All my “expertise” (NOT!) on twins is courtesy of Profs Wiki & Google in the last hour.
I had a rogue thought that twins, and specifically fraternal twins one of each gender might have some gender-ish influence on one another during gestation. Specifically that the female twin would get a very light dose of male-specific hormones or whatever leaking across and vice versa. With the expected outcome being that such pairs are more likely to contain more femme boys and more macho girls than is the norm for singleton pregnancies.
Clearly there are many general ways twins influence on another in utero. They’re competing for very finite space and finite maternal energy, nutrients, etc. I’m specifically interested in issues at the lower bio- & biochemical levels. With emphasis on sex/gender-related issues.
I couldn’t locate any good info to support or refute this idea. I did find that it seems (for fraternal twins at least) the “insulation” (amniotic sac(s), etc.) between the two fetuses is greater than is the case for most identical twins which directly share some of that uterine support hardware. But even so ISTM the fraternals still have a path through Mom to influence the other.
So the Q: Is there any support for this idea of cross-twin and specifically cross-gender influence in utero or is it just the random musings of some clueless dude on the internet? And before you ask: no, I wasn’t drinking.
Fraternal twins have different amniotic sacs and different placentas. Although occasionally the two placentas may fuse together, that happens later in pregnancy after the placentas have developed individual blood supplies. So there really isn’t much opportunity for leakage.
Heck, even in identical twins, the chances of one being gay and one straight are about 50/50. Hormones aren’t everything.
(I’m going to nitpick myself here. There were two reported cases in medical journals where fraternal twins shared the same placenta (with different amniotic sacs,) and one of them was a male/female pair, so anything is possible, I guess.)
A synthetic estrogen (DES) used to be prescribed to pregnant women post WW2, in the mistaken belief that it would prevent miscarriage (it was withdrawn from the market in 1971). The impact this had on females was often infertility & an increase in certain cancers & the effect on males was very small testes & other health issues. So clearly hormones can effect fetal development. Cite.
That said, I don’t know if hormones naturally present in amniotic fluid would leak and permeate the other sac. My hunch is that the levels would be so dilute as to not make any difference, but I don’t know for sure. I am not aware of any study on fraternal twins that would address this issue.
I’m not sure that developing fetuses produce very much in the way of hormones.
Looking around I see that the mother produces different levels of hormones depending on the sex of the baby. So you could expect some sort of influence.
Kunilou, I read that study as saying if one twin is gay then there is a 50% chance of the other being gay. Not " … in identical twins, the chances of one being gay and one straight are about 50/50."
Although the OP mentioned NOTHING about sexual orientation, it’s interesting how quickly the discussion turned to that. “Femme boys” and “macho girls” /= gay.
Steatopygia – your interpretation is certainly how I*** meant*** to write it. Bob++ – you may want to re-read the cite in light of that more precise description.
Right, but to** panache45’**s point, even if a fetus *is *exposed to hormones (lets use male fetuses exposed to estrogen, as in the DES cases), it has nothing to do with sexual orientation, so I’m not sure how your cite is relevant to this thread.
I think when one twin is gay and one isn’t, one twin just hasn’t some out yet.
Seriously. I know three sets of identical twins in which both are gay, and in no case did they come out at exactly the same time. In one case there was a gap of almost 10 years (22 & 32). The shortest gap was three years (16 & 19)*. If you’d asked any of those pairs during the “gap” period, you would have had another stat for one identical twin being gay, and one not.
*And in this case, while the second to come out was living a gay life at college, he had not come out to his family. He didn’t even come out to his brother for a full year, because when his brother “came out” (READ: was caught with a boyfriend by his fundamentalist parents), they put him in a re-education camp. He did not finally tell his parents until he was 24 or 25.
Anecdotal and barely tangential…but funny. I am a fraternal twin (both of us male). I have a bump near my sternum that the doctor diagnosed as a healed rib fracture from before I was born – presumably the work of my brother in utero.
What actually triggered this thought was my wife’s athletic trainer. She’s quite an Amazon woman with excellent genes and epigenetics ideally suited to being an athlete. And is a fraternal twin with her brother who’s apparently utterly ordinary as men go. Which made me wonder if she got a partial dose of whatever in utero magic makes a male a male.
Both she and her brother are heterosexual. But not together AFAIK
I with I could give a cite for this; maybe someone with more google-fu than me could find one.
I once read about a study of gayness in pigs. (Pigs can be gay? Learn something new.) It turns out that a male pig in a litter that was surrounded by two females in utero is likelier to be gay than one whose womb mates were both male or one of each. And the corresponding thing for females was also true, I believe, although my memory is vaguer on that point. So there can apparently be some leakage inside the womb. Note that this was just a statistical tendency. Biology always turns out to be more complicated than we think.
