Historically, Americans know that creating a group identity helps bridge the ethnic divide, according to Putnam. The introduction of the Pledge of Allegiance in classrooms at the turn of the century helped immigrants identify as Americans.
But, warns Larry Harrison, author of "The Central Liberal Truth: How Politics Can Change a Culture and Save It From Itself," today's immigrants need to assimilate better.
"We've got to emphasize the melting pot and put the kibosh on salad bowl," according to Harrison. "They need to become American."
He urges a crackdown on illegal immigration and a "calibration of the flow" of legal immigrants.
But, concedes Harrison, who has made some headway in getting his Brazilian neighbors to turn down their loud radios, "the issue of trust is complicated."
"I imagine that some WASP [white Anglo-Saxon Protestant] citizens reacted similarly to my grandparents, whom they may have regarded as 'loud Jews.'"
Putnam might agree.
"We have national amnesia on how complicated it was," he said of the early tides of immigration.
"Part of my story is meant to say to my fellow progressives, don't sneer at people who are discombobulated by diversity," he said. "It's real and we cannot let our political correctness get in the way of this serious issue. But we've dealt with this before."