Circumcision study from the Carrier show on morality
I looked up the study mentioned by Richard Carrier in the show on morality. Sombody had posed a question on the morality of circumcision and Richard responded that a study has shown that circumcision reduces the risk of AIDS transmission. So, I did some research and this is what I found.
There have been many studies on circumcision as it relates to HIV infection. There are 29 published articles dealing with this issue. Nevertheless, the data is inconclusive. There are lower rates of AIDS among circumcised African men, but there are other factors that can be used to explain this.
First of all, none of these studies have been experimental. Obviously, there may be an ethical dilemma involved in circumcising some men and not circumcising others, following them for twenty years and seeing whether or not they contract HIV. I don't have an answer to that problem, but since a randomized, controlled experiment is the gold standard, I feel that has to be mentioned.
Many of these studies were geographical analysis--meaning that the researchers gathered data on circumcision rates and AIDS rates in different regions of Africa. This is a problem because, first of all, you can't prove that the correlation isn't PURELY regional and that the same population in a different region wouldn't have the same AIDS rate. Also, most circumcised African men are Muslim, and although they may have multiple wives, they tend to not have extramarital sexual partners.
All the studies were retrospective and relied mostly on personal reports of sexual activity and circumcision status. Some of the studies were done on patients at STD clinics where they were infected with multiple STD's, often presenting with genital lesions which significantly increase the risk of AIDS transmission.
There are studies that have shown the opposite effect--that circumcision increases the risk of AIDS transmission. This may be because circumcision is sometimes used as a coming of age ritual, commonly followed by sexual intercourse. Obviously, an open wound will increase the risk of transmission, so that may or may not be relevant.
If one considers the data worldwide, the AIDS rates among uncircumcised men are actually lower. Again, much of this may be geographical, but it also corroborates the possibility that the correlation in Africa is also either geographical or due to cultural differences.
Here are two links - one to a meta-analysis done on the 29 published studies with available data and another that discusses the actual study mentioned by Richard Carrier on the show. (The latter is a subscription journal article, so if it doesn't work, let me know and I'll C&P it here. It is long, though.)
Hope that was informative.