The U.S. Census does not track religious identity because of constitutional restrictions. Trinity College fills that gap in data with the American Religious Identification Survey. It sampled more than fifty thousand Americans last year.
Seventy-six percent identified as Christians. That's a ten point decrease since 1990. The number claiming no religion has grown to fifteen percent of the population, behind only Catholics and Baptists.
That growth was most pronounced in New England. In Connecticut, Catholics still make up the largest share of the population at thirty-eight percent, but that's down from fifty percent in 1990.
Barry Kosmin is a professor at Trinity College and one of the study's authors.
"Most of those people have not joined another religion, the tendency is for people to actually go outside religion, not to do a lot of switching."
The total percentage of Catholics grew slightly nationwide as increases in the south and west made up for New England's decline.
There were other shifts among Christians. Identification with mainline protestant churches -- including Methodist, Presbyterian, and Episcopalian -- has steadily declined since 1990, while the share of nondenominational Christians has nearly tripled since 2001.
Kosman says overall, more than a third of those surveyed identified as born-again or evangelical Christians.
"That seems to have now become a feature even amongst the mainline Christian denominations, as well as 18 percent of the Catholic population, which is surprising theologically."
The survey also found that America's percentage of Muslims has doubled since 1990, though it's still less than one percent of the population.