Changing faiths: more Americans than ever are leaving childhood affiliations behind and making their own decisions about religion.

• Author: Banerjee, Neela
• Position: Cover story

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Americans have never been hesitant about moving among religions--or even starting new ones. Now, a new survey indicates that almost half of adult Americans have left the faith of their childhood to join another religion or have dropped affiliation with any organized religion.

The Roman Catholic Church has lost more adherents than any other group: About a third of respondents who were raised Catholic say they no longer identify as such. The group that shows the greatest gain is the unaffiliated.

The survey, conducted by the Pew Forum on Religion and Public Life, is particularly important because the U.S. Census does not track religious affiliation.

"Religion is the single most important factor that drives American belief, attitudes, and behaviors," says Michael Lindsay, a sociologist at Rice University in Houston. "It is a powerful indicator of where America will end up on politics, culture, family life. If you want to understand America, you have to understand religion in America."

Detailing the nature of religious affiliation--which groups are the largest, the best educated, and the most affluent--can signal who holds sway over the country's political and cultural life, says John Green, one of the authors of the Pew report.

In fact, the United States is unusual among industrialized democracies for how religious it remains. It's generally the case that as nations become more prosperous, healthy, and educated, demand for the support that religion provides declines.

The industrial democracies in Asia and Europe bear this out. According to the Pew Global Attitudes Project, only 20 percent of Germans, 12 percent of Japanese, and 11 percent of the French say religion plays a very important role in their lives.

But in the U.S., religious expression seems to have grown, not diminished, with socioeconomic development. According to Roger Finke, a sociologist at Pennsylvania State University, 45 percent of Americans were members of a church in 1890. By 2000, that figure was 62 percent.

Some sociologists have a novel explanation for why the U.S. hasn't followed this pattern: supply-side economics. Americans, they say, are more religious because there are so many churches competing for their devotion and finding ways to be more responsive to their needs--unlike in Europe, where there tend to be official state churches that in some cases had monopolies on religious life for centuries.

The supply-side theory is consistent with the...