Putnam set out not to study race, but to measure the levels of "social capital" and to go back and remeasure after community leaders enacted changes to see whether "the needle moved."
The most trusting places were "white places" like Duluth, Minn., the state of New Hampshire and Lewiston, Maine, according to Putnam.
Those at the lowest end of the spectrum were cities with large immigrant populations — Los Angeles, Atlanta, Houston and Phoenix.
"We were surprised," said Putnam, who thinks his critics have missed the point.
"The most successful immigrant societies like ours manage that problem over time by assimilating newcomers and creating a more encompassing sense of who we are," said Putnam.
Putnam said that immigration has strengthened the nation culturally and intellectually, but critics only concentrate on the border, "how high the wall should be and the numbers."
"These are legitimate questions, but we are not talking about how to reweave the fabric of our communities," he said.
Putnam's research doesn't "seal the argument" for less diversity, Clarence Page, Pulitzer Prize-winning columnist for the Chicago Tribune, told ABCNEWS.com.
"The reaction is predictable," said Page. "He's raised some interesting points that need to be looked at."
"The American experiment has been a remarkable success, perhaps more than we ever deserved," said Page. "Diversity is enriching, but at the same time, it means that people have a tougher time identifying with each other."
For 12 years, Philadelphia political scientist Harris Sokoloff has worked on the Great Expectations Project, encouraging civic engagement around mayoral elections.
"The greater diversity in the room, the more difficult it is to talk about issues together," according to Sokoloff, associate professor at the University of Pennsylvania's Graduate School of Education.
"We don't have a lot of experience talking to people who are not like ourselves," he said. "It's challenging and we're not practiced at it."
Sokoloff usually facilitates discussions by asking people to "tell stories about themselves. Once we find something we share, then we talk."
Putnam said that the part of his study that was largely ignored was the need to create a "group identity that for a moment trumps the racial identity."
"When you release a study, you lose control of it and it can be used by others in a way that can sometimes inaccurately present your findings," said Margaret Simms, a senior fellow at the Urban Institute, a nonpartisan economic and social policy research organization.
In her research on community engagement in California a decade ago, she found diverse groups were less likely to respond to "artificial interaction" like special festivals and more likely to unify on real issues like crime and schools.
Putnam's research also emphasized how the push-and-pull of ethnic differences strengthen market forces with increased productivity and creativity.
Peter Cappelli, director of the Center for Human Resources at The Wharton School at the University of Pennsylvania, has seen the power of diversity in industry.
"It is a bigger deal now and more of a plus," he said. "Most companies are under pressure to look like the customers they are serving — and most customers are not white guys."
Organizations can create a "powerful sense of identity," said Cappelli, citing cruise ships as an example of where diverse religious and ethnic groups get along.