If Gallup polls are anything to go by, half of Americans, and that includes Lady Gaga, believe that homosexuality is innate. A third put their money on what seems to be the opposite, which is that being gay is due to upbringing and environment. Finally, one in ten out themselves as pacifists in this particular culture war, crediting both nature and nurture. The claim that homosexuals are born gay, which philosophers have dubbed “nativism,” is on a high, at least compared to its antithesis, known as “environmentalism.” Nearly half a century ago, the very same Gallup poll, asking the very same questions, revealed a very different set of beliefs, with 13% of the respondents providing nativist and 56% environmentalist answers.

What Gallup never asked its participants, however, is what they mean by saying that homosexuality is innate, and where they hope to find the evidence for it. Many nativists seem to place their hopes in science, and sure enough, still according to the Gallup polls, the heyday of biological research on homosexuality, the 1990s, introduced both a rise in nativism and a decline in environmentalism. It seems, then, that we have two questions to answer. First: what is innateness? Second: is there scientific evidence that allows us to conclude that homosexuality is innate?

In the past two decades, philosophers have identified a number of features structuring the concept of innateness. One of these features is fixity. What is fixity? A trait is fixed if it “is hard to change; [if] its development is insensitive to environmental inputs in development; [and if] its development appears goal-directed, or resistant to perturbation.”[i] Many people think of homosexuality as something that science has shown to be very hard to change, and they call it “innate” for precisely that reason. Conversely, many websites devoted to “curing” homosexuality appear to deny that it is innate. Curing is, after all, a way of altering characteristics.

In the context of homosexuality, innateness seems to revolve around unalterableness or fixity, so the question is whether there are bona fide scientific concepts that allow us to measure the fixity of homosexuality. One such concept stands out: canalization.

In the early 1940s, the geneticist and developmental biologist Conrad Waddington coined the term “canalization” to capture the developmental fixity of a trait. More recently, and building on Waddington’s work, the philosopher André Ariew defined innateness as canalization and canalization as “the degree to which the developmental process [of a trait] is bound to produce a particular end state despite environmental fluctuations both in the development’s initial state and during the course of development.”[ii] For example, even in widely different environments, most adolescents grow pubic hair. However, some adolescents do not, including those who have an intersex condition like complete androgen insensitivity syndrome. In Ariew’s view, pubic hair is an innate characteristic in most humans, and the absence of pubic hair is innate in some intersex people.

Can canalization help us make sense of the claim that homosexuality is innate? Intuitively, yes. In Die Homosexualität des Mannes und des Weibes (1914), Magnus Hirschfeld already argued that homosexuality is innate because it also occurs in societies that are vehemently homonegative. Societal norms thus seem quite unable to alter sexual orientation[iii]. Moving beyond intuition, however, there are no scientific studies that directly measure canalization. Moreover, it is highly unlikely that such studies will become available in the future, as there are good moral reasons not to bring human developmental processes under experimental control. Of course, there is a lot of biological research that seeks to identify biological causal factors in the development of homosexuality, and some of this research bears indirectly on the issue of canalization. Maybe somewhat surprisingly, however, such research suggests that homosexuality is not strongly canalized. Here are two examples.

First, recent genome-wide association studies (GWAS) on homosexuality have identified a number of genes that are likely to affect the probability of its development. In the context of homosexuality, as in so many other contexts, genes fulfill their regular role as a probabilistic cause, increasing the odds for a given individual to become homosexual. It is perfectly possible, however, for a man to have all these identified genes and still be heterosexual. GWAS provide evidence against the strong canalization of homosexuality as they suggest that the environment plays at least some part in determining whether a given genotype develops into a homosexual or a heterosexual phenotype. Some genotypes are only a bit more likely to result in homosexual phenotypes, but as far as science currently knows, there is no genotype that results in a homosexual orientation in all natural environments of our species.

Second, there is the fraternal birth order effect (FBOE). One of the few reliable findings in contemporary research on male homosexuality, the effect holds that the number of older brothers is a good predictor of the sexual orientation of their later-born brothers. In other words, the odds of a man being homosexual increase with the number of older brothers he has[iv]. The effect occurs only with older brothers, not with older sisters, younger brothers, or younger sisters. Interestingly, research shows that the effect occurs only with biological brothers, not stepbrothers or adoptive brothers, suggesting its origins are to be found in the mother’s uterine environment, rather than in the family dynamics. More particularly, FBOE-researchers hypothesized that some forms or instances of male homosexuality are due to the progressive immunization of some mothers to the antigens produced by their successive sons during the first months of pregnancy. This immunoreaction then disturbs the sexual differentiation of the fetal brain, causing a deviation from the male-typical pattern of neurodevelopment. One result of this deviation would be that later-born sons sexually prefer men to women.

If this hormonal hypothesis makes sense, and if we include maternal wombs in our list of environments, then it seems fair to conclude that the fraternal birth order effect questions the strong canalization of male homosexuality. This conclusion may seem bizarre at first, because it involves a research domain that focuses on factors colloquially called “biological,” such as neurons and hormones, while at the same time providing munition against the nativist view of homosexuality. However, these biological factors include the fetus’s physiological response to a particular environment, that is, its mother’s womb. If FBOE-researchers are right, environmental fluctuations do impact the development of sexual orientation, which implies, at least according to Ariew’s definition of canalization, that homosexuality is not strongly canalized, and therefore not strongly innate.

One way to avoid this conclusion is to argue that Ariew’s definition is too strict, as it only applies to genotypes. If, for example, a mother’s diet or hormonal reactions would determine (some of) her sons’ homosexuality, then Ariew would not consider such sexual orientation to be innate. Why not loosen up the concept of canalization and redefine it as a more generic kind of developmental fixity? In that case, the fraternal birth order effect would prove homosexuality to be innate, or wouldn’t it?

Not quite. First, researchers have always been very open about the explanatory strengths and limitations of the fraternal birth order effect. In a recent meta-analysis, they repeat that “only about 15–29% of the men in any given homosexual group can attribute their sexual orientation to the FBOE.”[v] Many male homosexuals are firstborn or only children, or only have older sisters and no older brothers, and so their sexual orientation cannot be explained by their fraternal birth order or the putative underlying mechanism. We should be cautious in our claims, then: some research shows some kinds of homosexuality to be innate. Second, we cannot yet exclude the possibility that particular social or other environments are necessary for the fraternal birth order effect to kick in, as is the case with so-called “gay genes.” One prominent biological expert on homosexuality recently emphasized that it is indeed quite possible that “all the biological factors… only produce a predisposition to become homosexual, and [that] these predispositions can only develop in a specific set of psychosocial contexts that are not yet identified.”[vi] It remains to be seen whether the birth order-related kind of homosexuality is indeed strongly canalized.

But even if that turns out to be the case, another problem arises. If we focus on the fixity feature of innateness, and “operationalize” fixity as canalization, then it should be clear that we are only dealing with developmental fixity – the immutability of a trait during its development. Developmental fixity, however, does not always imply overall fixity. Some traits may be canalized, but not immutable after they have developed. This may seem like academic niggling, but it is not. There is a strong connection between the innateness debate and the debate about the value and efficacy of conversion therapy. Nativists often appeal to homosexuality’s innateness to oppose conversion therapy. If, however, innateness is nothing but developmental fixity, then it is of no use to them, for it may well be the case that homosexuality cannot be changed during its development, but that doesn’t mean it cannot be changed afterwards. Proponents of conversion therapy insist indeed that they can and do help homosexuals to become heterosexual, even though they must recognize, perhaps grudgingly, that they cannot prevent homosexuality from emerging altogether.

To be clear, we do not deny that sexual orientation is a feature that scores high on fixity, in that it is really hard to change once it is there. The same applies to heterosexuality, for that matter. As Sigmund Freud once put it: “In general to undertake to convert a fully developed homosexual into a heterosexual is not much more promising than to do the reverse, only that for good practical reasons the latter is never attempted.”[vii] We could not agree more. Much against Freud’s advice, however, psychoanalysts and psychiatrists have seriously harmed homosexuals by attempting to treat them, both psychologically and physiologically (Drescher et al. 2016). What we worry about is that innateness is too confused a concept to do the emancipatory work we want it to do. If it’s fixity or immutability that we are looking for, why not talk about canalization and other kinds of fixity, rather than innateness?


***Adapted and excerpted from Of Maybugs and Men: A History and Philosophy of the Sciences of Homosexuality by Pieter R. Adriaens and Andreas De Block, published by the University of Chicago Press. © 2022 by The University of Chicago. All rights reserved.***


[i] Griffiths, Paul, Edouard Machery, and Stefan Linquist. 2009. ‘The Vernacular Concept of Innateness.’ Mind & Language 24 (5), p. 609.

[ii] Ariew, André. 1999. ‘Innateness Is Canalization: A Defense of a Developmental Account of Innateness.’ In Biology Meets Psychology: Conjectures, Connections, Constraints, edited by Valerie Hardcastle, p. 117. Cambridge, MA: MIT Press.

[iii] Similarly, Arthur Schopenhauer argued that the universality of homosexuality is a proof of its innateness: “The complete universality and persistent ineradicability of the practice prove that it stems in some way from human nature itself.” (Confusingly, Schopenhauer also considered homosexuality “an unnatural monstrosity,” but that’s another story.) See Schopenhauer, Arthur. 2018. The World as Will and Representation. Edited by Christopher Janaway. Translated by Judith Norman and Alistair Welchman. Cambridge: Cambridge University Press, p. 577-578.

[iv] See, e.g., Blanchard, Ray. 2018. ‘Fraternal Birth Order, Family Size, and Male Homosexuality: Meta-Analysis of Studies Spanning 25 Years.’ Archives of Sexual Behavior 47:1-15.

[v] Ibid., p. 9.

[vi] Balthazart, Jacques. 2011. The Biology of Homosexuality. Oxford: Oxford University Press, p. 159.

[vii] Freud, Sigmund. 1955. ‘The Psychogenesis of a Case of Homosexuality in a Woman.’ In The Standard Edition of the Complete Psychological Works of Sigmund Freud, edited by James Strachey, 18:145–72. London: Hogarth Press, p. 151.