A Supreme Court ruling Thursday dealt a blow to schools that pick students by race to create diverse classrooms. But it didn't preclude less-racial means to achieve diversity. And it raises an old question: Is diversity good for America?

In theory, America's racial and ethnic mix can inspire idealism, based on the hope of a grand melting pot. In practice, however, it can cause many people who actually live with neighbors, students, or workers of different ethnicity or race to withdraw and retreat into their shells, producing what's called a "turtle effect."

That's the conclusion of a major new survey by an eminent scholar.

"Diversity, at least in the short run, seems to bring out the turtle in all of us," says Robert Putnam, a well-known political scientist at Harvard University and author of a massive five-year study on the effects of immigration and diversity on the United States.

Drawing on interviews with 30,000 Americans, Dr. Putnam finds that the greater the diversity in an area, the less trust neighbors have – even for those like themselves – and the more isolated they become.

That lack of trust, the study reports, shows up in a variety of ways. These residents are less likely to register to vote. They do less volunteering, give less to charity, have fewer close friends, and are less happy. They also spend more time watching television.

Yet don't be misled by the apparent challenge. Even though having different people around can be "genuinely unsettling" in the short term, Putnam is an optimist in the long run. He offers reassurance that diversity will give the US a vibrant future. He insists that it is wrong to assume that whatever tension might exist between ethnic or racial groups is a fact of life. Diversity is a challenge, not a threat.

"Over time, especially with some thought and care, we can get used to diversity," Putnam writes in Scandinavian Political Studies. "That's what the country has done in the past, and that's what the country is going through now."

His message of hope comes at an opportune time. As Congress wrestles with immigration, Putnam asks: How do we learn to live together to strengthen communities?

For starters, he would expand English-language training and build more community centers, playgrounds, and places to gather that can create opportunities for inclusiveness. Assimilation comes from sharing experiences, popular culture, and education. He points to the US Army and some evangelical megachurches as examples of institutions that open their doors to a variety of people and integrate them around common interests.

Ethnic and racial diversity is both inevitable and an asset. It spurs economic growth, brings new cultural influences, and increases creativity. Finding a "shared identity that transcends differences" is a continuing effort. Instead of becoming a nation of turtles, Americans will benefit by expanding their collective sense of "we" and "us."

Courts will always ask if diversity is a "compelling state interest" to justify the extent of the means to achieve it. In this latest case (see story) the high court found the ends didn't justify the means.

More broadly, the more Americans define themselves above superficial physical features, the easier it will be to go beyond old debates about race and ethnicity.