An August, 2007 study in Biological Psychology has come in under the radar. The study of twins provides some interesting findings of interest to discussions here. For now, I am going to post the abstract, discussion section and reference.
We investigated the potential to engage in homosexual behavior in 6001 female and 3152 male twins and their siblings finding that 32.8% of the men and 65.4% of the women reported such potential ( p < 0.001). 91.5% of these men and 98.3% of these women reported no overt homosexual behavior during the preceding 12 months. The potential to engage in homosexual behavior was influenced by genetic effects for both men (37.4%) and women (46.4%) and these overlapped only partly with those for overt homosexual behavior.
The results show, for the first time, prevalent potential for homosexual response in both men and women, even among individuals who do not report any overt homosexual behavior. Women reported more potential which is in accordance with previous findings showing that their sexual orientation may be more changeable (Kinnish et al., 2005). These results suggest that sexual identity, behavior and potential may often be quite disentangled as suggested by both Escoffier (2003) and Pathela et al. (2006). The phrasing of the question does not suggest that the homosexual sex would take place in the absence of possibilities for heterosexual sex, indicating that the responses refer to homosexual behavior by choice. Therefore, the concept of PHR should not be mixed with situational homosexuality occurring in all-male or all-female environments such as prisons.
PHR was also influenced by genes. These genetic effects overlapped only partly with overt homosexual behavior. Any alleles underlying the genetic effect on PHR should be relatively common and, therefore, also likely to have served some evolutionary purpose, such as limiting the aggressiveness of males and thereby making them more attractive to females (Miller, 2000). PHR was more prevalent among women, a result that is not in line with suggestions that the alleles influencing homosexuality would be predominantly linked to the X-chromosome (Hamer, 2002). It is also of interest to note that there was no evidence of any shared environmental effects on either phenotype. Such results suggest no role for neither intrauterine effects, arguing against hormonal theories of sexual orientation, nor for familial effects shared by all siblings to the same degree, arguing against simple parental personality or parenting style effects.
The results imply that we should rethink how the phenotype of homosexuality is defined. Finally, previous
psychological and genetic correlates of homosexual orientation may actually have more to do with why some people engage in homosexual behavior as opposed to being correlates of a potential to do so.