It is commonly claimed by atheists that they are under-represented in the prison population. This claim is not justified.
UK[]
According the UK's home office the March 2000 report, Religion in Prisons, 31.9% of inmates claimed to have "no religion", including 0.2% atheists and 0.1% answering agnostic. This compares with (taking data from the national census), 15.5% of people with answered "no religion" and 7.3% who did not answer the question. giving a total of between 15.5% and 22.8% of the general population who are atheists, well short of the numbers in the prison population.
USA[]
It is often claimed by atheists that US atheists make up 0.2% of the prison population, but around 10% of the population. This claim was seemingly started, by Rod Swift, who wrote it on his website from where it circulated around the Internet. He claims that that he contacted Denise Golumbaski, an employee of the Federal Bureau of Prisons, and got the following response:
Dear Mr. Swift:
The Federal Bureau of Prisons does have statistics on religious affiliations of inmates. The following are total number of inmates per religion category:
Response Number %
Catholic 29267 39.164%
Protestant 26162 35.008%
Muslim 5435 7.273%
American Indian 2408 3.222%
Nation 1734 2.320%
Rasta 1485 1.987%
Jewish 1325 1.773%
Church of Christ 1303 1.744%
Pentecostal 1093 1.463%
Moorish 1066 1.426%
Buddhist 882 1.180%
Jehovah Witness 665 0.890%
Adventist 621 0.831%
Orthodox 375 0.502%
Mormon 298 0.399%
Scientology 190 0.254%
Atheist 156 0.209%
Hindu 119 0.159%
Santeria 117 0.157%
Sikh 14 0.019%
Bahai 9 0.012%
Krishna 7 0.009%
Total Known Responses 74731 100.001% (rounding to 3 digits does this)
Unknown/No Answer 18381
Total Convicted 93112 80.259% (74731) prisoners' religion is known.
– Denise Golumbaski
It is concerning that there is no official source for this data. There's no way to verify any of these numbers, or to know how they were obtained.
The number of people in the general population answering that their religion was atheist at the time was around 0.5%, as compared with 0.2% identifying as atheist in the prison population. It seems plausible that many atheists simply did not answer what religion they were, or answered that they had none. Around 19.8% of people included here responded with "Unknown/No Answer", leaving the numbers of atheists in US prisons between 0.2% and 20.0% making conclusions about the number of atheists in US prisons hard to draw.
Finally, it's worth noting the inflated number of atheists Rod Smith claims in the US general population. In the general population Rod Swift claimed that the number of atheists was between 8% and 16%, whereas Gallop polls consistently show that between 94% and 96% of Americans believe in God.