A question for scientists is whether there are differences in the brains of gay men that relate to things other than sexual preference. A recent Swedish study is receiving much attention because it is the first to identify regions of the brain not directly involved in sexuality that seem to be feminized in gay males. [Ivanka Savic and Per Lindström. PET and MRI show differences in cerebral asymmetry and functional connectivity between homo- and heterosexual subjects. Proc. Natl. Acad. Sci. USA, 10.1073/pnas.0801566105, published online, 16 June 2008.]

Scientists at the Karolinska Institute studied brain scans of 90 gay and straight men and women, and found that the size of the two symmetrical halves of the brains of gay men more closely resembled those of straight women than they did straight men. A denser network of nerve connections was found, for example, in the amygdala, the emotional centre of the brain. Yet not all regions of the brain are affected in such a way.

Earlier studies found that cerebral responses to putative pheromones and objects of sexual attraction differed between homosexuals and heterosexual subjects. Although this observation may have merely mirrored perceptional differences, it raised for the authors "the intriguing question as to whether certain sexually dimorphic features in the brain may differ between individuals of the same sex but different sexual orientation."

They addressed this question "by studying hemispheric asymmetry and functional connectivity, two parameters that in previous publications have shown specific sex differences."

The authors say that "The results cannot be primarily ascribed to learned effects, and they suggest a linkage to neurobiological entities." However,

The present study does not allow narrowing of potential explanations, which are probably multifactorial, including interplay between pre- and postnatal testosterone and estrogen, the androgen and estrogen receptors, and the testosterone-degrading enzyme aromatase. It nevertheless contributes to the ongoing discussion about sexual orientation by showing that homosexual men and women differed from the same-sex controls and showed features of the opposite sex in two mutually independent cerebral variables, which, in contrast to those studied previously, were not related to sexual attraction. The observations cannot be easily attributed to perception or behavior. Whether they may relate to processes laid down during the fetal or postnatal development is an open question. These observations motivate more extensive investigations of larger study groups and prompt for a better understanding of the neurobiology of homosexuality.

All this is part of the ‘nature vs. nurture’ debate concerning the origins of sexual preference. Some would argue that if sexual preference is physically inherent from the womb onward, it is unreasonable to describe expression of that preference as immoral. Yet, it is unclear (at least to me in my ignorance) just how much, and in what ways, even the organisation and structure of the human brain can be influenced by experience and training.

These questions are scientifically interesting, no doubt. However, I am not convinced of the usefulness and validity of moral and ethical reasoning based on a mind-body duality-although I acknowledge it to be a large and complex question.