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Yet Another Twin Study Dr N.E.Whitehead Twin studies on same sex attraction (SSA) continue to be performed. A sixth major one has just emerged (Langstrom et al. 2008) and nicely confirms the main results of the others, viz that genetic influences are modest. The paper’s main conclusion is that non-shared influences predominate in the development of SSA, ie the predominant influence in homosexuality is the environment that twins do not have in common. This study is Swedish, and has the distinction of including UK author Qazi Rahman on its author list. Rahman was the co-author of Born Gay, which argued strongly, mainly on the basis of twin studies, that all SSA people are born that way. (A book review may be found on this site.) Perhaps he has learnt much more about twin studies since; the current paper contains not a hint of the arguments in his book. In comparison the paper is almost “traditional”. Langstrom’s paper shows again that most events leading to SSA are so specific to the individual that even identical twins react differently. This conclusion has now been affirmed by 6 major studies. At this stage we should be saying, “This conclusion is not going to change now – the main cause of SSA is individualistic reactions”. It is also time to say that there is no way that SSA is an example of something inexorably taking people over from birth onwards. There is far too much chance involved. Rather, SSA is a good example of a trait which is not genetically enforced. Langstrom’s study is the second large study of its kind to come out this year. The first was (Santtila et al. 2008) which was Finnish. Conclusions from that are already incorporated in My Genes Made Me Do It! (download the book from this site). Although Langstrom et al. claim their study is the largest so far with 3826 twin pairs, the Santtila et al study was actually the largest, with about 4500 twin pairs. (Each group of researchers was working simultaneously, unaware of the other, each thinking their study was the largest to date.) Previous studies have tried to classify people as opposite sex attracted (OSA) or SSA either on the basis of their attractions, their activities, or their identity. In general studies have shown a greater “genetic contribution” towards activities than attractions. This is puzzling. However, the Langstrom study is also activity- based but the genetic contribution is a little lower than some previous studies. Of particular interest to me are the identical twin pairwise concordances (ie how often a co-twin is also homosexual if one twin is). Bear in mind identical twins are genetically identical. Here the results were very similar to previous studies. For identical male twins 7/71 were both SSA, (10%), for females 26/214 concordant (12%). This compares with 11% and 14% respectively for the Bailey et al (2000) study, so it's very similar. Again I repeat a point I have to make a lot: This concordance for homosexuality, reflecting all possible factors in common between the twins, pre- and post-natal, is unusually low for traits studied using the twin method.
Reference List Langstrom, N., Rahman, Q., Carlstrom, E. and Lichtenstein, P. (2008) Genetic and Environmental Effects on Same-sex Sexual Behavior: A Population Study of Twins in Sweden. Archives of Sexual Behavior http://dx.doi.org/10.1007/s10508-008-9386-1
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