Research Article

Sexually Antagonistic Selection in Human Male Homosexuality

  • Andrea Camperio Ciani mail,

    Affiliation: Dipartimento di Psicologia Generale, Università di Padova, Padova, Italy

  • Paolo Cermelli,

    Affiliation: Dipartimento di Matematica, Università di Torino, Torino, Italy

  • Giovanni Zanzotto

    Affiliation: Dipartimento di Metodi e Modelli Matematici per le Scienze Applicate, Università di Padova, Padova, Italy

  • Published: June 18, 2008
  • DOI: 10.1371/journal.pone.0002282

Reader Comments (4)

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Fraternal Birth Order Threatens Research into the Genetics of Homosexuality

Posted by tanyakh on 21 Jul 2008 at 15:36 GMT

The theory that the probability that someone will be gay is dependent on the fraternal birth order impacts in several interesting ways on the whole field of research into the genetics of homosexuality.

Let us create a theoretical model where homosexuals would be born randomly with a fixed probability if they are the first sons of a woman, and increasing probability for subsequent sons. In this model there would be two interesting consequences:

* Mothers of homosexual men would be more fertile on average than mothers of men in general. Remember, this random consequence can be misinterpreted as a genetic correlation between homosexuality and fertility.
* If a homosexual person has a brother, then the probability that this brother is homosexual might be very different from the probability that a random person is homosexual. Again, this might create the suggestion that there is a genetic component, when it is not there.

More details in my blog entry: http://blog.tanyakhovanov...

RE: Fraternal Birth Order Threatens Research into the Genetics of Homosexuality

sigfridciani replied to tanyakh on 24 Jul 2008 at 06:21 GMT

Dear Tanyakh,
the question you posed is interesting, and is related to the fact that rare traits (as homosexuality) have a higher probability to appear in larger families were there are more chances just because there are more cases. This possible fallacy of the genetic interpretation was adressed by me and other colleagues in detail in a previous work in 2004, and quoted in the target article. Yes, as a first view, your argument works perfectly: it could be just a false correlation due to the sampling method. We used to put it in this way, if there is a mother with a million sons and a mother with only one, in the whole population, then there are a million more probabilities that a homosexual is born in the million son family than in the single born one.
However this fallacy was disproved in our first paper:
There are two answer solving this dilemma.
The first was suggested by my daughter Giorgia age 15 at the time that was helping now and then with the research. She said that if this (the fallacy) is true for homosexual it should be true also for priest, or students following a special course at university and so on, because all these are rare individuals in a large population with higher chances to be appearing in families with a lot of offsprings. We verified it and found it is true: priest come from larger families, students of special course also and so on.... this is true always when you sample a small population within a larger one,as homosexuals in the population at large. So, Giorgia suggested, it is not correct to compare fecundity of the mother of homosexuals with the population at large but with a small population with equally rare traits, exactly as we did in our studies, following her suggestion. Then, when controlling for rarity of the trait, If the difference in fecundity (family size) persist then it should not be imputable to the sampling method but has underling biological meaning to be studied....
The second answer might be more satisfing for your direct question, and it is that your argument is no more true when we find, as we did in a very significant way, higher fecundity in the aunts (maternal) of the homosexual subject, but these aunts do not have any homosexual in their sons (except a little portion) but still produce more offsprings that the control sample. This cannot be due by any sampling of increase probability in fraternal order effect given that these females do not have at all homosexual subjects but still are more fecund, this can be only explained by a biological fact promoting fecundity, running in the family as homosexuality is running in the family, hence we first derived a real possibility of a genetic factor. I hope this answer satisfies your question please let me know your comments. Thank You Andrea camperio Ciani