There is little doubt any more among the research community that sexual preference is a combination of both nature and nurture. In other words, it comes about because of both genes and the environment.
The next questions to answer have more to do with how much each contributes and which genes and environmental factors are involved. A new study has begun to shed light on the genetic side of the equation.
In this study, researchers looked at over 400 pairs of brothers who were both gay and found two regions in the genome that may play a role in male sexual orientation. These regions were found on chromosome 8 and on one of the sex chromosomes, the X.
As expected for something as complex as sexual attraction, neither region is the whole story. There are undoubtedly many, many genes in many different parts of the genome involved in this part of brain development. There will be heterosexual men who have the genetic variations identified in this study and homosexual men who do not.
And of course genes aren't the whole story. The environment plays an important role as well.
For example, men with lots of older brothers are more likely to be gay than first born sons. This is true even for brothers reared apart which means it isn’t the older brothers themselves influencing the younger brother directly. Rather it is the effect on the mom of having multiple sons which triggers some effect in the next male fetus as he develops in her womb. This is just one of many environmental influences like this.
Again, this obviously does not mean all younger brothers are gay and all first born men are straight. It just means that men with lots of older brothers are more likely to be gay and that first born sons are less likely.
This all points to sexual orientation in men being a complex mix of genetics and the environment. Some men will have a genetic predisposition towards being gay but will not have encountered any of the environmental triggers while others will have no genetic predisposition but will be gay because of multiple environmental triggers. And everything in between.
One thing this means is that even if scientists find every DNA region that plays a role in a man’s sexual orientation, they still probably won’t be able to predict if a boy will grow up to be attracted to men or women. A boy might have all the variations that point to being gay and still be attracted to women because of how the environment has influenced his genes, his development, his cells and so on.
It is important to note that this study only looked at homosexual men, not women. This is because a growing amount of research points to sexual orientation being determined differently in each sex. When researchers look at both men and women in the same studies, there is a big chance that subtle effects like the ones seen in this study will be missed. Scientists need to do a separate study in women to understand the genetics behind their sexual orientation.
Regions Not Genes
The researchers in this study did not find any specific gene involved in male sexual preference. As is common in these studies, they are at the first step where they have narrowed down parts of the genome that may be involved. Now they can focus on these regions and find the specific differences responsible for these brothers' sexual orientation.
The next step will be to figure out whether these regions translate to more gay men or whether these researchers have stumbled onto something that happens to be specific for this set of brothers. This will not necessarily be a slam dunk.
The data in this study is definitely suggestive that these regions on chromosome 8 and the X chromosome are important but they are nowhere near overwhelming. More work will need to be done to confirm that these regions play a role. And then the real work of identifying which genetic differences affect which genes can begin.
X Marks the Spot
The region on the X chromosome is particularly intriguing. It matches up with a study done back in 1993 that found the same region, Xq28, that was involved in male sexual orientation.
The X chromosome has been a favorite of scientists because it may help to explain how being gay has survived in the population as a sizeable minority. After all, most genetic differences that lead to having fewer kids should fade away over time. Unless, that is, there is some way that it can also lead to more children.
One idea scientists have had is that women who have the genetic differences on their X that increase a man’s chances of being gay have more babies. Her increase offsets his decrease and so the genetic variants stay in the population. There is even data to support this hypothesis.
This is a good theory but definitely needs more testing. And keep in mind that this region on the X is one of at least two and probably more regions involved in male sexual preference. It is unlikely that every region that might lead to homosexuality in men has some sort of compensatory effect in women.
Instead it may just be that because there is such a complex relationship between these genes and the environment, that these genes don’t have a big impact on the number of kids over time. In each generation, only a few of the men with these differences go on to be homosexual. This might mean that the genetic differences do not have a significant effect on the number of kids in each generation. Obviously more research needs to be done.