This is the third post in an ongoing series on the topic of biology and sexual orientation.
Having dealt with what I felt were some necessary preliminaries, we now turn our attentions to the main questions about the biological mediation of sexual orientation, starting with issues of genetics and heritability.
To what extent is sexual orientation passed from parent to child, and how much of that inheritance can be attributed to genes?
Since we are restricting ourselves to human studies, that limits the kinds of genetic research that can be done. In those cases, scientists rely on what they call natural experiments — in other words, observing what occurs naturally rather than deliberately establishing test conditions of interest. One of the most popular natural experiments in genetics is the birth of identical (monozygotic – from the same fertilized egg) twins or fraternal (dizygotic – from different fertilized eggs). Since twins tend to be raised in environments as similar as we can muster, evidence of greater similarity in identical twins compared to fraternal twins is generally attributed to their genetic similarity.
The most recent large twin study on this topic seems to be one by Niklas Långström et al from 2010. In addition to being relatively large (over 7000 twins participated), it was based on a population sample rather than a self-selected one, in order to reduce bias. It found that identical twins were more likely to have the same sexual orientation than fraternal twins, from which it is inferred that some portion of sexual orientation is genetic. The analysis, based on a mathematical model of gene inheritance, attributed 39% of orientation to genetics in men and 19% in women. Given the uncertainty in such estimates, this is not entirely at odds with figures around 20% reported elsewhere.
This particular study did not attempt to identify which gene or genes are involved; no DNA was collected from the study participants. Other studies have attempted to find such genes, with a variety of results, including the possibility that same-sex orientation may have different genetic factors in males and females. A leading theory based on work such as Iemmola & Ciani’s 2009 study links male homosexuality to gene(s) on the X chromosome. This theory is popular because it also suggests an explanation for why natural selection hasn’t completely eliminated such traits — when present in women, the same markers are associated with having more children. However, other studies have failed to reproduce that link, and some instead point to genes elsewhere. Research in females has similarly turned up candidate genes, but a full picture has not yet emerged.
The twin studies also suggest a possible role for shared environmental factor to explain some of the similarity between twins. However, the exact nature of these environmental factors is not well established. A strong candidate is the environment of the womb — there is evidence that hormone exposure during development may have an influence on orientation, and there may also be a role played by an immune response from the mother in some cases. But from the evidence of the twin studies, these environmental factors could also include anything else up to the point at which children can no longer be said to have a shared environment.
This might seem to allow for the possibility that orientation is partially learned based on environmental exposures. There does not appear to be much evidence supporting this notion, however. Various learning scenarios involving parents, siblings, or peers have been examined and no connections have been found between homosexuality and any particular scenario, as summarized by Rahman in a 2005 review. That doesn’t completely eliminate the possibility that counterintuitive learning models are involved, for example that same-sex orientation can be learned equally well from parents of any orientation in some non-obvious way, but this is deemed unlikely.
Finally, the most recent avenue for research into the heritability of orientation is epigenetics. The field of epigenetics is the current new hotness in biology in general; the basic concept is that extra-genomic factors which influence gene expression can sometimes be passed from parent to child. These factors are influenced by the lifetime exposures and experiences of the parent. They are also generally “erased” during the process of gamete formation, but recently it has been discovered that that process is imperfect. The variability introduced by this imperfect erasure is part of what makes epigenetics an attractive answer; it could potentially explain the inconsistent results of the various genetic studies, and the lower-than-expected level of genetic influence calculated from the twin studies. However, it is still early days for epigenetics; no strong conclusions can be drawn yet.
So, what then are we to make of all of the evidence thus far? I think it’s best to think about that question in terms of necessary and sufficient conditions. Given the variety of findings from genetic linkage studies, and the estimates of genetic contributions in twin studies, it seems reasonable to conclude that there is no gene or set of genes which is sufficient to determine sexual orientation in humans. Whether any genes are necessary is less clear at this point, at least from human studies alone. Given the ambiguity, the answer can’t be ‘yes’ for any one gene, but it may still be the case that it is necessary to have one of several different genetic markers.
All of which leads me to think that genetics can influence orientation, but does not determine it. In other words, your orientation is not completely decided at the moment of your conception. Which, when you think about what genes, and the proteins they encode, can and cannot do, is not a terribly surprising conclusion. On the other hand, there is still plenty of room for additional biological factors to be involved even if they aren’t inherited. In the next installment, we will take a look at the data on the biological mechanisms associated with orientation, including how the prenatal conditions mentioned above are involved.