Gay genes questioned by university biologist

December 9th, 2011 

A university biologist who contributed to the Anglican Communion and Sexuality book for Lambeth 2008 says  “a thoroughgoing review of the contributions of various psychosocial factors to homosexuality is urgently needed, as this was a glaring omission from The Anglican Communion and Homosexuality and “The claim that homosexuality is “mostly genetic” in origin is scientifically unfounded.”

Letter to Church of England Newspaper from The Rev Dr David de Pomerai, Associate Professor in the School of Biology, University of Nottingham


Michael Davidson’s response (25 November) to Stuart Walton’s article quotes from the Lambeth 2008 book The Anglican Communion and Homosexuality to the effect that the search for genes related to homosexuality “has lessened with the weakness of genetic links to homosexuality implied by recent twin studies.” As the author of that quotation, I would like to reaffirm it and to explain how the scientific data may sometimes be misunderstood.

I tabulated (on p 277) a summary of recent twin studies – an important tool in trying to distinguish between the influences of nature and nurture. The important study of Bailey et al (2000) found male identical twins to be between 20 per cent and 38 per cent concordant for homosexuality, meaning that if one is homosexual, there is a 20-38 per cent likelihood that the other will be also. This seems like ‘almost 40 per cent’, but these numbers may mislead the unwary. The 20 per cent figure represents three out of 27 pairs (as Michael Davidson noted). Of course 3/27 is one in nine, or 11.1 per cent, so how does it become 20 per cent? A statistical concept called ‘probandwise concordance’ requires that each concordant pair be double-counted, since twin A is concordant with twin B and vice versa. So (3+3)/(27+3) = 20%. The 38 per cent figure is calculated similarly, but using a looser definition of homosexuality (1 or more on the Kinsey scale – which, arguably, is not really homosexual at all). So, the straightforward (‘pairwise’) concordance is just 11.1 per cent — near the opposite end of the scale from Stuart Walton’s 100 per cent aspiration.

The claim that homosexuality is “mostly genetic” in origin is scientifically unfounded. Indeed, Bailey said his study “did not provide statistically significant support for the importance of genetic factors for sexual orientation.” There is no ‘gay gene’ as such, and media reports to this effect have been very misleading. Other recent studies vary in their estimates of the percentage contribution of genetic factors towards human homosexuality, but the overall consensus is between 10 per cent and maybe 25 per cent (at most). This is very substantially less than the contribution of genetic factors towards conditions such as autism, but even the lower figure quoted is roughly 2 to 3 times greater than the prevalence of homosexuality in the general population (3-5 per cent). Thus the genetic influence is not zero (ie 100 per cent nurture, as Davidson seems to imply), but neither is it substantial – let alone the 100 per cent (all nature) view suggested by Walton.


Very similar arguments could be made for most of the other “nature explanations” reviewed in my 2008 article – involving maternal hormonal effects on the unborn foetus, fraternal birth order, or differences in pheromone perception. The evidence for all of these biological “nature” explanations is weak, but neither is it so negligible as to be dismissed out of hand. It seems more likely that different nature factors contribute in varying measures to homosexual tendencies, and that nurture factors play at least as important a role (possibly more so;
see below).

This is inconvenient for a simple Christian theology in relation to homosexuality; there may well be individuals for whom homosexuality is a lifestyle choice, others who have been affected by childhood trauma, and yet others who can legitimately claim that they were “born that way”, literally by nature.

Moving now from nature to nurture, I must state that this is not my field and I shall not comment on specific factors, but it is evident that environmental influences are important in the development of sexual orientation. It is not within my competence as a biologist to explore this area (my remit was solely to review the biological factors contributing to homosexuality), but a thoroughgoing review of the contributions of various psychosocial factors to homosexuality is urgently needed, as this was a glaring omission from The Anglican Communion and Homosexuality. Nature has been extensively analysed; nurture has not. Until that gap has been filled, there is a grave danger that Anglicans may find themselves misinformed about this key topic in relation to homosexuality.

Finally, I should note my own frustration as a scientist at the sloppy way in which terms like “homosexual” are used, even in reputable scientific journals. Like Humpty Dumpty, it seems that authors can get away with meaning whatever they like by this term – ranging from ‘those who engage actively and exclusively in homosexual practices’ to ‘those who might, at some stage in their lives, have entertained the occasional homosexual fantasy’. Few studies make use of the Kinsey scale (published over 60 years ago; the Bailey article cited above is one of the few that does), in part because such subdivision of their homosexual sample group would expose the perilously low sample sizes involved (Bailey’s three out of 27, as cited above, is by no means atypical)! You don’t need to be a biostatistician to see the perils of basing conclusions on a few actual cases.

To a considerable extent, different definitions of homosexuality underlie a great many contradictions in the literature. To cite but one example, the study of Bearman and Bruckner (2000, American Journal of Sociology) is widely held to establish the primacy of nurture factors in homosexuality, by systematically undermining all of the purported nature factors (twin concordance, fraternal birth order etc), on the
basis of impressively large sample numbers.

Unfortunately, this critique is itself vitiated by the very broad definition of homosexuality used; it is not at all clear whether these same conclusions would apply equally to the minority of exclusive homosexuals within that sample.

In conclusion, suffice it to say that scientific studies in this area have so far failed to establish any single biological factor as the major cause of human homosexuality. As far as I am aware, there is similarly no overwhelming evidence for any single psychosocial factor either. There is, perhaps, a reluctance to engage with the nurture issues for fear that this would label homosexuality as purely a matter of (sinful?) choice. However, nothing could be further from the truth.

Arguably, exposure of a foetus to high or low levels of testosterone within the womb is more of a nurture rather that a nature factor – since it is the mother’s rather than the foetus’ own genes that are responsible. Many other possible nurture factors (such as childhood trauma, or homosexual abuse, or even early childhood play preferences) are equally not subject to the rational choices of the individual who may (perhaps – in some cases) subsequently develop homosexual tendencies. We should beware of the false equation that nurture = choice. A multiplicity of nature and nurture factors (inconvenient as this may be theologically) seems much more likely.

If so, it behoves Christians in general, and Anglicans in particular, to tread warily in this area. I have considerable sympathy with Stuart Walton’s plea for tolerance and respect, particularly since some (but not all) homosexuals may indeed be born that way. That said, we are each of us responsible before God for how we choose to act on our natural impulses and inclinations (as Michael Davidson points out). We cannot know how Jesus would have dealt with this issue, but we can at least learn from his handling of the case of the woman taken in

The Rev Dr David de Pomerai,
Associate Professor in the School of Biology,
University of Nottingham.
Associate Minister, Walcross United Benefice
and Science Adviser, in the Diocese of Derby

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