Evangelical Protestants have become more devoted to their religious beliefs over the last three decades, even as Catholics have become less attached to their faith, new research finds.

The denominational differences come even as religious affiliations have decreased overall in America, with the number of people who claim no religious affiliation at all doubling from 7 percent in 1990 to 14 percent in 2000, said study researcher Philip Schwadel, a sociologist at the University of Nebraska, Lincoln.

Nevertheless, Schwadel said, these unaffiliated individuals seem to be dropping out of religious institutions that they were previously ambivalent about. People who feel strongly about their faith are as numerous as ever.

"The proportion of Americans who say they have a very strong religious affiliation over time is very stable," Schwadel told LiveScience.

Strength of faith

Schwadel based his findings on a major questionnaire called the General Social Survey, which has been administered to a cross-section of Americans yearly or every other year since 1974. Among the questions on this survey are several about religion, including one that asks how strongly affiliated people feel about their denomination. [8 Ways Religion Impacts Your Life]

By analyzing about 40,000 responses over the decades, Schwadel was able to track changes in how strongly tied people felt to their religion. He found that the total number of strongly affiliated people stayed basically steady around 37 percent, with a small, short-lived bump to 43 percent in 1984 and 1985.

The people who identified as religious but said they weren't strongly tied to their religion became less common over time, however, dropping from 56 percent in 1990 and 1991 down to 45 percent between 2008 and 2010. This coincided with the uptick in unaffiliated Americans.

"The tremendous growth in being unaffiliated came, I think not very surprisingly, from the relatively uncommitted Americans," Schwadel said.

Story continues

Changing attitudes

On a denomination-by-denomination level, the picture gets more complex. While the overall number of strongly affiliated people has stayed stable, that's because Evangelical Protestants have become more tied to their churches, while Catholics identify less strongly with their faith.

In the 1970s, there was only about a 5-percentage-point difference in how strongly Catholics and Evangelicals felt about their religion, Schwadel said. Today, it's around 20 percentage points. About 56 percent of Evangelicals currently say they're strongly affiliated with their religion, while only 35 percent of Catholics say the same.

A few things could be driving these trends, Schwadel said, though the survey did not ask people for their reasons behind their religious devotion. Priest sex abuse scandals in the Catholic Church could have shaken people's trust in the institution, so that they still call themselves Catholic but distance themselves from the Church. Likewise, the demographics of Catholicism are changing, Schwadel said. There are more Latino Catholics in America today than in the past, and Latinos may be less likely to strongly identify with the institution of the Church than white Catholics.

On the Protestant side, Evangelicals became more visible during the 1990s, as well as more politicized, Schwadel said. It's possible that broader social influence encouraged more people to identify strongly with the faith.

Both the increase in Protestant devotion and the decrease in Catholic faith happened gradually over time rather than sharply from generation to generation, Schwadel noted in the Autumn 2012 issue of the journal Sociology of Religion. That should offer some reassurance to Catholic leaders disappointed to see their flock less devout.

"When it's that rapid and not really generationally motivated, it may be possible to reverse people's views," Schwadel said.

Follow Stephanie Pappas on Twitter @sipappas or LiveScience @livescience. We're also on Facebook & Google+.

Copyright 2012 LiveScience, a TechMediaNetwork company. All rights reserved. This material may not be published, broadcast, rewritten or redistributed.