Only 35 per cent of American Catholics said that they felt strongly affiliated with their religion, in contrast to 56 per cent of Evangelical Christians.
In the 1970s there was only a 5 per cent difference between the two groups.
The figures are the result of an analysis of 40,000 recent and historic responses to the General Social Survey, which has been conducted annually in America since 1974.
Philip Schwadel, a sociologist at the University of Nebraska in Lincoln, collated the research.
Mr Schwadel found that the number of people with religious affiliation has also decreased overall in America.
In 2000, 14 per cent of Americans said they had no religious affiliation - a figure that was double that of a decade before.