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We have identical boys, James and Charlie, about to turn 5. Got a DNA test for a holiday present that confirmed it. They're very healthy and happy kids, and are

We have identical boys, James and Charlie, about to turn 5. Got a DNA test for a holiday present that confirmed it. They're very healthy and happy kids, and are each other's best friend. When they were about 3, James asked to wear a dress. Now James wears a dress every day, and is -- for lack of a better word -- extremely flamboyant. At first Charlie wanted to wear a dress too, but that seems to have just been FOMO because now he never wears them. Charlie is maybe more cerebral and cautious than your average boy, but still very clearly feels like a boy. This has all been really interesting and mystifying to us. My wife and I and most of the people we interact with are pretty ordinary cisgendered, heterosexual people. We've never consciously pushed the boys toward any particular social norm. And we both figured one's sexuality and gender identity was hardcoded in one's DNA. Now we have living proof that doesn't seem to be the case. Our best uneducated guess is that something in utero caused the boys to develop with different levels of hormones, and that caused their "wiring" to diverge.

guntri 4y ago
Please don't trans your kid just because he likes wearing dresses.
- robertakarobin 4y ago
  Yes, we're careful to not put any labels on him. In his words, he doesn't like it when people call him a girl, and wants people to know he's a boy, but he does like to do things that girls do.
thaumasiotes 4y ago
> And we both figured one's sexuality and gender identity was hardcoded in one's DNA. There are some good reasons not to believe this. From a Darwinian perspective, we can easily observe that homosexuality is one of the worst possible phenotypes an organism can have, so we'd expect genes to be adamantly opposed to it. Or in other words, heterosexuality definitely should be hardcoded in, in several different ways, but homosexuality should not. And the evidence, such as it is, tends to bear that out: concordance of homosexuality in identical twins is, if I remember correctly, about 40%. That sounds high, but it isn't -- concordance for randomly chosen traits in identical twins is much, much higher than 40%.
- robertakarobin 4y ago
  Yes, that certainly all makes sense. If I recall correctly, growing up in the 90s we were sort-of put under the impression that DNA determines almost everything about a person. It's only more recently that science and society have discovered how much is actually determined by things like gut bacteria.
  - thaumasiotes 4y ago
    The rule of thumb for what determined any given trait, from summaries of the relevant science in the early 2000s, was "genetics 50%, unique environment 40-50%, shared environment 0-10% [heavily weighted toward the 0% end of things]". Of course that division of influence (1) varies by the particular trait; and (2) is itself dependent on environmental conditions; when the general background is extremely impoverished, shared environmental influences may matter in a way that they don't when the background is lush. Steven Pinker writing in The Blank Slate pounded on the message that it is essentially impossible for parents to have any predictable non-genetic influence on their children, but that there was still plenty left to be determined after the genes had their say. He recommended greater acceptance of the concept of fate.
- tzs 4y ago
  Would we still expect such adamant evolutionary opposition to homosexuality if we took into account kin selection or that sexuality is a complex thing whose genetic component is likely to be influenced by many different genes and it is possible that the set of genes that in some particular combinations give homosexuality give in other combinations heterosexuality plus some additional survival benefits that make it overall worthwhile to preserve those genes that can give homosexuality?
  - thaumasiotes 4y ago
    > Would we still expect such adamant evolutionary opposition to homosexuality if we took into account kin selection or that sexuality is a complex thing whose genetic component is likely to be influenced by many different genes Yes. For example, a kin selection argument would need to assume that the number of additional surviving children produced by the homosexual's siblings was at least double the number of children forgone by the homosexual. This does not come anywhere close to being plausible. Things only get worse as the relationship becomes more distant. > and it is possible that the set of genes that in some particular combinations give homosexuality give in other combinations heterosexuality plus some additional survival benefits that make it overall worthwhile to preserve those genes that can give homosexuality? Is it possible? Sure; that's the sickle-cell anemia model. But as previously noted, what evidence we have tells us that homosexuality is already one of the least genetically-determined traits we're aware of.† And this theory shares the same problem as your first idea: the cost of homosexuality is so staggeringly high that we would never expect an offsetting benefit to be able to make up for it. Suppose there is a genetic complex that arranges for various effects, and if everything goes wrong it causes homosexuality. Assume this failure occurs 5% of the time. Our homosexual produces zero children. But a decline in relationship coeffiecients, which trashes the kin selection idea, is not a problem here: we're worried about comparisons of self-to-self, so as long as the expected number of children produced by this genetic complex is greater than otherwise, the complex can persist despite the occasional catastrophic failure. In other words, we need 19 out of every 20 people expressing this genetic complex to have as many children as 20 baseline people would, an increase of 5.2%. Compare this discussion of the inhabitants of Madagascar ( https://razib.substack.com/p/the-unreal-voyages-and-trials-of https://razib.substack.com/p/the-unreal-voyages-and-trials-o... ): > Might adaptive selection pressures have reshaped the genomes of the Malagasy, altering the proportion of African and Asian ancestry and leaving a signature in their very DNA? > A research group tested this hypothesis by sampling the genomes of 700 Malagasy from 257 villages across the island. What they immediately remarked on was a sharp drop in Asian ancestry across much of chromosome 1: > The target of selection on chromosome 1 in this case is the Duffy locus; and specifically, the null allele that fails to produce the Duffy antigen is under selection. This gene is well known in population genetics because of a marked contrast in variants between African and non-African populations; most Africans are Duffy-negative while most non-Africans are Duffy-positive. Duffy-negative individuals lack the receptor used by the Plasmodium vivax malarial parasite to invade red blood cells, a deficit that confers malaria resistance on its bearer. In Malagasy populations today, the region of Chromosome 1 around Duffy spikes to over 90% African in local ancestry whereas across the rest of the genome, the African ancestry proportion hovers around only 60%. Researchers calculate that this change was driven by a selection coefficient on the order of 0.20. > We can translate this into plain English as an individual carrying the Duffy-negative allele has 20% more surviving offspring than one who does not. > This is a staggeringly powerful selection coefficient; very few genes have been subject to such drive in humans; indeed a coefficient of 0.01 or above is by convention usually considered strong selection. So the effect on non-homosexual bearers of this gene complex would need to be very powerful indeed, far more powerful than almost any selective effect we know of in humans, just for the complex as a whole to break even on reproduction. It would also have to be quite recent; over time the disastrous side effect would be separated from the beneficial primary effects. The primary effects would persist and the side effect would vanish. † It's worth being careful here. Heterosexuality is strongly genetically determined. Homosexuality is not strongly genetically determined. As a result, it doesn't really mean anything to say that "sexual orientation is [or isn't] genetically determined" - different orientations are not the same by this metric, and so referring to an umbrella group doesn't make much sense. By analogy, it should be easy to understand that people who have two arms are mostly that way for genetic reasons, while people with one arm are mostly that way for non-genetic reasons. Is "number of arms" genetically determined? Well, yes, but only when the number is two (or in fantastically exotic scenarios).
malux85 4y ago
Very interesting! I went to high school with identical twins, and even though they were quite hard to tell apart in terms of looks, they were wildly different in terms of intelligence - one was in the class for the gifted and the other was in the class for the struggling, they had quite different personalities too. Really fascinating.

guntri 4y ago

Please don't trans your kid just because he likes wearing dresses.

robertakarobin 4y ago
Yes, we're careful to not put any labels on him. In his words, he doesn't like it when people call him a girl, and wants people to know he's a boy, but he does like to do things that girls do.

thaumasiotes 4y ago

> And we both figured one's sexuality and gender identity was hardcoded in one's DNA. There are some good reasons not to believe this. From a Darwinian perspective, we can easily observe that homosexuality is one of the worst possible phenotypes an organism can have, so we'd expect genes to be adamantly opposed to it. Or in other words, heterosexuality definitely should be hardcoded in, in several different ways, but homosexuality should not. And the evidence, such as it is, tends to bear that out: concordance of homosexuality in identical twins is, if I remember correctly, about 40%. That sounds high, but it isn't -- concordance for randomly chosen traits in identical twins is much, much higher than 40%.

robertakarobin 4y ago
Yes, that certainly all makes sense. If I recall correctly, growing up in the 90s we were sort-of put under the impression that DNA determines almost everything about a person. It's only more recently that science and society have discovered how much is actually determined by things like gut bacteria.
- thaumasiotes 4y ago
  The rule of thumb for what determined any given trait, from summaries of the relevant science in the early 2000s, was "genetics 50%, unique environment 40-50%, shared environment 0-10% [heavily weighted toward the 0% end of things]". Of course that division of influence (1) varies by the particular trait; and (2) is itself dependent on environmental conditions; when the general background is extremely impoverished, shared environmental influences may matter in a way that they don't when the background is lush. Steven Pinker writing in The Blank Slate pounded on the message that it is essentially impossible for parents to have any predictable non-genetic influence on their children, but that there was still plenty left to be determined after the genes had their say. He recommended greater acceptance of the concept of fate.
tzs 4y ago
Would we still expect such adamant evolutionary opposition to homosexuality if we took into account kin selection or that sexuality is a complex thing whose genetic component is likely to be influenced by many different genes and it is possible that the set of genes that in some particular combinations give homosexuality give in other combinations heterosexuality plus some additional survival benefits that make it overall worthwhile to preserve those genes that can give homosexuality?
- thaumasiotes 4y ago
  > Would we still expect such adamant evolutionary opposition to homosexuality if we took into account kin selection or that sexuality is a complex thing whose genetic component is likely to be influenced by many different genes Yes. For example, a kin selection argument would need to assume that the number of additional surviving children produced by the homosexual's siblings was at least double the number of children forgone by the homosexual. This does not come anywhere close to being plausible. Things only get worse as the relationship becomes more distant. > and it is possible that the set of genes that in some particular combinations give homosexuality give in other combinations heterosexuality plus some additional survival benefits that make it overall worthwhile to preserve those genes that can give homosexuality? Is it possible? Sure; that's the sickle-cell anemia model. But as previously noted, what evidence we have tells us that homosexuality is already one of the least genetically-determined traits we're aware of.† And this theory shares the same problem as your first idea: the cost of homosexuality is so staggeringly high that we would never expect an offsetting benefit to be able to make up for it. Suppose there is a genetic complex that arranges for various effects, and if everything goes wrong it causes homosexuality. Assume this failure occurs 5% of the time. Our homosexual produces zero children. But a decline in relationship coeffiecients, which trashes the kin selection idea, is not a problem here: we're worried about comparisons of self-to-self, so as long as the expected number of children produced by this genetic complex is greater than otherwise, the complex can persist despite the occasional catastrophic failure. In other words, we need 19 out of every 20 people expressing this genetic complex to have as many children as 20 baseline people would, an increase of 5.2%. Compare this discussion of the inhabitants of Madagascar ( https://razib.substack.com/p/the-unreal-voyages-and-trials-of https://razib.substack.com/p/the-unreal-voyages-and-trials-o... ): > Might adaptive selection pressures have reshaped the genomes of the Malagasy, altering the proportion of African and Asian ancestry and leaving a signature in their very DNA? > A research group tested this hypothesis by sampling the genomes of 700 Malagasy from 257 villages across the island. What they immediately remarked on was a sharp drop in Asian ancestry across much of chromosome 1: > The target of selection on chromosome 1 in this case is the Duffy locus; and specifically, the null allele that fails to produce the Duffy antigen is under selection. This gene is well known in population genetics because of a marked contrast in variants between African and non-African populations; most Africans are Duffy-negative while most non-Africans are Duffy-positive. Duffy-negative individuals lack the receptor used by the Plasmodium vivax malarial parasite to invade red blood cells, a deficit that confers malaria resistance on its bearer. In Malagasy populations today, the region of Chromosome 1 around Duffy spikes to over 90% African in local ancestry whereas across the rest of the genome, the African ancestry proportion hovers around only 60%. Researchers calculate that this change was driven by a selection coefficient on the order of 0.20. > We can translate this into plain English as an individual carrying the Duffy-negative allele has 20% more surviving offspring than one who does not. > This is a staggeringly powerful selection coefficient; very few genes have been subject to such drive in humans; indeed a coefficient of 0.01 or above is by convention usually considered strong selection. So the effect on non-homosexual bearers of this gene complex would need to be very powerful indeed, far more powerful than almost any selective effect we know of in humans, just for the complex as a whole to break even on reproduction. It would also have to be quite recent; over time the disastrous side effect would be separated from the beneficial primary effects. The primary effects would persist and the side effect would vanish. † It's worth being careful here. Heterosexuality is strongly genetically determined. Homosexuality is not strongly genetically determined. As a result, it doesn't really mean anything to say that "sexual orientation is [or isn't] genetically determined" - different orientations are not the same by this metric, and so referring to an umbrella group doesn't make much sense. By analogy, it should be easy to understand that people who have two arms are mostly that way for genetic reasons, while people with one arm are mostly that way for non-genetic reasons. Is "number of arms" genetically determined? Well, yes, but only when the number is two (or in fantastically exotic scenarios).

malux85 4y ago

Very interesting! I went to high school with identical twins, and even though they were quite hard to tell apart in terms of looks, they were wildly different in terms of intelligence - one was in the class for the gifted and the other was in the class for the struggling, they had quite different personalities too. Really fascinating.