Although acceptance of atheists is increasing, their fellow Americans still eye them with considerable suspicion. The percentage of Americans who declare themselves religiously unaffiliated has risen from 5 percent in 1972 to 25 percent now. But depending upon the poll, the share of Americans who call themselves atheists varies from a low of 3 percent to around 11 percent.

Given the social stigma attached to atheism, researchers at the Public Religion Research Institute (PRRI) hypothesized that polls might underreport the number of Americans who are nonbelievers. To test this hypothesis, they used the unmatched count technique, in which poll respondents are randomly divided into two groups. The control group is asked how many of a number of harmless statements—"I am a vegetarian," "I can drive a motorcycle," "I own a dog," etc.—are not true statements about them. The second group is asked to respond to one additional, more sensitive statement: "I believe in God."

Respondents are not specifically indicating which statements are true for them, only the total number that is. This type of polling has been used, for example, to estimate the size of the LGBT community and the extent of antigay feeling.

The researchers ran two slightly different unmatched count technique surveys involving 4,000 Americans. In their report, "How many atheists are there?," they conclude that about 26 percent of Americans likely do not believe in God.

Over at FiveThirtyEight, PRRI research director Daniel Cox notes that public attitudes toward the LGBT community have become more accepting as more Americans report having a gay friend or family member has increased. He suggests that the same dynamic is happening as more atheists come out of their nonbelief closets.

Interestingly, PRRI's 2013 American Values Survey reported that "fewer than 6-in-10 (58%) libertarians believe that God is a person with whom one can have a relationship, one-quarter (25%) believe God is an impersonal force in the universe, and 16% report that they do not believe in God."

For more background, see my article, "The New Age of Reason: Is the Fourth Great Awakening finally coming to a close?"

Also see ReasonTV's report on the Rally for Reason, a 2012 gathering of nonbelievers on the National Mall: