This post originally appeared at Pew Research Center's Fact Tank blog.

Estimating the number of atheists in the U.S. is complex.

Some adults who describe themselves as atheists also say they do believe in God or a universal spirit, according to Pew Research Center surveys. At the same time, some people who identify with a religion (e.g., say they are Protestant, Catholic or Jewish) say they do not believe in God.

The debate over the definition and identity of atheists came up recently during Oprah Winfrey’s interview with Diana Nyad, a self-identified atheist who recently gained attention for her Cuba-to-Florida open swim.

Here’s what we know about self-described atheists and their beliefs:

1. The number of people who identify themselves as atheists in the United States has been rising, modestly but steadily, in recent years. Our aggregated data from 2012 show that 2.4% of American adults say they are atheists when asked about their religious identity, up from 1.6% in 2007.

2. Atheists, in general, are more likely to be male and younger than the overall population; 67% are men, and 38% are ages 18-29(compared with 22% of all U.S. adults). About four-in-ten atheists (43%) have a college degree, compared with 29% of the general public.



3. Although the literal definition of “atheist” is “a person who believes that God does not exist,” according to the Merriam-Webster dictionary, 14% of those who call themselves atheists also say they believe in God or a universal spirit. That includes 5% who say they are “absolutely certain” about the existence of God or a universal spirit. Alternatively, there are many people who fit the dictionary definition of “atheist” but do not call themselves atheists. More Americans say they do not believe in God or a universal spirit (7%) than say they are atheists (2.4%).



4. Not all atheists see a contradiction between atheism and spirituality. A quarter (26%) say they think of themselves as spiritual people, and 3% consider themselves religious people. Four-in-ten atheists (41%) say they often think about the meaning and purpose of life.



5. Among atheists, 82% say they either often (52%) or sometimes (30%) feel a deep connection with nature and the earth; among all American adults, 85% either often (58%) or sometimes (26%) feel such a connection.

Michael Lipka is Assistant Editor at the Pew Research Center’s Religion & Public Life Project.

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