From an article in Vox entitled How many American atheists are there really?
How to find “closet atheists”
So if you can’t ask people outright whether they’re atheist and get an honest response, how do you go about finding them?
Gervais and Najle set up a very subtle test. They sent a nationally representative poll to 2,000 Americans, who were randomly assigned to two conditions.
The first condition asked participants to read through a bunch of statements like, “I am a vegetarian,” “I own a dog,” and, “I have a dishwasher in my kitchen.”
All the participants had to do was simply write down the number of statements that were true for them.
The value of this method is that participants don’t have to directly say, “I am a vegetarian,” or, “I’m a dog owner” — they only have to acknowledge the number of statements that apply to them. That alone should zero out any embarrassment or hesitance to admit to a particular item.
That’s important because the other 1,000 or so participants saw the exact same list — but with one statement added: “I believe in God.”
By comparing the responses between the two groups, Gervais and Najle could then estimate how many people don’t believe in God. (Because both groups of 1,000 poll takers should, in theory, have the same number of vegetarians, dog owners, and so on in each group, any increases in the number of agreed-to statements from the first group to the second should be reflective of the number of people who don’t believe in God.)
One thing is clear from the results: Much more than 10 or 11 percent of the country (as assessed in Gallup and Pew polling) does not believe in God. “We can say with a 99 percent probability that it’s higher than [11 percent],” said Gervais.
His best estimate: Around 26 percent of Americans don’t believe in God. “According to our samples, about 1 in 3 atheists in our country don’t feel comfortable disclosing their lack of belief,” Najle explains in an email.
Gervais admits this method isn’t perfect, and yields an answer with a wide margin of error. (On the other end of the margin of error, around 35 percent of Americans don’t believe in God.) But the most fundamental question he and Najle are asking here is do polling firms like Gallup and Pew undercount atheists? And it seems the answer is yes.