How many Americans are atheists?

A headline from Slate caught my eye this week: “Poll Shows Fivefold Increase in Ranks of U.S. Atheists.” But the details of the story didn’t really bear that out. The article reported that a new study found that 1 in 20 Americans now consider themselves to be atheists. That’s 5 percent.

But since at least 2006, most researchers have been estimating the American atheist population at 3 percent to 10 percent—not the mere fraction of a percent that the Global Index of Religiosity and Atheism poll had previously found. From an article I wrote for the LA Daily News:

Depending on the definition, researchers estimate atheists make up 3 to 10 percent of the U.S. population.

Most likely to be educated and men and ranging from liberals to libertarians, experts say, atheists’ chief interest shifts from promoting science to fighting religious influence on politics, depending on the cultural climate.

Part of the reason that researchers have struggled to peg the atheist population is that there is no clear definition of who is an atheist. It might not be as vexing a question as Who is a Jew? But it’s a similarly difficult question to answer.