RALEIGH, N.C. – Despite the prominence of religious believers in politics and culture, America has shrinking congregations, growing dissatisfaction with religious leaders and rising numbers of people who do not think about faith, according to a new study by a Duke University expert.

In “American Religion: Contemporary Trends,” author Mark Chaves argues that over the last generation or so, religious belief in the U.S. has experienced a “softening” that affects everything from whether people go to worship services regularly to whom they marry. Far more people are willing to say they don’t belong to any religious tradition today than in the past, and signs of religious vitality may be camouflaging stagnation or decline.

“Reasonable people can disagree over whether the big picture is one of essential stability or whether it’s one of slow decline,” said Chaves. “Unambiguously, though, there’s no increase.”

Chaves, who directs the National Congregations Study, used data from that research and from four decades’ worth of General Social Survey results to draw what he aims to be an overview of contemporary American religion.

Today, as many as 20 percent of all Americans say they don’t belong to any religious group, Chaves found, compared with around 3 percent in the 1950s. Yet, those people aren’t necessarily atheists, agnostics or others. Instead, about 92 percent of Americans still profess belief in God, they just don’t use religion as part of their identity.

“It used to be that even the most marginally active people wouldn’t say they have no religion, they’d say I’m Catholic or I’m Baptist or I’m Methodist or whatever,” Chaves said. “That’s not the case today.”

Even signs of robust religious faith may not be what they appear, Chaves found. The strength of religious conservatives in politics, for example, has coincided with a growing disillusionment about faith’s role in the public square. Chaves found that between 1991 and 2008, the percentage of Americans who strongly agreed that religious leaders should stay out of politics rose from 20 percent to 44 percent.

At the same time, those who remain devout have become more conservative. In the mid-1970s, knowing that someone attended church regularly wouldn’t reveal much about their political leanings; today, regular churchgoers are far more likely to be Republicans than Democrats.

“It’s not random who’s leaving churches,” said Bradley Wright, a University of Connecticut sociologist who studies American Christianity and wrote the 2010 book “Christians Are Hate-Filled Hypocrites … and Other Lies You’ve Been Told.”

“As Christians affiliated more through the Republican Party, liberal, marginal churchgoers became offended and left,” she said.

The notion of decline misses important developments, like the enthusiastic devotion of Christian immigrants, argues Leith Anderson, president of the National Association of Evangelicals.

“Much of our immigration is coming from countries where Christianity is blossoming,” he said. “I think God’s doing some great things in African-American churches and among Hispanic immigrants.”

Anderson thinks the change is better described as a shift than a decline, as people become more willing to leave the denominations or faiths in which they were raised and look elsewhere for spiritual nourishment.