For generations, the United States has been considered a religious outlier -- its citizens more dedicated to their faith and houses of worship than the rest of the developed world.

But new research from Duke University and University College London (UCL) suggests that American devotion to religion is waning, a decline mirrored across the Western world.

The study published in the American Journal of Sociology finds a slow, steady drop in the number of Americans who claim religious affiliations, attend church regularly and believe in God. It also finds that these drops are driven by generational differences.

“None of these declines is happening fast, but the signs are now unmistakable,” said David Voas, a social scientist with UCL and co-author of the study. “It has become clear that American religiosity has been declining for decades, and the decline is driven by the same dynamic -- generational differences -- that has driven religious decline across the developed world.”

The study examined U.S. data from the General Social Survey, which is conducted every two years, and compared it with similarly broad data from Great Britain, Canada, Australia and New Zealand. Across the board, people have slowly become less religious over time; the U.S. decline has been so gradual that until recently scientists haven’t had enough data to be sure the trend was real, said Mark Chaves of Duke, the study’s other co-author.

“The U.S. has long been considered an exception to the modern claim that religion is declining,” said Chaves, a professor of sociology, divinity and religion. “But if you look at the trajectory, and the generational dynamic that is producing the trajectory, we may not be an exception after all.”

The study didn’t draw comparisons between religions or religious denominations.

This slow drip is generational. A few examples:

-- 94 percent of Americans born before 1935 claim a religious affiliation. For the generation born after 1975, that number drops to 71 percent.

-- 68 percent of Americans 65 and older said they had no doubt God exists, according to the study. But just 45 percent of young adults, ages 18-30, had the same belief.

-- 41 percent of people 70 and older said they attend church services at least once a month, compared to just 18 percent of people 60 and younger.

The data are consistent over a long stretch of time, Chaves said.

“If you break it down over five-year chunks, each age group is a little less religious than the one before it,” he said.