In 2012, 17.8 percent of respondents said they had no explicit religious identification, up from 14.6 percent in 2008, results indicated.
The increase from 2011 to 2012 in religious "nones" -- from 17.5 percent in 2011 to 17.8 percent last year -- was the smallest such year-to-year increase during the past five years of Gallup Daily tracking of religion in America, the Princeton, N.J., polling agency said.
Religious "nones" are those who respond "no religion" as well as those who say they don't know or refuse to answer.
The rise in the religious "nones" over time is one of the most significant trends in religious measurement in the United States, Gallup said. Surveys conducted in the 1950s and 1960s indicated virtually all Americans had a religious identity. The percentage who did not report such an identity began to rise in the 1970s and has increased in the years since.
Results are based on nationwide telephone interviews conducted as part of Gallup Daily tracking from 2008 through 2012, with random samples of 311,588 adults in 2008; 353,849 adults in 2009; 352,842 adults in 2010; 353,492 adults in 2011, and 353,571 adults in 2012. The margin of error is 1 percentage point.