor disparities of wealth explain this result. “Americans raised in the 1970s,” he writes, “seem fully as unnerved by diversity as those raised in the 1920s.” And the “hunkering down” occurred no matter whether the communities were relatively egalitarian or showed great differences in personal income. Even when communities are equally poor or rich, equally safe or crime-ridden, diversity correlates with less trust of neighbors, lower confidence in local politicians and news media, less charitable giving and volunteering, fewer close friends, and less happiness.

Putnam has long been aware that his findings could have a big effect on the immigration debate. Last October, he told the Financial Times that “he had delayed publishing his research until he could develop proposals to compensate for the negative effects of diversity.” He said it “would have been irresponsible to publish without that,” a quote that should raise eyebrows. Academics aren’t supposed to withhold negative data until they can suggest antidotes to their findings.

Nor has Putnam made details of his study available for examination by peers and the public. So far, he has published only an initial summary of his findings, from a speech he gave after winning an award in Sweden, in the June issue of Scandinavian Political Studies. His office said Putnam is in Britain, working on a religion project at the University of Manchester, and is currently too busy to grant an interview.

Putnam’s study does make two positive points: in the long run, increased immigration and diversity are inevitable and desirable, and successful immigrant societies “dampen the negative effects of diversity” by constructing new identities. Social psychologists have long favored the optimistic hypothesis that contact between different ethnic and racial groups increases tolerance and social solidarity. For instance, white soldiers assigned to units with black soldiers after World War II were more relaxed about desegregation of the army than were soldiers in all-white units. But Putnam acknowledges that most empirical studies do not support the “contact hypothesis.” In general, they find that the more people are brought into contact with those of another race or ethnicity, the more they stick to their own, and the less they trust others. Putnam writes: “Across local areas in the United States, Australia, Sweden Canada and Britain, greater ethnic diversity is associated with lower social trust and, at least in some cases, lower investment in public goods.”

Though Putnam is wary of what right-wing politicians might do with his findings, the data might give pause to those on the left, and in the center as well. If he’s right, heavy immigration will inflict social deterioration for decades to come, harming immigrants as well as the native-born. Putnam is hopeful that eventually America will forge a new solidarity based on a “new, broader sense of we.” The problem is how to do that in an era of multiculturalism and disdain for assimilation.