I'm bringing this to the top, because if people do want to discuss it, it doesn't need to overwhelm an already lengthy thread about something entirely different below. This is a short primer on the complex relationship of genes to sexuality.
A few preliminary notes:
1. As Kinsey first demonstrated, sexuality isn't as simple as gay-straight-bisexual. Some people are sexually attracted to both sexes, but strongly prefer one over the other. Others are attracted to their own sex only in certain circumstances (e.g., the large number of men who have sex with men in prison, but only have sex with women outside of prison). Others are attracted only to pre-pubescent children. Still others are attracted to no one.
2. Sexual orientation and sexual behavior are not the same thing. Both can be considered traits, and potentially inheritable. But, being a behavioral trait, there is a stronger element of choice - for the lack of a better word - in sexual activity than there is in sexual orientation. Both may be partially affected by genes, environment and circumstance, but not necessarily in the same ratios.
3. "Genes" are not identical to "determined." Just because something is not genetic doesn't mean that it is a choice. Some of sexuality may be determined by genes, and some by environment, but even the environmental factors may not be a choice. Chemicals in the amniotic fluid, parenting choices, etc., are examples of non-genetic factors that may still diminish the "choice" an individual has.
Now, the first and primary way to study genetics is in two types of studies known collectively as "twin studies." Both can provide evidence of a genetic factor in behavior. The first way is by comparing identical to fraternal twins, and then comparing both to genetically-unrelated siblings (i.e., adopted). A trait that is at least partially genetic should show up the most among identical twins, who share the same genome. It should show up less often among fraternal twins, who share the same environment, but only 50% of their genes. And it should show up even less among adoptive siblings, who share no more genes between them that two random humans, but do still share (to an extent depending on their age when adopted, and difference in age between each other) some environment. When it comes to sexuality, lo and behold, that is exactly what we have found.
I cannot express how big of a deal those studies are. They pretty much seal the deal on their own. They are almost irrefutable proof that sexuality is at least partially genetic. All that is needed to turn an almost slam dunk into an epic Victor Oladipo highlight is to add other confirming studies, as well as some genetic studies to try to find potential genes at play. Well, studies have confirmed that there is a genetic role. The second type of twin study is to compare twins who were separated at birth with twins who have not been, and also with non-twins. Those types of studies have confirmed the genetic link to sexuality, along with a whole host of other traits, some of which are fairly disturbing, like religiosity, or a tendency to enjoy Salem cigarettes.
The next step is to do genetic studies. Well, scientists have identified at least 5 SNPs, or single-nucleotide polymorphisms - essentially points where a mutation has changed exactly one nucleotide base in the genetic code - associated with homosexuality.
This is what you might call home-run science. If sexuality is genetically-determined (at least to an extent), science predicts certain results in studies. And those studies confirm those exact results.
There is one more step in any study of inheritable traits, and that is to explain it. At first glance, one might think that homosexuality would be an evolutionary failure. Surely, it doesn't increase the fitness of an organism, since it dramatically reduces the likelihood said organism reproduces. I can think of at least three common explanations, listed here from least interesting to most interesting.
1. "Bad" luck. Perhaps the types of mutations that lead to homosexuality are just, for various chemical reasons, very common mutations. Some mutations are like this. The mutation that codes for sickle-cell disease, for example, probably arose spontaneously in at least five different human communities. It, of course, is a positive trait in malaria-heavy regions, so its survival was virtually guaranteed once it appeared.
2. Benefits outweigh the reduction in sexual activity likely to result in procreation. This is especially likely if bisexuality and homosexuality are related to the same genetic mutations. If bisexuality offers a survival advantage, bisexual individuals will be very likely to pass down the genes that, in at least some of their descendants, will express as homosexuality instead of bisexuality. This also works with the idea that sexuality is a scale, rather than a binary state.
3. Kin selection. This is by far the most viable and elegant solution. Kin selection is based on the idea that an individual's close relatives share many genes with that individual. If some of those shared genes lead to behavior that increases the likelihood of those close relatives surviving, then they can contribute to the passing down of genes, even if the individual does not reproduce directly. I'll give you a simple example. My sister shares 50% of my genes. If some of those genes result in behavior on my part that directly helps my sister survive and thrive to the point she is able to have at least one child, then I have helped ensure that at least 25% of my genetic code, up to closer to 50%, depending on how many children she has, will be "passed down" through my nieces and nephews.
For homosexuality, the theory of kin selection is applied in what is cleverly called "The Gay Uncle Theory." The theory states that a gay male will spend less (or no) energy raising his own offspring, and instead will help raise his sister's offspring. This increases the chance that the sister's children will survive, and each child is likely to share 25% of the gay uncle's genetic code. In other words, they have a 25% chance of carrying the gay gene themselves. This especially works with the idea that sexuality is only partially genetically determined, and also is affected by multiple genes, because it increases the likelihood that someone may carry some of the gay genes, but still themselves be straight, thus making it easier for those genes to be passed down.
So, here's what we have. We have studies which show a direct correlation between genetic relationships and sexuality. We have studies that pinpoint specific genetic mutations associated with those correlations. And we have a solid evolutionary explanation for why said mutations and behaviors may be positively selected for. In short, we have enough evidence to say that it is almost certain - as certain as science can be - that genes play at least some role in sexuality.
goat
A few preliminary notes:
1. As Kinsey first demonstrated, sexuality isn't as simple as gay-straight-bisexual. Some people are sexually attracted to both sexes, but strongly prefer one over the other. Others are attracted to their own sex only in certain circumstances (e.g., the large number of men who have sex with men in prison, but only have sex with women outside of prison). Others are attracted only to pre-pubescent children. Still others are attracted to no one.
2. Sexual orientation and sexual behavior are not the same thing. Both can be considered traits, and potentially inheritable. But, being a behavioral trait, there is a stronger element of choice - for the lack of a better word - in sexual activity than there is in sexual orientation. Both may be partially affected by genes, environment and circumstance, but not necessarily in the same ratios.
3. "Genes" are not identical to "determined." Just because something is not genetic doesn't mean that it is a choice. Some of sexuality may be determined by genes, and some by environment, but even the environmental factors may not be a choice. Chemicals in the amniotic fluid, parenting choices, etc., are examples of non-genetic factors that may still diminish the "choice" an individual has.
Now, the first and primary way to study genetics is in two types of studies known collectively as "twin studies." Both can provide evidence of a genetic factor in behavior. The first way is by comparing identical to fraternal twins, and then comparing both to genetically-unrelated siblings (i.e., adopted). A trait that is at least partially genetic should show up the most among identical twins, who share the same genome. It should show up less often among fraternal twins, who share the same environment, but only 50% of their genes. And it should show up even less among adoptive siblings, who share no more genes between them that two random humans, but do still share (to an extent depending on their age when adopted, and difference in age between each other) some environment. When it comes to sexuality, lo and behold, that is exactly what we have found.
I cannot express how big of a deal those studies are. They pretty much seal the deal on their own. They are almost irrefutable proof that sexuality is at least partially genetic. All that is needed to turn an almost slam dunk into an epic Victor Oladipo highlight is to add other confirming studies, as well as some genetic studies to try to find potential genes at play. Well, studies have confirmed that there is a genetic role. The second type of twin study is to compare twins who were separated at birth with twins who have not been, and also with non-twins. Those types of studies have confirmed the genetic link to sexuality, along with a whole host of other traits, some of which are fairly disturbing, like religiosity, or a tendency to enjoy Salem cigarettes.
The next step is to do genetic studies. Well, scientists have identified at least 5 SNPs, or single-nucleotide polymorphisms - essentially points where a mutation has changed exactly one nucleotide base in the genetic code - associated with homosexuality.
This is what you might call home-run science. If sexuality is genetically-determined (at least to an extent), science predicts certain results in studies. And those studies confirm those exact results.
There is one more step in any study of inheritable traits, and that is to explain it. At first glance, one might think that homosexuality would be an evolutionary failure. Surely, it doesn't increase the fitness of an organism, since it dramatically reduces the likelihood said organism reproduces. I can think of at least three common explanations, listed here from least interesting to most interesting.
1. "Bad" luck. Perhaps the types of mutations that lead to homosexuality are just, for various chemical reasons, very common mutations. Some mutations are like this. The mutation that codes for sickle-cell disease, for example, probably arose spontaneously in at least five different human communities. It, of course, is a positive trait in malaria-heavy regions, so its survival was virtually guaranteed once it appeared.
2. Benefits outweigh the reduction in sexual activity likely to result in procreation. This is especially likely if bisexuality and homosexuality are related to the same genetic mutations. If bisexuality offers a survival advantage, bisexual individuals will be very likely to pass down the genes that, in at least some of their descendants, will express as homosexuality instead of bisexuality. This also works with the idea that sexuality is a scale, rather than a binary state.
3. Kin selection. This is by far the most viable and elegant solution. Kin selection is based on the idea that an individual's close relatives share many genes with that individual. If some of those shared genes lead to behavior that increases the likelihood of those close relatives surviving, then they can contribute to the passing down of genes, even if the individual does not reproduce directly. I'll give you a simple example. My sister shares 50% of my genes. If some of those genes result in behavior on my part that directly helps my sister survive and thrive to the point she is able to have at least one child, then I have helped ensure that at least 25% of my genetic code, up to closer to 50%, depending on how many children she has, will be "passed down" through my nieces and nephews.
For homosexuality, the theory of kin selection is applied in what is cleverly called "The Gay Uncle Theory." The theory states that a gay male will spend less (or no) energy raising his own offspring, and instead will help raise his sister's offspring. This increases the chance that the sister's children will survive, and each child is likely to share 25% of the gay uncle's genetic code. In other words, they have a 25% chance of carrying the gay gene themselves. This especially works with the idea that sexuality is only partially genetically determined, and also is affected by multiple genes, because it increases the likelihood that someone may carry some of the gay genes, but still themselves be straight, thus making it easier for those genes to be passed down.
So, here's what we have. We have studies which show a direct correlation between genetic relationships and sexuality. We have studies that pinpoint specific genetic mutations associated with those correlations. And we have a solid evolutionary explanation for why said mutations and behaviors may be positively selected for. In short, we have enough evidence to say that it is almost certain - as certain as science can be - that genes play at least some role in sexuality.
goat