The Importance of Being Genetic

Before I get into the possible whys of homosexuality, it’s probably best to go into the topic of genetics.

(In the following an alteration needs to be made; I'd hope to know if it’s clear before you get to the end :-))

I first want point out how difficult it is to understand who we’re talking about in these studies. Are they people who identify as homosexual or choose homosexual actions? As Perelle et al. point out in their review, An International Study of Sexual Orientation: The Data (1):

“there is no agreement among researchers as to who can be considered a homosexual person, what is the etiology of homosexuality, or what the proportion of homosexuality is in the world's population”

By defining people by the actions they choose, we do find that just under 10% of the US population is homosexual (2). With such a high occurrence of homosexual behavior, one should expect a genetic role in the associated orientation. This fact has been readily accepted and without controversy, for many decades. As Rief explains way back in 1939 (my emphasis) (3):

“WHILE the occurrence of hetero- and homo-sexuality does not appear to conform to any simple Mendelian formula [a simple genetic mode of inheritance], the familial incidence of homosexuality rather definitely indicates a genetic basis.”

More recently, in a review by Orlebeke, 1996, we see this observation repeated with much supporting data (4). Simply, homosexuality clearly runs in families, and no serious researcher disputes that.

But finding the extent and mechanism of the involved gene or genes has been a bit of a problem. To do this and better separate nature from nurture, twin studies have typically been used.

Firstly, similar to what we saw in my past posts (here and here), gay twins also show a birth weight effect, as reported by Orlebeke et al. in a study of 1,700 twin pairs (4). They report “very low” birth weight correlated with homosexuality, and theorized that the “possibility that exposure to prenatal…hormones” made these children gay. These finding were repeated by James et al. in 2002 (5). It should also be noted that same-sex twins were more often gay than opposite sex twins (6).

But back to the genes in twin studies.

On the general topic, Sicotte et al., in 1999, did an excellent job of reviewing and compiling 28 studies, which represented almost 10,000 twin pairs (!), in “the largest meta-analysis of twins and singletons conducted to date” (I, of course, see no need to go into each study separately :-)) (7). They found, in part, for dizygotic twins (fraternal, not sharing the exact same genes) that if one twin were gay the other had a 9.4% chance of being gay themselves (choosing gay activities). This is about the same as the background, thus suggesting no upbringing effect, as the twins were raised together.

But, for monozygotic twins (twins who do share the same genetic code), the other twin had a 14.9% chance of being gay. So clearly they found a genetic effect. When a child shares the same genes as his brother, he’s about 1.6 times more likely to be gay himself.

Also clearly, with only a 15% concordance, genes are not the whole story. It’s clear this is where effects such as hormone exposure, immune response, and prenatal experience come in. Regardless, the authors conclude (my emphasis):

“Although the frequent occurrence of monozygotic twins who are discordant for sexual orientation is clearly inconsistent with genetic models that would define hetero and homosexuality as dominant or recessive traits, discordance among monozygotic twins does not automatically preclude straightforward inheritance of one or more genes that strongly influence sexual orientation.”

Sure, there are many discordant identical twins who do not share their brother or sister’s orientation, and there are some interesting findings regarding the non-gay sibling (8). Still, discordance, gratefully, causes no one to rule out a strong genetic effect or claim homosexuality is a chosen lifestyle.

But the genetic cause must be complicated, and a handful of studies bring to question the results of Sicotte, questioning the existence of any significant genetic cause. For example Derom, was unable to find any significant difference between monozygotic and dizygotic twins using 808 twin pairs (9), and a similar finding was reported by Ross in 1999 with over 2,000 twin pairs (10). Simply these two studies were unable to find any statistically significant difference between the appearance of homosexuality in fraternal and identical twins.

This apparent inconsistency in the data, coupled with the low concordance found even in studies purporting it, has lead prominent researchers in the field to refer to twin studies as the Achilles heal of the hypothesis of there being a genetic cause of homosexuality (11). Fortunately it really doesn’t matter much, not to homosexuals, not to the general public, not to politicians. While, in my dad’s day, they’d be persecuted for their behaviors, even by public school teachers in front of the whole class, no kid questions or tries to alter his orientation these days. It’s really only of interest to biologists and psychologist who care to study such things. And among them, even those questioning the genetic involvement know there is a biological reason involved and it would take willful ignorance and a denial of one’s own experience with sexuality to think homosexuality is a matter of choice.

…

Okay, enough. I must confess I stole this idea from a book I read a long while ago (but I put a lot more work into it :-)). In short, the above in this post only is false, and I hope you can see the reason I did it this way. None of those studies refer to what I say they do. To make it true, though, is quite easy: replace all references to sexual orientation with handedness, and those to homosexuality with left-handedness.

I’ll wait….

There, now this post true; look at the references if you don’t trust me ;-). But read it again and see how it sounds, all true, but now minus the political, social, and religious baggage we all, both sides, carry into the topic of homosexuality and issues of “born that way”.

As we’ll see in a future post, for a genetic role in being gay a shocking amount of the above applies (for example, the birth weight findings again), and where it deviates from handedness it deviates into the direction of homosexuality being more a matter of genes than handedness (perhaps surprising with the apparent difference in effect the two have with regards to reproduction). Both the biological parallels and differences are surprising to me, not to mention the political and social differences (which I just mentioned ;-)).

Again, I hope your trust doesn't feel betrayed in my trying to illustrate the importance of being genetic :-).

1. Perelle, I. and L. Ehrman (1993). "An international study of human handedness: The data." Behavior Genetics 24(3): 217-227.
2. Estevez-Gonzalez, A., C. Garcia-Sanchez, et al. (1996). "Neuropsychology of left-handedness: current knowledge." Revista de neurologia 24: 515-522.
3. Rife, D. (1939). "HANDEDNESS, WITH SPECIAL REFERENCE TO TWINS." Genetics 25: 178-186.
4. Orlebeke, J., D. Knol, et al. (1996). "Left-handedness in twins: genes or environment?" Cortex 32(3): 479-490.
5. James, W. and J. Orlebeke (2002). "Determinants of handedness in twins." Laterality: Asymmetries of Body, Brain, and Cognition 7(4): 301-307.
6. Carter-Saltzman, L., S. Scarr-Salapatek, et al. (1976). "Left-handedness in twins: Incidence and patterns of performance in an adolescent sample." Behavior Genetics 6(2): 189-208.
7. Sicotte, N., R. Woods, et al. (1999). "Handedness in Twins: A Meta-analysis." Laterality: Asymmetries of Body, Brain, and Cognition 4(3): 265-286.
8. Gurd, J., J. Schulz, et al. (2006). "HAND PREFERENCE AND PERFORMANCE IN 20 PAIRS OF MONOZYGOTIC TWINS WITH DISCORDANT HANDEDNESS." Cortex 42(6): 785-970.
9. Derom, C., E. Thiery, et al. (1996). "Handedness in twins according to zygosity and chorion type: A preliminary report." Behavior Genetics 26(4): 407-408.
10. Ross, D. C., J. Jaffe, et al. (1999). "Handedness in the NAS/NRC Twin Study." Laterality: Asymmetries of Body, Brain, and Cognition 4(3): 257-264.
11. Corballis, M. (1997). "The genetics and evolution of handedness." Psychological Reviews 104(4): 714-727.