As I have blogged about before, a big part of sexual preference is biological. Research shows that some people are hardwired to prefer the same sex. This is true in animals as well.

What scientists haven’t had much luck at yet is finding out why this is. There have been studies that have implicated the X chromosome (although not convincingly). And others that have pointed to brain structure. But none of these studies really gets at what is going on in these folks’ brains that makes them prefer relationships with the same sex.

A new study out suggests that, at least in mice, the neurotransmitter serotonin may play a big role. Scientists created a strain of mice that lacked most of a certain kind of serotonin receptors in the brain (central serotonin receptors). These mice were healthy and happy. And the males were not at all fussy about whom they hooked up with.

When scientists put wild type* male mice in a cage with other males, they mostly ignored the other mice. The male mice lacking their central serotonin receptors reacted differently. They got busy with the male mice almost every time.

These mice aren’t homosexual though. Given a choice of a male or female, they didn’t really care; they went after both at about the same rate. The genetically altered mice were more bisexual than homosexual.

The researchers did lots of other experiments as well that showed that these mice were not oversexed or lacking anything in particular. They just liked the boys as much as the girls.

What this study tells us is that in mice, serotonin plays a big role in sexual preference through these particular brain neurons. What it doesn’t tell us is if the same thing is true in people. After all, picking a mate is very different in mice as compared to people.

But there are hints that serotonin works differently in the brains of bisexual and homosexual men. For example, certain selective serotonin reuptake inhibitors (or SSRIs) have different effects in bisexual and homosexual men compared to heterosexual men. Still, this isn’t yet enough to finger serotonin use as the main driver of sexual preference in people.

What it does do though is provide scientists some direction for their research. Instead of wading through all 20,000+ genes, they can start out focusing on those that deal with serotonin. This will greatly simplify the research and if serotonin does play a role, then scientists will find the genetic variations involved sooner rather than later.

And frankly, given the slow progress thus far, focusing on serotonin genes won’t set the field back too far. It is probably worth taking the research in this direction.

* Wild type just means a mouse (or any living thing) that hasn’t been tampered with. In this case, it is a run of the mill lab mouse.

A more in depth look at the story from blogger Ed Yong at Not Exactly Rocket Science.

37.7749295 -122.4194155

Gay Pride Parade in Brazil

For example, one study showed that if one identical twin was gay, then the other was 50% of the time. If the two brothers were fraternal twins, then the odds dropped down to 22%. And if one brother was adopted, the odds were only 11%.

Another more comprehensive study gave similar results with both twins in an identical twin pair being gay around 32% of the time as compared to 13% of the time for fraternal twins. Lots of other studies have been done that show the same trend even if they don't have the exact same numbers.

These results strongly suggest genes are involved because identical twins share the exact same DNA. If both twins in an identical twin pair have some trait in common more often than do fraternal twins, then odds are that genetics plays a role.

These twin studies show that being gay is at least partly genetic. So the new study is really just looking to explain explain the older results.

That said, I am torn about whether I want this study to succeed or not. If it is successful, that'll shut up all those people who maintain that sexual preference is purely the result of personal choice. In my opinion, quieting those folks is a good thing. But finding the gay gene(s) may have unintended consequences. Some people may see being gay as akin to a genetic disease like cystic fibrosis or sickle cell anemia. As something to be minimized through carrier screening, embryo selection, or even abortion.

Scary, but I have heard people say things like this. Just the other day someone asked a friend of mine if homosexuality was genetic. They weren't merely being curious. They were hopeful that there was a gay gene so that homosexuality could be eliminated from the human population at some point. Yikes!

And that was here in the San Francisco Bay Area. Who knows what they are saying in Wyoming or Texas. Do we want to find the gene(s) responsible for homosexuality? Or are some things better left unknown? Comment below.

Dr. Barry Starr is a Geneticist-in-Residence at The Tech Museum of Innovation in San Jose, CA.

latitude 37.3316, longitude -121.89