This popularly referenced survey gives statistics about the number of atheists in US prisons.
It suggests just 0.21% of the prison population is atheist. There are more scientologists! This is surprising because atheists represent about 15% of the US population.
Now, this survey is often used to show that atheists are more moral. (I lean towards this idea, being an atheist myself.) However, I have been told that such a survey is useless - many "closet" atheists would be worried about reprisal from checking the "atheist" box, especially in a predominantly Christian environment. This would tend to bias the results.
Has any research been performed about this?