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C. Corax is curious about my response to this study which suggests there may be a genetic component to religiosity. Unfortunately, I don't have on-line access to the Journal of Personality so I can only comment on this reporter's summary. Nevertheless, this is a good opportunity to make some general observations about, uh, observational studies in general, and twin studies in particular.

This study was based on questionnaires administered to sets of fraternal and identical twins -- all male, for some reason, and all born in Minnesota, for some other reason. They were asked to remember how often they went to religious services, prayed and discussed religious teachings when they were children, and how often they did those things today. The basic conclusion was that both kinds of twins were very similar in these respects as children, but the fraternal twins became less similar as adults while the identical twins retained their similarity. The summary doesn't give much quantitative information about these effects, except to say that the fraternal twins were "1/3 less similar [as adults] than they were as children," a statement which is essentially uninterpretable, but presumably the original report makes sense.

The researchers conclude that it is the identical genes of identical twins that keeps them similarly religious as they grow older. However, I can think of numerous objections to this conclusion. First, if genes are what makes people religious, the fraternal twins would not have reported similar religiosity as children. Obviously, the first and most cogent conclusion one can draw from this study is that it is not genes, but the family environment, that determines how religious people are as children. Odd, then, that all the spin is the other way.

Second, it is completely inappropriate to conclude that a genetically determined predisposition to religion keeps the identical twins more similar as adults. If genes are behind this, then it seems much more likely to me that it's a genetic predisposition to believing whatever the heck it is your parents told you to believe, not to religion specifically, that keeps the identical twins more similar. What is more, identical twins not only share the same genes, they have a more similar environment as children than do fraternal twins, because other people treat them more similarly; and it is plausible to suppose that they continue to have a more similar environment as adults, and also to remain closer to each other, than do fraternal twins. So I must say I feel the investigators, and reporters who have covered this study, are overinterpreting results which are merely suggestive and could have various explanations.

Finally, this might apply only to males from Minnesota.

Observational studies, alas, are very often misleading. There is a major article in the December 5 JAMA (subscription only) by Athina Tatsioni and colleagues, about the persistence in the literature of claims based on observational studies which have been contradicted by subsequent experimental evidence. These include the once widely touted claims that Vitamin E and beta Carotene prevent cancer, and that estrogen can prevent Alzheimer's disease. Alas, randomized controlled trials have found these supplements, on the contrary, to be harmful. (Stop taking them if you haven't alrady!) The problem is that the world is very complicated and when we try to look at the relaionship between X and Y observationally, we can't rule out that X is associated with a whole of other things -- call them A through W -- that are also influencing Y. Therefore the perceived association between X and Y may be entirely spurious.

There are some profounder things I could say about genes and personality. Essentially, genes don't determine anything. Genes interact with the environment through developmental processes, and it could be that a gene that predisposes to being fat in one environment predisposes to being thin in another. We are no doubt born with predispositions to various personality traits but that is assuming a particular context.

In the end, it is obvious that the principal source of religious belief is indoctrination. Children who grow up in Christian households become Christians, and children who grow up in Yezidi households become Yezidis. People who are inclined to figure things out for themselves, who have the courage and critical thinking skills to abandon old beliefs when they encounter new evidence, who have access to accurate information about the world, and who live in societies that tolerate independent thinking and do not impose excessive conformity, stop being religious. People who lack those advantages do not.