The first scientific studies of homosexuality date back to the early 1800s. The original theories suggested that homosexuality was a disease and not unexpectedly these initial theories influenced the outlook of the scientific community and society in general for the next hundred years or so. In fact, during the first half of the twentieth century psychiatrist tended to consider homosexuality as a treatable disease that could be cured through psychotherapy.
Needless to say, these treatments were not always successful. It was not until the late twentieth century that the theories of a hormonal and genetic origin of homosexuality arose and became generally accepted. Besides the scientific outlook, these new ideas also influenced the political and social arenas. In fact, homosexuality stopped being classified as a mental disorder in 1973 and since then the scientific definition of homosexuality has evolved progressively.
Both nature and nurture contribute towards defining a person’s sexual orientation. Unfortunately, even though there are a number of studies investigating the medical, genetic and psychological basis of homosexuality, there is no conclusive study which has identified the underlying cause. What has become increasingly clear is that sexual orientation is influenced by both biological factors as well as environmental factors. Biological factors may be due to genetic variation whilst environmental factors include personal experiences but also the hormonal environment in the mother’s womb.
Thus there are factors shaping the person even before birth. In fact scientific research has shown that identical twins are more likely to both have the same sexual orientation than non-identical twins or sibling pairs; thus implicating a genetic component. The hormonal involvement has been demonstrated by studies looking at the length ratio of the ring and index fingers. The length ratio of these fingers is determined by in utero hormonal exposure and is therefore determined before birth. Correlates have been found between sexual orientation and finger length ratio, thus implicating hormonal exposure in sexual orientation.
Interestingly, there is on-going debate as to whether or not homosexuality is a healthy trait in a species. (As an aside, it is worth noting that homosexuality is not an exclusively human characteristic.) Two relatively recent articles published in the journal Evolution and Human Behaviour present data illustrating a possible mechanism by which genes that predispose towards homosexuality make heterosexual carriers more likely to produce offspring. Put simply, it takes a collection of genetic variants to influence one’s sexual orientation enough to make an individual homosexual.
Men, or women, with only a few of these variants will be heterosexual but will have a number of characteristics which make them attractive to the opposite gender. So men with a few of these variants may be more handsome than normal, they might be kind, compassionate, sensitive, tender… These characteristics are usually the prerogative of females, but many women would give a lot for a man who is drop dead gorgeous, and kind and sensitive on top of that. It can be taken as given that such men will produce offspring. And thus such variants will accumulate from one generation to the next.
At some point the number of variants which influence sexual orientation will increase enough between one generation and the next to tip the balance from an appealing heterosexual individual to an individual with homosexual preferences. A number of studies have shown that this is a viable model but it far from explains the whole picture and there is definitely not enough data available to make this more than a theory at this point in time.
To complicate matters even further, some people have a genetic identity that is the opposite of their anatomical identity. Typically individuals with Chapelle syndrome appear male at birth but are genetically female. Conversely individuals with Swyer syndrome are anatomically female but genetically they are male. In both cases hormone therapy is needed to establish secondary sexual characteristics which develop at puberty. Whilst this might not enter directly into the homosexual debate it highlights the complexity of the human body and further muddies the waters when one tries to define what is ‘normal’ and what is not.
I italicised ‘normal’ on purpose because in science we do not like to use the word ‘normal’ when describing a characteristic. Because, after all, what is normal? Black, brown, red, yellow, grey and white hair is normal… green hair is not. Why? Because one does not usually come across naturally occurring green hair! So if someone developed a variant that gave them green hair, would they suddenly become abnormal? So where does that leave us when considering individuals whose inherited genetic variants or hormonal exposure has predisposed them towards homosexuality? I don’t think there is a clear cut answer to that question, but it is certainly worth thinking about before forming an opinion one way or another.