5X5 Episode 26
|5X5 Episode 26
|Sexuality: Genetic vs environmental contributions
|29th June 2008
|S: Steven Novella
|R: Rebecca Watson
|B: Bob Novella
|J: Jay Novella
|E: Evan Bernstein
New study sheds light on genetic vs environmental contributions to sexuality
Voice-over: You're listening to the Skeptics' Guide 5x5, five minutes with five skeptics, with Steve, Jay, Rebecca, Bob and Evan.
S: This is the SGU 5x5 and the topic for this evening is a new study which shows that homosexual behavior is due partly to genetic and partly to environmental factors. This is a study published in the scientific journal "Archives of Sexual Behavior". Researchers from the Queen Mary School of Biological and Chemical Sciences and the Karolinska Institute in Stockholm. A report on the results of a very large twin study. Now, a lot of twin studies are done in Sweden because they have an excellent database of basically everyone in Sweden, so it's the - the data is easy to get to, it's easy to do these kinds of studies. That's why you're always going to see these genetic twin studies coming out of Sweden.
B: Also, twin studies is uniquely suitable for teasing out the difference between genetic contributions and environmental contributions.
S: That's right, and that's exactly what they did in this study. They looked at 3826 same gender twin pairs. They looked at both fraternal and identical twins. So, identical twins share 100% of their genes and 100% of their environment. Fraternal twins share 50% of their genes and 100% of their environment. So they were able to then do statistical analysis to say how much relative influence is genetics playing on a behavioral measure--like a self-reporting behavior measure--of sexuality, of homo- versus heterosexuality and how much of the - did the environment play.
R: One thing I think it's important to point out is that when you're talking about environment in this case, it's not necessarily - you're not talking about the way they're raised, like family or parenting things, but you're talking about fetal development and biological factors like that, right?
J: Oh, because I actually read it that way, Rebecca, I really did.
R: Yeah, it-
R: -can be kinda confusing if you don't.
R: And I think that's kind of the important thing of this is that it sorta - doesn't it sorta rule out the idea that your family life plays a role in determining whether or not you're a homosexual?
S: Well, actually you are correct in that the environmental factors that they're talking about includes the environment of the womb as well as other biological factors, although it doesn't rule out other environmental factors, so it could include the home situation, for example.
J: They're not mutually exclusive.
S: Yeah, so it just - all environmental factors were kind of pooled together in the way this analysis was done; they really didn't have a way of splitting apart biological environment versus social environment or family environment.
J: But Steve, was there anything determined about the environment of the womb that was considered to be irregular or anything?
S: Well, not in this study. Again, this is just-
S: -this was really just asking a very specific question. What relative contribution is genetics playing versus the environmental factors. It wasn't really exploring any other aspect of those environmental factors-
R: Steve I have to-
S: -but there's other studies that we can look at.
R: -correct you though, one thing, is the researchers specifically say that genetics accounted for around 35% of the differences between men and homosexual behavior and other individual specific environmental factors, that is, not societal attitudes, family, or parenting, which are shared by twins, accounted for around 64%. So they did rule out the family side of environmental factors.
S: Yeah, but I'm not sure how they did that. Because-
R: Well, twins are raised together by the same family so I think if-
S: Yeah, but they also are in the same womb.
R: But, doesn't fetal development differ from child to child, even if they're sharing a womb?
S: No, not in this - no, no and in the context of this study, I think that's confusing in the article that you're reading, because both fraternal and identical twins share a womb environment and that's critical to the analysis that they did, because you can separate the genetic from that womb environment because that's the difference between fraternal and identical twins. Fraternal and identical differ only in the number of percentage of genes that they share, but they're the same in that they completely share a womb environment. But they also share, unless you're doing a separated at birth study, which I was trying - I read this - I didn't see anything commenting on that, but maybe that's something that's just missing from the accounts that I've read. If you then look at twins that have been separated at birth, then and only then, can you distinguish social family parenting factors from biological factors.
J: Steve, what do they mean in the article, "In other words, men become gay or straight because of different developmental pathways, not just one pathway." What is the pathway?
S: So, what they're talking about there is that there's not a gay gene, right. There's not one genetic factor or one gene that's determining it. By developmental pathways, they're talking about developmental biology, as they're developing as an embryo, a fetus, etcetera. And that's determined partly by genes, partly by the biological environment and as Rebecca said, in this study, for men, it was 35% genes, 64% environment, but you have to take those statistics with a grain of salt. It doesn't mean that in an individual 35% of their whatever, their sexual orientation was determined by genes, it's just a statistical way of referring to the degree of concordance, the degree to which you can explain the differences based upon one factor versus the other. So-
E: One of the co-authors of the study acknowledged that there's limitations to this study that they did and I'm quoting him, he says, "We used a behavioral measure of sexual orientation which might be ok to use for men, but less so for women. Despite this our studies prove the most unbiased estimates presented so far of genetic and non-genetic contributions to sexual orientation."
S: Yeah, it was a good study. Also, it's interesting that the genetics only explained about 18% of the variation for women and 35% for men, but that could be an artifact of, as they said, that - of in women, their attitudes and orientation are not as closely tied to their behavior as it seems to be for men.
B: There's also another point that I think we need to stress even a little bit more. According to Dr. Qazi Rahman, the study co-author, she said that this study puts cold water on any concerns that we're looking for a single gay gene or a single environmental variable which could be used to select out homosexuality. Even one of the original proponents of gay gene theory, Dr. Dean Hamer, according to gaywired.com, now concedes that it's unlikely that something as complex as human sexuality can be explained solely in terms of genetic inheritance.
S: Right, and that belief that there was like a gay gene, and as you said, that it could be selected out - also, there's an unstated premise there, that it's a quote-unquote problem from an evolutionary point of view which has led to what is called the homosexual paradox: why hasn't evolution selected out the genetics that would prevent somebody from passing on their genes? And this is an interesting area of study. One possibility is that the genes that make a male more likely to be a homosexual also make females more fertile, have more kids. So, this could just be an epiphenomenon of genetics that make women more fertile and have more kids. But there's also other studies which show that having males that are not competing for females in a population may actually confer an advantage to the group. So, this is still a complex area that hasn't completely been fleshed out, but you can't assume that homosexuality is an evolutionary disadvantage when you look at all the factors.
S: SGU 5x5 is a companion podcast to the Skeptics' Guide to the Universe, a weekly science podcast brought to you by the New England Skeptical Society in association with skepchick.org. For more information on this and other episodes, visit our website at www.theskepticsguide.org. Music is provided by Jake Wilson.