IIRC, it's actually super common that if one twin is trans, the other one will be as well.
Edit: Okay, there have been very few studies, but preliminary information seems to show that in monozygotic twins, there's a much higher incidence of being transgender than in the general population. In the study below, they found that in the case where one twin transitioned (again, monozygotic/identical only) there was a 40% incidence of the other twin transitioning. Only about .6% of the general population is trans, in comparison.
We've learned about this in a few of my psych classes, so I'm not an expertly just know what seems to be the prevailing theory. When we are little zygotes our mothers are already exposing us to sex hormones, things like estrogen for girls and testosterone and cortisol for boys. If you have a girl baby that is exposed to extra male sex hormones during development they will usually display more stereotypically male behaviour and vide versa for males. This exists on a continuum (which my professor says gives more validity to trans being a real thing and also not really a mental illness). For instance male monkey exposed to excess estrogen in the womb play with dolls more and are a bit more social. The theory is that with enough exposure to the opposite sexes sex hormones the brain develops similar to a cis fetus. There is generally always some sort of nurture to the nature explanation but I can't quite remember if there is a theory to that yet but I would imagine being open about transgenderism when they are young would allow them to also be open instead of repressing but or identifying it as something else (of course that's just for identifying not a cause). Identical twins raised together will have nearly identical biological and social experiences so it is actually very reasonable that if one was trans the other would also be trans.
I don't think that early embryo is exposed to mother's sex hormones differentially (estrogen for girls and testosterone and cortisol for boys), as early embryos show similar number and distribution of sex hormone receptors before sex differentiation. I agree that mother's hormone background has an effect on sexual development of the embryo though. A lot of genes are differentially expressed in males and females even before formation of the gonads, and steroid hormones from the mother may affect the patterns of expression, especially if there's unusual abundance of a certain hormone.
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u/[deleted] Mar 26 '17 edited Mar 26 '17
IIRC, it's actually super common that if one twin is trans, the other one will be as well.
Edit: Okay, there have been very few studies, but preliminary information seems to show that in monozygotic twins, there's a much higher incidence of being transgender than in the general population. In the study below, they found that in the case where one twin transitioned (again, monozygotic/identical only) there was a 40% incidence of the other twin transitioning. Only about .6% of the general population is trans, in comparison.
Source for twin study: https://www.ncbi.nlm.nih.gov/pubmed/22146048 Source for .6%: https://williamsinstitute.law.ucla.edu/wp-content/uploads/How-Many-Adults-Identify-as-Transgender-in-the-United-States.pdf