IT HAS BECOME increasingly popular to speak of racial and ethnic diversity as a civic strength. From multicultural festivals to pronouncements from political leaders, the message is the same: our differences make us stronger.

But a massive new study, based on detailed interviews of nearly 30,000 people across America, has concluded just the opposite. Harvard political scientist Robert Putnam -- famous for "Bowling Alone," his 2000 book on declining civic engagement -- has found that the greater the diversity in a community, the fewer people vote and the less they volunteer, the less they give to charity and work on community projects.

In the most diverse communities, neighbors trust one another about half as much as they do in the most homogenous settings. The study, the largest ever on civic engagement in America, found that virtually all measures of civic health are lower in more diverse settings.

"The extent of the effect is shocking," says Scott Page, a University of Michigan political scientist.

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.

Click for Page 2 of the story >>