2012 was a reasonably good year for the International Gay and Lesbian Rights Movement. A number of countries and some states in the US have decided to legalize same sex marriages, and various openly gay politicians have been elected or re-elected for the highest political offices in their countries or states.

These trends mirror the scientific advancement that has been made in 2012 on the biology of homosexuality. The converging findings suggest that exclusive homosexuality is not a “life style choice” but a perfectly natural sexual orientation. While there is still much debate on the exact causes of homosexuality and a “gay gene” remains elusive, biological scientists agree that there is a strong genetic influence to gayness.

First, across various nations and cultures there is a relatively stable minority of people – about 8% -- with an exclusive homosexual orientation. There are twice as many men as women in this category.  Second, there is converging evidence from twin studies that homosexuality “runs in families.” Third and finally, biologists have discovered forms of homosexuality in many other animal species to which humans are related to varying degrees, from baboons to dolphins and from penguins to worms.1

Although these findings make a reasonably strong claim that homosexuality is part of someone’s genotype, there is still much speculation as to how it got there. In an informative and entertaining book, titled “Homo Mysterious”, evolutionary biologist and psychologist David Barash offers a number of different scenarios for the evolution of predominantly male homosexuality, whihc is considered a moe stable trait. Here is a list of the 5 most likely evolved functions of homosexuality in humans.

1. Kin helping

This hypothesis suggests that homosexuality serves the function of providing assistance to siblings and other relatives. By refraining from producing off-spring themselves, homosexuals can invest more time and resources in rearing the offspring of close relatives with whom they share portions of their genes. The evidence for this kin helping hypothesis is mixed, however. A study in the UK comparing homosexual and heterosexual men found no significant differences in their assistance and generosity to kin such as babysitting or willingness to invest resources. Yet, research in traditional societies such as on the island of Samoa found that male homosexuals invest significantly more than male heterosexuals to support their extended family, and that they are fully accepted members of their societies.

Another version of the kin selection hypothesis is that homosexuals (as non-married individuals) occupied high status positons within their societies -- think of the monks and nuns in medieval times -- and so indirect benefits might accrue to their families.

It is thus possible that genes for homosexuality could spread in early traditional societies because of the family benefits they produced.

2. Group selection

It is also possible that homosexuality emerged because it produced benefits for the entire group. One possibility is that groups containing significant numbers of homosexual individuals got fewer children and these groups would be less likely to exhaust available resources such as food and water. Another possibility is that groups with proportionally more homosexuals would function more smoothly, because there would be fewer conflicts over access to mates (males or females). In all fairness, I do not know of any empirical evidence for this group selection hypothesis.

3. Sexual attraction

A third possibility that homosexuality–promoting genes were selected for because of women’s sexual preferences. The argument is that some women may have been favorably disposed towards homosexually inclined men because of their social, cooperative and empathic qualities. Thus, forming partnerships with those men may have ensured better care for children. Research indeed shows that many women find gay men appealing because they are less threatening than (some) heterosexual men who might be openly aggressive and predatory. Again this hypothesis awaits empirical investigation.

4. Balanced selection

A fourth evolutionary explanation for the stable frequency of homosexual genes in human populations is that these genes provide some ulterior benefit. The argument is that although there is negative selection for homosexual genes (because their bearers do not produce as much offspring as individuals with heterosexual genes) there may be compensating benefits. For instance, if the genes for same sex preferences would also cause an individual to have a greater intelligence or a better physical health then there would be compensating benefits, leading to the spread of homosexuality genes in a population.

5. Sexually antagonistic selection

A final possibility is that homosexuality genes might produce different effects for males versus females. It could be that when homosexual genes reside in male family members this would result in them having fewer off-spring. Yet when these same genes reside in the female family line they could result in them getting more offspring to compensate for the loss of fitness in males. There is some support for this. One study found that the mothers of homosexuals had, on average, more children than mothers of heterosexual children. And the family members of the mothers’ line in homosexuals also sired more offspring. A review study just published in the Quarterly Review of Biology provides further support for this hypothesis. It suggests that particular (epigenetic) mechanisms that suppress androgens in female fetuses -- that enable them to grow into more feminine bodies with feminine brains -- also suppress androgens in male fetuses, which has the side effect of turning them into less masculine men. If these feminine women do better than the average female in getting more children such a mechanism could result in the propagation of homosexual genes.

These are some of the evolutionary hypotheses that are out there to support the relatively well-established scientific claim that homosexuality is a natural (normal) sexual orientation. More research is needed and hopefully in 2013 scientists will come closer to solving the mystery of homosexuality.

The biological science of homosexuality suggests that rather than discussing how we feel about homosexuals and how society should treat them, we should ask ourselves why homosexuality exists and what its functions are (or were).

1. Barash, D. P. (2012). Homo Mysterious: Evolutionary puzzles of Human Nature. Oxford University Press.

Follow me on Twitter: @ProfMarkvugt

About the Author

Mark van Vugt, Ph.D.

Mark van Vugt, Ph.D., is a professor of social and organizational psychology at the VU University Amsterdam and a research associate at Oxford University.

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