This is the fourth post in an ongoing series on the topic of biology and sexual orientation.
Last month we talked about the evidence for the heritability of sexual orientation. The upshot of that seems to be that it is partially correlated with certain genes, or with inheritance in general (as estimated by twin studies), but that genetics only tell part of the story. There is still plenty of room for contributions from other factors post-conception. However, that doesn’t mean that there aren’t other biological determinants. So this month we’ll take a look at what is know about the biological mechanisms of sexual orientation.
Our general intuition is that orientation and attraction are functions of our brain and/or our mind. (Recall from the first post in this series that many scientists in these fields think of the brain and the mind as effectively the same thing, while others consider them separate.) So that is a natural place to look. We don’t have great tools for examining the mind (to the extent that it is separate) but we can look at the brain; that investigation began with this study by Le Vay from 1991. Brains of people of different orientations and genders have been examined, and differences have been observed between the brains of heterosexual men and homosexual men, although the exact details of the differences differ. For example, this 2001 study by Byne et al only partially reproduced Le Vay’s findings.
Now, as with all correlations, the challenge is determining which direction the arrow of causation points. Do the differences in brain structure mediate the difference in what we find attractive? Or do the choices we make result in changes to how our brain is organized? Since these brain examinations are being performed on deceased adults, there is some room for discussion on that point. One way to differentiate between the two options is to identify the mechanisms for creating those differences, and when in development they operate. The earlier those changes develop, the less likely it is that conscious choices can be influencing them.
A leading theory about how those brain differences might arise biologically, reviewed by Morris et al in 2004, is that they driven by responses to hormone exposure in the womb. In this scenario, differences in testosterone exposure during development signal to a brain whether it is supposed to be male or female. In homosexual individuals, either the hormone levels themselves or the neurons’ sensitivity to them is different, resulting in a brain which develops according to the opposite path than would be expected based on just a look at chromosomes. Those hormone levels may be influenced by environmental exposures experienced by the mother, or the genes of the mother or the baby; sensitivity to those hormones may be a/the place where genetic or epigenetic factors are expressed.
Another theory, generally considered complementary to the first, is that women have an immune response to male children that also impacts brain development. She develops antibodies which inhibit the masculinisation of the baby’s brain. This immune response can get stronger with each subsequent male child. This is used to explain the observation that men are more likely to be homosexual the more older brothers they have; see Blanchard (2004) for more details.
If the sex/gender/orientation-correlated differences in brain structure are caused by these factors encountered in the womb, then it seems likely that the causal arrow goes from structure to orientation, not the other way around. However, that still doesn’t explain just how a different brain structure actually mediates differences in orientation. To answer that, we need to consider what we mean, biologically, by orientation. At least part of orientation means that we are attracted to those who are physically members of a particular sex. More technically, this means that sensory inputs associated with a particular sex stimulate various arousal responses.
Differences have been observed between how homosexual and heterosexual individuals respond to such stimuli as pheromones or the voices of members of one sex or the other. That these responses could develop biologically seems fairly plausible. There are specific receptors for particular pheromones or particular sound frequencies; it is not impossible to imagine a developmental program that “hardwires” those sensory neurons to the neurons that activate arousal pathways. The one I struggle with is how visual attraction could be programmed developmentally. Visual identification of a male or female would seem to involve at least one layer of abstraction from direct sensory input, which adds complexity to what information would need to be stored in genes.
I suppose it’s possible that some of our sensory responses are more strongly influenced by development than others. I suspect few of us would argue for a conscious response to pheromones, since we are generally not consciously aware of them at all. (A fact that, by itself, supports the idea that those neurons are connected to another part of the brain than the one which assigns a smell to olfactory neuron input.) Perhaps those responses prime us to be attracted to a particular sex, and then visual attraction is more of a learned response. This is also consistent with the observation that certain kinds of visual stimulation can be habit forming, and that those habits can be modulated by positive or negative reinforcement.
So, we have biological pathways that can causes differences in brain organization and structure that occur early in development, we have observations of actual differences, and we have observations of neurological differences in the response to stimuli. There is certainly room for criticism of individual studies — small samples, self-selected participants, etc. — but the totality of the evidence is suggestive. If nothing else, hopefully at this point we can agree that a biological explanation for sexual orientation can be expressed in an attractive and potentially compelling narrative. Choosing to believe in a compelling narrative is something that I would hope we Christians can empathize with.