The poll was conducted by WIN-Gallup International and is based on interviews with 50,000 people from 57 countries and five continents. Participants were asked, "Irrespective of whether you attend a place of worship or not, would you say you are a religious person, not a religious person, or a convinced atheist?"
The seven years between the polls is notable because 2005 saw the publication of The End of Faith by Sam Harris, the first in a wave of best-selling books on atheism by Christopher Hitchens, Richard Dawkins, Daniel Dennett and other so-called "New Atheists."
"The obvious implication is that this is a manifestation of the New Atheism movement," said Ryan Cragun, a University of Tampa sociologist of religion who studies American and global atheism.
Still, Cragun does not believe the poll shows more people are becoming atheists, but rather that more people are willing to identify as atheists.
"For a very long time, religiosity has been a central characteristic of the American identity," he said. "But what this suggests is that is changing, and people are feeling less inclined to identify as religious to comply with what it means to be a good person in the U.S."
Another possible factor may be the number of atheists within organized efforts by American atheist groups to encourage those who do not believe in God to say so publicly. The Out Campaign, a project of the Richard Dawkins Foundation for Reason and Science, was launched in 2007 and has since been endorsed by several national atheist groups.
The current poll confirms a declining religiosity at home and abroad that's been detected in other polls. The 2008 American Religious Identification Survey found that 15 percent of Americans said they have no religion different from being a "confirmed atheist," but nonetheless up from 8 percent in 1990.
Barry Kosmin, the principal investigator for the ARIS report, said he's skeptical of the new study.
"The U.S. trends are what we have found and would expect, but the actual numbers are peculiar to say the least," he said. "The drops in religiosity seem too sharp for the time period people just don't change their beliefs that quickly. Most of the trend away from religion has demographic causes and demography moves glacially."
Specifically, he points to the poll's finding that Vietnam, while showing a sharp 23 percent drop in religiosity since 2005, also shows no atheists. "Eight million Communist Party members but zero atheists?" he said. "That statistic makes me very doubtful of the accuracy of the survey overall and some of the international comparisons."
Other findings from the poll include:
• Besides Vietnam, Ireland had the greatest change in religiosity, down from 69 percent to 47 percent.
• China has the most "convinced atheists," at 47 percent, followed by Japan (31 percent), Czech Republic (30 percent) and France (29 percent)
• The most religious countries are in Africa (Ghana, Nigeria, Kenya), South America (Brazil, Peru) and Eastern Europe (Macedonia, Romania, Armenia).
• Countries with the same percentage of atheists as the U.S. are Poland, Moldova and Saudi Arabia.