Catholics in the Northeast, Protestants in the Midwest, Christian evangelicals in the South, with isolated pockets of Jews, Mormons, Muslims and Buddhists: from the outside (and sometimes from the inside), the US looks like a weather map of religious storm fronts poised to erupt into a full-blown tempest.
Similar religious divisions have, after all, led to turbulence in other nations, up to and including genocidal civil wars. Moreover, US politics looks increasingly polarised along what appear to be religious lines - the so-called "God gap" - with the intensely religious seemingly inclined to support the conservative Republican Party and the most secular, the more progressive Democrats.
But a respected Harvard scholar, backed by the two most in-depth surveys ever conducted on the topic, has concluded - to what he confesses was his own surprise - that if there are divisions in the US, they are political first, and not principally religious.
In a country in which 35 to 40 per cent of citizens switch religions at some point in their lives, people are more likely to choose their religion based on their political beliefs than pick their political affiliations because of their faith, writes Robert D. Putnam in his new book, American Grace: How Religion Divides and Unites Us.
"I would have thought that the increasing overlap, for example between being religious and voting Republican, was because people chose their political outlook based on their religious outlook. We couldn't imagine that the causation would go in the other direction - that people would choose their religion based on their prior political behaviour," Putnam recalls. "People are sorting themselves out by their politics. The underlying driver is not so much religion as these seemingly intractable ideological and political debates."
In fact, says Putnam, a political scientist and professor of public policy at Harvard's John F. Kennedy School of Government, the US turns out to be as religiously tolerant as it is religiously diverse, for the simple reason that its religious plurality means that almost everyone has friends and relatives of other faiths. Today, half of US citizens have a spouse whose religious background differs from theirs, for instance, meaning that most people have in their extended families someone they love who worships in a different church, temple, mosque or synagogue.
Putnam and his co-author David Campbell, John Cardinal O'Hara C.S.C. associate professor of political science at the University of Notre Dame, call this the "Aunt Susan" principle. It is hard, they contend, to demonise your relatives and friends.
Both academics are living proof. Raised a Methodist, Putnam converted to Judaism when he married a Jew, while his sister married a Catholic and had three children who became evangelicals. Campbell is a Mormon whose ancestors included Protestants and Catholics.
It is an increasingly American phenomenon. The marriage last year of Chelsea, the daughter of Secretary of State Hillary Clinton and former president Bill Clinton, attracted international media attention. But almost no one commented on the fact that Clinton, a Protestant, was marrying a Jew.
"Despite the talk of culture wars in the US over religion, most Americans are remarkably tolerant and open-minded about people who have different views as far as religion is concerned," Putnam says.
"Even the 10 per cent who are the most religious say that even a person without religious faith can be a good American. Their own view is deeply religious, and they may disagree with non-religious people, but they're willing to say they can be good Americans."
And among the growing number of Americans on the other end of the spectrum who have no religious affiliation - the "nones", as Putnam calls them, who presently make up 17 per cent of the population and outnumber Protestants at 14 per cent - "even they say that religion is a force for good in American life".
The surveys on which Putnam's book is largely based asked a question that is also frequently posed by pollsters elsewhere in the world: "Are all religions false, is one religion true and all the others false, or are there basic truths in more than one religion?"
"If you read the American media, if you listen to talk radio and watch cable television and read commentary, especially in the British Isles, about religion in US life, you might believe that all Americans are in the first category or the second category," Putnam says.
"But the overwhelming number of Americans choose the option that there are basic truths in many religions."
What all of this means, he adds, is that "despite the increasing polarisation, Americans are pretty open-minded in terms of religious tolerance. The bottom-line message of our book for Americans and others viewing Americans is, cool it. Relax."
However, that may not be the message of Putnam's still-untitled forthcoming book, due out next year and based on a similar survey that has already been conducted in Britain, Northern Ireland and the Republic of Ireland. It will compare religious attitudes in the US and the British Isles, and the results are starkly different.
That is because most immigrants to the British Isles and the US are intensely religious, says Putnam, who attended Balliol College, Oxford and is visiting professor and director of the University of Manchester's Graduate Summer Programme in Social Change.
But in the US, immigrants find a society that is also deeply religious, and while they may face hostility because of language and other differences, it is not over their religion. In the British Isles, by contrast, Putnam says, immigrants and their faith stick out in ways that lead to tension more often than tolerance.
"It is certainly true that there is increasing religious diversity in the British Isles, especially involving Muslims but also Hindus and Sikhs and Afro-Caribbean religions," he says. "In that sense, over the long term, what is true in the US may also become true in Europe.
"But there is one very important difference. In the British Isles, native-born individuals are much less religious, so when highly religious Muslims arrive, it is not that they are a different religion but that they are religious at all that is odd."
Putnam is author of a landmark 1995 best-seller, Bowling Alone: The Collapse and Revival of American Community, which argues that Americans have become more isolated from one another, and cites the example that there are more bowlers but fewer bowling leagues. The book suggests that social connections are essential to viable multi-ethnic societies, but that such "bridging" behaviour, or social relationships among people who are different, is in decline in the US.
"But it turns out that in religion that's not true," he says. "The reason we did this research on religion in the US is because I recognised that religion deserved more extensive treatment than I gave it in Bowling Alone, and this is one example where the more I examined the data on bridging in religion, the more I became convinced by the evidence that there has been a lot of religious bridging over the past 50 years."
Faith in that period has ebbed and flowed, Putnam says. US society after the Second World War was largely made up of moderate, nonpolitical religious believers. In the counter-cultural 1960s, religious observance declined dramatically. But that resulted in a backlash of gradual growth among conservative religions, which by the early 1990s converged with political conservatism.
Recently there has been another backlash, this one away from organised religion altogether, although not necessarily against belief in God. This trend is particularly marked among the young and stems in part from a discomfort with the confluence of religion and conservative politics.
Most Americans, in fact, wish to keep a wall between religion and politics. But it also turns out that conservative congregations, contrary to popular perception, are less politically involved than liberal ones.
Nor do all faiths benefit from the interaction that allows familiarity to breed respect. Muslims and Buddhists, whose numbers are among the smallest in the US, are viewed equally coolly by other citizens. Putnam says the fact that the two groups are held in the same low regard confirms his theory that this is because most Americans don't know Muslims or Buddhists personally, rather than having anything to do, in the case of Muslims, with an association with terrorism.
"When did you ever hear about a Buddhist terrorist?" Putnam asks. "But very few Americans have a Muslim Aunt Susan, and we think that's the main part of the story."
Jews, on the other hand, are today the most broadly popular religious group in the US, despite a national history of anti-Semitism.
"As Muslims become more integrated into American society, we expect them to follow the same path that was followed historically in terms of the attitudes of Protestants toward Catholics and of Christians toward Jews," Putnam says.