The study comes at a time when the future of the American melting pot is the focus of intense political debate, from immigration to race-based admissions to schools, and it poses challenges to advocates on all sides of the issues. The study is already being cited by some conservatives as proof of the harm large-scale immigration causes to the nation's social fabric. But with demographic trends already pushing the nation inexorably toward greater diversity, the real question may yet lie ahead: how to handle the unsettling social changes that Putnam's research predicts.

"We can't ignore the findings," says Ali Noorani, executive director of the Massachusetts Immigrant and Refugee Advocacy Coalition. "The big question we have to ask ourselves is, what do we do about it; what are the next steps?"

The study is part of a fascinating new portrait of diversity emerging from recent scholarship. Diversity, it shows, makes us uncomfortable -- but discomfort, it turns out, isn't always a bad thing. Unease with differences helps explain why teams of engineers from different cultures may be ideally suited to solve a vexing problem. Culture clashes can produce a dynamic give-and-take, generating a solution that may have eluded a group of people with more similar backgrounds and approaches. At the same time, though, Putnam's work adds to a growing body of research indicating that more diverse populations seem to extend themselves less on behalf of collective needs and goals.

His findings on the downsides of diversity have also posed a challenge for Putnam, a liberal academic whose own values put him squarely in the pro-diversity camp. Suddenly finding himself the bearer of bad news, Putnam has struggled with how to present his work. He gathered the initial raw data in 2000 and issued a press release the following year outlining the results. He then spent several years testing other possible explanations.

When he finally published a detailed scholarly analysis in June in the journal Scandinavian Political Studies, he faced criticism for straying from data into advocacy. His paper argues strongly that the negative effects of diversity can be remedied, and says history suggests that ethnic diversity may eventually fade as a sharp line of social demarcation.

"Having aligned himself with the central planners intent on sustaining such social engineering, Putnam concludes the facts with a stern pep talk," wrote conservative commentator Ilana Mercer, in a recent Orange County Register op-ed titled "Greater diversity equals more misery."

Putnam has long staked out ground as both a researcher and a civic player, someone willing to describe social problems and then have a hand in addressing them. He says social science should be "simultaneously rigorous and relevant," meeting high research standards while also "speaking to concerns of our fellow citizens." But on a topic as charged as ethnicity and race, Putnam worries that many people hear only what they want to.

"It would be unfortunate if a politically correct progressivism were to deny the reality of the challenge to social solidarity posed by diversity," he writes in the new report. "It would be equally unfortunate if an ahistorical and ethnocentric conservatism were to deny that addressing that challenge is both feasible and desirable."

Putnam is the nation's premier guru of civic engagement. After studying civic life in Italy in the 1970s and 1980s, Putnam turned his attention to the US, publishing an influential journal article on civic engagement in 1995 that he expanded five years later into the best-selling "Bowling Alone." The book sounded a national wake-up call on what Putnam called a sharp drop in civic connections among Americans. It won him audiences with presidents Bill Clinton and George W. Bush, and made him one of the country's best known social scientists.

Putnam claims the US has experienced a pronounced decline in "social capital," a term he helped popularize. Social capital refers to the social networks -- whether friendships or religious congregations or neighborhood associations -- that he says are key indicators of civic well-being. When social capital is high, says Putnam, communities are better places to live. Neighborhoods are safer; people are healthier; and more citizens vote.

The results of his new study come from a survey Putnam directed among residents in 41 US communities, including Boston. Residents were sorted into the four principal categories used by the US Census: black, white, Hispanic, and Asian. They were asked how much they trusted their neighbors and those of each racial category, and questioned about a long list of civic attitudes and practices, including their views on local government, their involvement in community projects, and their friendships. What emerged in more diverse communities was a bleak picture of civic desolation, affecting everything from political engagement to the state of social ties.

Putnam knew he had provocative findings on his hands. He worried about coming under some of the same liberal attacks that greeted Daniel Patrick Moynihan's landmark 1965 report on the social costs associated with the breakdown of the black family. There is always the risk of being pilloried as the bearer of "an inconvenient truth," says Putnam.

After releasing the initial results in 2001, Putnam says he spent time "kicking the tires really hard" to be sure the study had it right. Putnam realized, for instance, that more diverse communities tended to be larger, have greater income ranges, higher crime rates, and more mobility among their residents -- all factors that could depress social capital independent of any impact ethnic diversity might have.

"People would say, 'I bet you forgot about X,'" Putnam says of the string of suggestions from colleagues. "There were 20 or 30 X's."

But even after statistically taking them all into account, the connection remained strong: Higher diversity meant lower social capital. In his findings, Putnam writes that those in more diverse communities tend to "distrust their neighbors, regardless of the color of their skin, 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."

"People living in ethnically diverse settings appear to 'hunker down' -- that is, to pull in like a turtle," Putnam writes.

In documenting that hunkering down, Putnam challenged the two dominant schools of thought on ethnic and racial diversity, the "contact" theory and the "conflict" theory. Under the contact theory, more time spent with those of other backgrounds leads to greater understanding and harmony between groups. Under the conflict theory, that proximity produces tension and discord.

Putnam's findings reject both theories. In more diverse communities, he says, there were neither great bonds formed across group lines nor heightened ethnic tensions, but a general civic malaise. And in perhaps the most surprising result of all, levels of trust were not only lower between groups in more diverse settings, but even among members of the same group.

"Diversity, at least in the short run," he writes, "seems to bring out the turtle in all of us."

The overall findings may be jarring during a time when it's become commonplace to sing the praises of diverse communities, but researchers in the field say they shouldn't be.

"It's an important addition to a growing body of evidence on the challenges created by diversity," says Harvard economist Edward Glaeser.

In a recent study, Glaeser and colleague Alberto Alesina demonstrated that roughly half the difference in social welfare spending between the US and Europe -- Europe spends far more -- can be attributed to the greater ethnic diversity of the US population. Glaeser says lower national social welfare spending in the US is a "macro" version of the decreased civic engagement Putnam found in more diverse communities within the country.

Economists Matthew Kahn of UCLA and Dora Costa of MIT reviewed 15 recent studies in a 2003 paper, all of which linked diversity with lower levels of social capital. Greater ethnic diversity was linked, for example, to lower school funding, census response rates, and trust in others. Kahn and Costa's own research documented higher desertion rates in the Civil War among Union Army soldiers serving in companies whose soldiers varied more by age, occupation, and birthplace.

Birds of different feathers may sometimes flock together, but they are also less likely to look out for one another. "Everyone is a little self-conscious that this is not politically correct stuff," says Kahn.

. . .

So how to explain New York, London, Rio de Janiero, Los Angeles -- the great melting-pot cities that drive the world's creative and financial economies?

The image of civic lassitude dragging down more diverse communities is at odds with the vigor often associated with urban centers, where ethnic diversity is greatest. It turns out there is a flip side to the discomfort diversity can cause. If ethnic diversity, at least in the short run, is a liability for social connectedness, a parallel line of emerging research suggests it can be a big asset when it comes to driving productivity and innovation. In high-skill workplace settings, says Scott Page, the University of Michigan political scientist, the different ways of thinking among people from different cultures can be a boon.

"Because they see the world and think about the world differently than you, that's challenging," says Page, author of "The Difference: How the Power of Diversity Creates Better Groups, Firms, Schools, and Societies." "But by hanging out with people different than you, you're likely to get more insights. Diverse teams tend to be more productive."

In other words, those in more diverse communities may do more bowling alone, but the creative tensions unleashed by those differences in the workplace may vault those same places to the cutting edge of the economy and of creative culture.

Page calls it the "diversity paradox." He thinks the contrasting positive and negative effects of diversity can coexist in communities, but "there's got to be a limit." If civic engagement falls off too far, he says, it's easy to imagine the positive effects of diversity beginning to wane as well. "That's what's unsettling about his findings," Page says of Putnam's new work.

Meanwhile, by drawing a portrait of civic engagement in which more homogeneous communities seem much healthier, some of Putnam's worst fears about how his results could be used have been realized. A stream of conservative commentary has begun -- from places like the Manhattan Institute and "The American Conservative" -- highlighting the harm the study suggests will come from large-scale immigration. But Putnam says he's also received hundreds of complimentary emails laced with bigoted language. "It certainly is not pleasant when David Duke's website hails me as the guy who found out racism is good," he says.

In the final quarter of his paper, Putnam puts the diversity challenge in a broader context by describing how social identity can change over time. Experience shows that social divisions can eventually give way to "more encompassing identities" that create a "new, more capacious sense of 'we,'" he writes.

Growing up in the 1950s in small Midwestern town, Putnam knew the religion of virtually every member of his high school graduating class because, he says, such information was crucial to the question of "who was a possible mate or date." The importance of marrying within one's faith, he says, has largely faded since then, at least among many mainline Protestants, Catholics, and Jews.

While acknowledging that racial and ethnic divisions may prove more stubborn, Putnam argues that such examples bode well for the long-term prospects for social capital in a multiethnic America.

In his paper, Putnam cites the work done by Page and others, and uses it to help frame his conclusion that increasing diversity in America is not only inevitable, but ultimately valuable and enriching. As for smoothing over the divisions that hinder civic engagement, Putnam argues that Americans can help that process along through targeted efforts. He suggests expanding support for English-language instruction and investing in community centers and other places that allow for "meaningful interaction across ethnic lines."

Some critics have found his prescriptions underwhelming. And in offering ideas for mitigating his findings, Putnam has drawn scorn for stepping out of the role of dispassionate researcher. "You're just supposed to tell your peers what you found," says John Leo, senior fellow at the Manhattan Institute, a conservative think tank. "I don't expect academics to fret about these matters."

But fretting about the state of American civic health is exactly what Putnam has spent more than a decade doing. While continuing to research questions involving social capital, he has directed the Saguaro Seminar, a project he started at Harvard's Kennedy School of Government that promotes efforts throughout the country to increase civic connections in communities.

"Social scientists are both scientists and citizens," says Alan Wolfe, director of the Boisi Center for Religion and American Public Life at Boston College, who sees nothing wrong in Putnam's efforts to affect some of the phenomena he studies.

Wolfe says what is unusual is that Putnam has published findings as a social scientist that are not the ones he would have wished for as a civic leader. There are plenty of social scientists, says Wolfe, who never produce research results at odds with their own worldview.

"The problem too often," says Wolfe, "is people are never uncomfortable about their findings."

---Michael Jonas is acting editor of CommonWealth magazine, published by MassINC, a nonpartisan public-policy think tank in Boston.