The percentage of Americans who say they are strong in their religious faith has been steady for the last four decades but a new sociology analysis claims that religious groups who have become more staunchly devout have surged while others, notably Roman Catholics, who have sought to become more liberal under Vatican II in that time, have faded in popularity.

Catholics now report the lowest proportion of strongly affiliated followers among major American religious traditions. The drop in intensity could present challenges for the Roman Catholic Church in the U.S., the study suggests, both in terms of church participation and in Catholics' support for the Church's social and theological positions.

"On the whole, the results show that Americans' strength of religious affiliation was stable from the 1970s to 2010," said Philip Schwadel, a University of Nebraska-Lincoln sociologist who authored the analysis to be published in Sociology of Religion. "But upon closer examination, there is considerable divergence between evangelical Protestants on the one hand and Catholics and mainline Protestants on the other."

Schwadel modeled data from nearly 40,000 respondents to the General Social Survey from 1974-2010 and created a measure for Americans' strength of religious affiliation over time. Overall, the proportion of Americans who said they were "strongly affiliated" with their religion increased from 38 percent in the 1970s to a high of more than 43 percent in the mid-1980s but then that number slid to 37 percent by the end of the '80s and has remained about the same ever since.

The changes were instead within the nation's various denominations and religious traditions – most noticeably between Catholics and evangelicals. Since the 1980s, an intensity gap emerged between the groups, the study found. By 2010, about 56 percent of evangelicals said they considered themselves strong adherents to their faith but for Catholics, it was just 35 percent, which was also 4 percentage points lower than mainline Protestants.

Schwade suggests the changes are related to "period-based" effects, like political or social events or other occurrences that can lead to changes among certain groups of people during a specific time period. In Catholics' case, there was an abrupt decline in strength of affiliation starting in 1984 and ending in 1989, which could be due to the growing number of Latino Catholics responding to the survey. Prior research has shown Latino Catholics to be unlikely to report a strong religious affiliation compared with other Catholics. Sex scandals and an inconsistent response to pederasts have not helped.

Meanwhile, evangelicals' strength of affiliation began to swell in the early 1990s.

Survey results also found that though there has been a steady deterioration in strength of religious affiliation over time among Catholics, strength of affiliation was less strongly associated with church attendance among younger generations. This means that declines in Catholics' strength of affiliation do not necessarily lead to equivalent declines in their church attendance.

"That could be seen as good news and bad news for the Catholic Church," Schwadel said. "Younger Catholics are not being driven away from going to church, but they do still feel less strongly committed to their religion than they did a few decades ago."

The study also found:

• Similar to evangelicals, African American Protestants report a high proportion of strongly affiliated members – about 57 percent in 2010.
• Mainline Protestants' devoutness fell to lows of roughly 30 percent in the late 1970s and late 1980s before gradually climbing to 39 percent in 2010.
• The proportion of Americans who say they adhere to no religion climbed from about 6 percent in the 1970s and 1980s to 16 percent in 2010. The increase is roughly equivalent in the decline of people who say they were "somewhat" or "not very strongly" affiliated with their religion over the same time period.