Homosexuality has an evolutionary function

The Darwinian Paradox of Homosexuality

One of the achievements of the last century is the removal of taboos on homosexuality. Little is known about their causes, however. From a biological point of view, two theories are in the foreground, which at least partially explain the male variant.

For certain sections of society, homosexuality is still a sensitive issue. For evolutionary biology, on the other hand, it is above all fascinating: How can it be that such behavior, which is conceivably unfavorable for one's own reproduction, can persist? Because purely homosexual women and men have no offspring, they are at a dead end from an evolutionary point of view: their genes are not passed on and their line is dying out.

Evidence from twin studies

Genetic studies on homosexuality are still rare. Since the 1990s, however, twin studies have shown that homosexuality is at least partially genetic. For example, in identical twin pairs, in which one of the twins is homosexual, the brother or sister is also homosexual or lesbian in about 30 percent of the cases, while this occurs in only about 10 percent of the cases in the case of dizygotic twins. This is due to the fact that identical twins have identical genes.

Such a genetic predisposition to homosexuality is difficult to reconcile with Charles Darwin's theory of natural selection. This means that only genes which are beneficial (or at least not disadvantageous) for the survival or reproduction of their carrier persist in a species. According to this theory, «gay or lesbian genes», which are obviously detrimental to the reproduction of their carriers, should not occur at all. They should have already been eliminated by natural selection. In this context one speaks of the Darwinian paradox of homosexuality.

How this paradox could be solved, at least for male homosexuality, was shown by Andrea Camperio-Ciani from the University of Padua with a team of psychologists for the first time in 2004. The scientists in Italy had asked numerous homosexual and heterosexual men about their relatives. She was particularly interested in how many other cases of homosexuality occur in the respondents' families and whether these are more likely to be found on the mother's side or on the father's side. As it turned out, the maternal kinship of homosexual men had significantly more cases of male homosexuality than the paternal and also the maternal side of heterosexuals. Above all, however, it was shown that the female ancestors on the maternal side of homosexual men gave birth to more children on average than the female ancestors on the paternal side or those of heterosexual men.

Positive influence on fertility

If one now assumes the existence of a homosexuality gene, then such a gene would take on different functions, depending on whether it is in a woman's or in a man's body. In women, this gene would have a positive influence on fertility, while in men, at least in certain cases, it led to homosexuality. There would therefore be a conflict of interest, a so-called “trade-off”, between maternal fertility and male homosexuality: the more fertile the mother, the greater the probability that her offspring will also have a homosexual son. The aforementioned evolutionary impasse, in which purely homosexuals find themselves as individuals, would thus be lifted in the family by this higher fertility on the maternal side. That in turn means that your own line continues, at least indirectly.

Camperio-Ciani's study has since been confirmed several times by him and two other research teams from London. A statistical analysis published two months ago also showed that this theory of female fertility can explain almost all of the findings of the documented studies on male homosexuality. How exactly a "homosexuality gene" could develop its effect, however, is still in the dark. There are only indications of its location. Because such a gene is always inherited through the mother's line, it could be on the X chromosome inherited from the mother's side. However, since one of the few larger-scale studies on the homosexual genome in 2005 came to the conclusion that homosexual men and their mothers not only have a typical section on the X chromosome, but also on other chromosomes, the localization of such a gene remains not clear.

The order of births

The other common explanation for male homosexuality today is provided by Canadian Ray Blanchard of the University of Toronto Psychiatric Institute. According to his theory, being gay is related to birth order. Using several studies dating back to the 1980s, he was able to show that the greater the number of older brothers, the greater the likelihood of a man being homosexual: it is up to 30 percent in the second-born and 40 percent in the case of third-born sons. Blanchard explains this with an immune reaction from the mother. Since half of every fetus is made up of paternal genes, half of it is basically a foreign body in the womb; this strangeness is even more pronounced in a male fetus. According to Blanchard, every male fetus triggers a defense reaction in the womb using substances whose blueprints are determined by genes on the Y chromosome. This in turn influences the development of the fetus, especially the development of its growing brain. As with a vaccination, such an immune reaction would be modest in the first son, but would increase in every further male offspring.

There are several molecular candidates for substances defined by the Y chromosome, above all gender-specific hormones or molecules on the cell surface could play a role. Blanchard himself refers to a work from 1987 in which Indian researchers demonstrated that female mice immunized with male spleen cells produced male offspring who were not sexually active. However, because a lack of sexual activity is not enough to call a mouse “homosexual” and because this work has never been repeated and no corresponding substance has yet been found, the existence of such a molecule remains questionable.

Little about female homosexuality

In principle, both theories could also apply to female homosexuality, but there is no evidence for this. For example, family studies in homosexual women could not show any increased fertility in their relatives either on the maternal or on the paternal side. The order at birth does not seem to matter either. The specialist literature shows large gaps here, which is surprising in view of the similarly frequent occurrence of female homosexuality.

As far as male homosexuality is concerned, Blanchard's theory of fraternal birth order and Camperio-Ciani's theory of female fertility are by no means mutually exclusive, especially since mothers who bear several sons can also be considered fertile. Neither do the two theories contradict a possible influence of childhood and adolescence on sexual orientation. It is more likely that genetic and prenatal as well as postnatal factors play a role and that existing predispositions to homosexuality are accentuated to different degrees by later experiences. Interestingly, this would come close to Sigmund Freud's postulate that all people are bisexual from birth.