There is a simple question the anti-gay ask: If a gene creates a homosexual orientation that decreases human reproduction. Wouldn’t evolution weed out this unproductive gene?
Logical as that question seems, the answer is not that hard if we remember the obvious fact that each one of us adds more to humanity’s success than just our ability to reproduce. Women contribute more than the fruit of their wombs, and men contribute more than their sperm. To think otherwise is daft. Put even more simply, the overal survival of humanity is predicated on more than just our birthrate.
To provide the simplest example, it is not the number of babies that determines reproductive success in a species, it is the number of children who successfully make it to adulthood and are able to have children of their own. Any trait that benefits the net success of reproductively viable adults should thrive.
More specifically, genetic scientists Gavrilets and Rice found three possible reasons for the persistence of a genetic origin for homosexuality:
Answer #1: Overdominance
Overdominance says that if there is a gay gene, having two parents with that gene may make a child gay and decrease their reproduction, while getting the same gene from only one parent may increases that child’s reproductive success, resulting in increased reproduction over time.
We know this dynamic from Sickle Cell Anemia, where the gene from one parent makes you immune to malaria, but the gene from two parents gives you a crippling disease. A gene for homosexuality might work the same way.
Answer #2: Sexually antagonistic selection
This theory says that the same gene has different effects depending on the child’s gender. The gene that makes a brother gay and less sexually reproductive, may make his sister more reproductively successful, for a net increase.
The sexually antagonistic selection theory gains added credence when we note the sisters of gay men do, in fact, have more children than women who do not have gay brothers. The gay gene, or more likely, combination of genes, appears to be a net positive for the family.
There is also increasing evidence that male homosexuality may be have stronger genetic origins
Answer #3: Kin altruism
I personally like this one, as kin altruism posits that a gay gene may run in families, and the presence of a gay family member helps the overall reproductive fitness of the family, so the gene perpetuates.
This one is the most obviously logical, without the genetic complexities of the previous two. Clearly having a few confirmed bachelor uncles and spinster aunts around can be a helpful for large families, especially in stressful times. A related theory from the social sciences says that families benefit from having a certain percentage of spare adults around who are not tied to specific children.
Hard for us to imagine in our times when family means nuclear family, but families used to be large and complex social systems of interrelated people that benefited from a certain amount of diversity as they worked together for the common good.
- S. Gavrilets and W.R. Rice, Genetic models of homosexuality: generating testable predictions, Proc. R. Soc. Lond. B Biol. Sci. 273 (2006), pp. 3031–3038.