"A more general issue affects American surveys on religious beliefs, namely, the "social desirability effect," in which respondents are reluctant to give an unpopular answer in a society in which being religious is the norm. What happens when questions are framed to overcome this distortion? The FT/H poll tried to counteract it by allowing space not only for the customary "Not sure" but also for "Would prefer not to say" -- and 6 percent of Americans chose this as their answer to the question of whether they believed in God or a supreme being. Add to this those who declared themselves as atheists or agnostics and, lo and behold, the possible sum of unbelievers is nearly one in four Americans."Ronald Aronson, The Nation, via Alternet June 16, 2007
The article deals with much more than statistics, and is well worth reading. See also: Are Nearly One-Fourth of Americans Non-Believers?