That is a question that Cynthia Nixon's claim to be gay by choice has sparked anew. I don't claim to have any special scientific expertize on the matter, but there's an interesting debate on the subject at First Things. Stanton Jones - who opposes the gay and lesbian movement in both the church and the culture at large, as I noted last week - has an interesting post about conflicting studies of the sexual orientation of identical twins. Be sure to read the comments, including the one by Stephen Barr.
The gist of it is that one study found that, if a twin is gay, there is a 50% chance the other twin will be as well. Another study, however, found the probability to be just 10%. Part of the difference may be explained by volunteer bias. It may also be explained by emerging scientific evidence that identical twins are not, in fact, genetically identical. But either way, the evidence to date suggests that neither nature nor nuture - both of which identical twins more or less tend to share in common - can fully explain why some people are gay and others are straight (or bi or asexual).
Not that this means that sexual orientation is a choice for most people. Proponents of epigenetic theory, for example, argue that many human traits are the result of a dynamic interaction between genetics and environmental forces. In other words, the exact same genes may express themselves differently depending on a person's environment, including, presumably, the unique environment created when identical twins grow up together and, self-consciously or not, learn to assert their unique individuality.
This theory would go a long way toward explaining not only why identical twins do not always share the same sexual orientation, but also why they have distinct personalities and interests. It might also explain why some people can "choose" to be gay (or, presumably, straight), while others cannot. Perhaps the truth is that, given any particular genetic and environmental mix, sexual orientation may be experienced as either a choice or a command.
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