Leiden Psychology Blog

When girl meets boy in utero: The twin testosterone transfer (TTT) hypothesis

When girl meets boy in utero: The twin testosterone transfer (TTT) hypothesis By Thomas Hawk

Female fetuses gestated together with a male co-twin are thought to be ‘masculinized’ in their development. Is this really the case? And how does it work?

Between week 8 and 24 of pregnancy, a male fetus is exposed to testosterone, whereas a female fetus is not. This prenatal testosterone peak in males is thought to lead to early ‘masculinization’ of the brain and behavior. That is to say, it enhances the development of typically male traits. For example, increased aggression has been attributed to relatively high prenatal testosterone exposure.

Because of ethical considerations direct measurements of prenatal testosterone in healthy human fetuses are carried out only rarely. Therefore, researchers have tried to find ways to estimate prenatal testosterone exposure using indirect measures. One of these ways is to study opposite-sex (boy/girl) twins: fetuses gestated together with a male co-twin are thought to be ‘masculinized’ in their development, arguably due to the influence of prenatal testosterone exposure. According to this so-called twin–testosterone-transfer (TTT) hypothesis, female fetuses with a twin brother are being exposed to higher levels of prenatal testosterone than fetuses with a twin sister. The hypothesis is based on the idea that within the uterus twins exchange hormones either via the maternal bloodstream or directly through the amniotic membrane. Even though hormonal transfer between fetuses has already been established in animals, it is still unclear whether a similar mechanism exists in humans.

Empirical evidence for the TTT hypothesis was reviewed by Tapp and colleagues (2011). The results of more than 25 studies using opposite-sex twins were examined for three domains: behavioral, cognitive and physiological/morphological traits. With respect to behavior, evidence for the TTT hypothesis is scarce. Aggression and toy preference, for instance, do not seem to be more ‘masculine’ in girls with a twin brother than girls with a twin sister. On the other hand, eating disorders (more prevalent in girls) seem to be less frequent in girls with a twin brother, whereas autistic traits (more prevalent in boys) occur less in boys with a twin sister.

In further support of the TTT hypothesis, physiological/morphological characteristics such as otoacoustic emissions (sounds produced by the inner ear either in response to a sound or in the absence of any stimulus (F>M)), tooth size (M>F) and brain size (M>F) also seem ‘masculinized’ through the intrauterine presence of a brother.

Interestingly, the TTT hypothesis holds not only for girls gestated with a brother: boys gestated together with another twin brother are also more masculinized than boys with a twin sister. However, the effects of gestation with a male co-twin are more pronounced in females than in males, possibly because females produce little testosterone themselves and therefore may be more susceptible to external testosterone.

The TTT hypothesis has been criticized because social factors such as growing up/playing together with a brother masculinize behavior as well and, in fact, have little to do with a possible prenatal hormone transfer. In an attempt to distinguish prenatal from postnatal (social) factors, some studies also include an additional singleton brother in the analyses. The results of these studies indicate that the masculinizing effects of a male co-twin can exceed the influence of the singleton brother, suggesting more pronounced prenatal than postnatal effects. Finally, the question whether the effects are only observable during early development and may be ‘overwritten’ by hormonal effects during puberty remains an important issue that needs to be addressed in future studies.

In sum, studying opposite-sex twins seems an interesting approach to studying the effects of prenatal testosterone on brain development, behavior, and psychopathology.


Doe Cartwright
Posted by Doe Cartwright on December 15, 2017 at 21:40

I am a female with a twin brother. I grew up not accepting that he had privileges that I did not.  Concerned with his anger and resentment towards me, I have been taking a closer look at gender and it’s relative issues

Posted by Sara on October 23, 2017 at 17:25

This article and the comments are very interesting.  I have fraternal boy/girl twins and I definitely see an increased aggressiveness in my daughter.  She can be very girly, but she is also an incredible athlete, loves watching more aggressive TV than most of her friends, and has a shorter index finger than ring.  She is only 9 but I can see that something is slightly different about her.  I have often wondered if she will turn out to be gay.  Also, my son is slightly more effeminate than his peers.  He is gentle and soft-spoken, and sometimes moves effeminately.  I often wonder about their shared hormone exposure and how it affected their in-utero development.

Jeana Pethel
Posted by Jeana Pethel on October 13, 2017 at 13:39

I would love to start a closed fb group where people can give and get information on this. I am having trouble coming up with a name that would be user friendly. Any ideas?

Posted by Heather on October 12, 2017 at 19:36

And, yes, my relative digit lengths are “masculine.”

Posted by Heather on October 12, 2017 at 19:34

I know this is an old thread, but I would love to be in touch with you, Heather. (As you can see, I’m a Heather too…) I’m actually writing about a similar issue. I learned several years ago that there was a second placenta at birth, suggesting a vanishing twin. There is an abundance of circumstantial evidence that would corroborate my intuition that a male twin died in utero (including the fact that as a toddler, I would look at my baby pictures and cry, feeling that something was lost). My father says I have more testosterone than most men (kind of a joke), but I was always an athlete and “tomboy” type. I’m fascinated by these issues and would love to chat about your research. Thanks!

faye tatum
Posted by faye tatum on April 25, 2017 at 01:03

I just remembered another thing about myself that is equally significant. After I reached young adulthood I had a very distinct body. I have shoulders as wide as my twin. I had 9% body fat. I had six-pack abs without trying. I can’t begin to tell you how many times I was asked if I was a body builder. I am Gay and have had no children. And yes, as with other articles I’ve read, I too have shorter index than ring fingers with both hands. Now I really feel like a freak of nature. All my life I knew something was different about me. My aggressiveness was certainly affected by my evil brother and growing up with brothers in general, but a great part of it is due to my fraternal brother. At least I think so. Who knows?

faye tatum
Posted by faye tatum on April 25, 2017 at 00:49

I am completely shocked by the comment from Jeana Pethel. That is my life exactly except one thing: my brother came second, and the Doctors didn’t know it, but he was born breech with the cord around his neck for 11 minutes, and thus was born brain damaged. I grew up with only brothers to play with but my older brother was what you would call, a psychopath. He beat us daily and I fought back while my twin never did. My physiology is different than most women I’ve ever seen. I’ve had surgery to correct a defect. I was the one who joined the Volunteer Fire Department, was coached in softball by my dad, and joined the military. Yes, I believe I’ve always been more aggressive than probably 90% of other females. Oh yeah, I’m smarter too! LOL

Posted by Heather on September 8, 2016 at 03:26

I would love to hear more about your experience. My twin brother died early in the pregnancy. I am writing about the issue and especially interested in how a male twin might affect a female’s development. Thanks!!!!

Jeana Pethel
Posted by Jeana Pethel on September 3, 2016 at 18:24

I am a 70 year old female from a male/female twin birth. With this new research into the testosterone transfer, I believe that I am finally able to explain my life. 
My brother and I both weighed 7.5 pounds, and I was born first. The doctors did not know there was a second baby, and finally William was born, but fluids got into his lungs and he died later that day. My entire childhood was spent trying to kiss my elbow….my mom had told me that I could turn into a boy if I could do that. I spent my days playing with the neighborhood boys. My dad did all the guy things with me that he would have done with William….hunting, fishing, golf, baseball….so I attributed my sports ability to that and ultimately got two degrees in kinesiology.  I later went into sales and preferred to work with men. I had all kinds of opportunities to join women’s clubs, sororities, but I was never interested and always gravitated to men’s groups professionally. I competed with men in business for over 25 years and usually came out on top.  I would consider myself more aggressive than any of the women friends I know. Since I didn’t grow up with a brother, those traits don’t come from that association. I would love to be part of any future studies that are done. I am sure there are keys here to the gender identity issue as well. Thank you for the article.

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