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

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

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.


Lisa R.A.

Female twin here with Male twin brother. I was second born. My Mother is also a female twin with a twin brother. (She was first born). Her Mother had a set of twins as siblings but they died at birth ( I am not sure what their sex was.) My mother is physically strong and more "masculine" in her appearance than her older sister. Her brother was more delicate but matured to be fairly average, regarding his sex traits. My Mother grew up to be more feminine despite her strong characteristics. I have been mistaken as male since puberty and tend to be told things like "I am in the wrong bathroom" and TSA stops me routinely because I do not match the expected appearance for my sex. I have had lots of problems with my reproductive system and have required a hysterectomy, oopherectomy and removal of my breasts (no cancer but fibroids, endometriosis, cysts, extreme breast tissue density and issues caused by a prolactinoma). My mother and I both have prolactinoma pituitary adenomas. My mom is straight and I am lesbian. The comments here seem to match up with my experiences. My Mom and I were always very protective of our brothers when we were children. I was always protecting my brother and other boys from bullies. (In adulthood, my Uncle and brother seem fairly average in regard to their sex characteristics - although, they are both slightly more emotionally sensitive than most men. I don't want to disclose too much about my bother out of respect for his privacy.)

Il-Keun Kong

I want to get this kinds of information to expand my knowledge, if possible.


Phoebe Brule

fascinating -I am 67 and had a twin brother til he died 10 years ago . Today is my first day of researching this topic -always felt like a B-list twin as the identicals get all the research . Can relate to many of the comments -have long ring fingers and was never pink/frilly as a child though hetero . Never wore make up or high heels and men always preferred to be friends and comrades rather than romantic -annoying , I always felt I was a failed girl -never married , no kids .So I love sites like this one and feel inspired to look for more .

Ttt Effect

Glad a group for TTT females was started on facebook! It seems TTT females really do share similar traits, or, at least to a much greater extent than solo born females. It appears to have little to do with modeling after a twin brother. Two TTT females moderate the group. One is a twinless twin, the other has a co-twin. Here is a link to the group--we would love to have you join! https://www.facebook.com/groups/203161770572091/

Jeana Pethel

Please check out our new facebook page and ask to join if this is of interest to you. This is not a support group but a venue for educational purposes and personal enlightenment. We can learn from each other!
TTT Effect ~ Twin Testosterone Transfer Female ~ Male Co-twin ~ Hypothesis
Closed group


Here are a few more Facts about me that may/may not be related to TTT; which I would like to share in the name of research. They are mostly physical traits. More mental traits can be seen in the post before this one. It's a little personal, but I think it will help further study.
I have a retroverted uterus (indicated on my first pelvic exam).
Studies have shown hormones have an effect on fingers. More elevated testosterone levels in the womb create longer ring fingers. My ring fingers are equal, or a fraction longer than my index fingers. Most women have shorter ring fingers.
Partners have commented I have a "hard" body and that my clitoris is farther from my vaginal opening than "other girls."
I've been a 36 A most of my life.
I have broad shoulders (great for swimming)..
I was mistaken for a boy often, up to puberty (even in a girl's Speedo).
People tell me I have "sharp" facial features and "strong jaw."
I don't like dressing up,or wearing high heels. I'll do it for theater, or a costume party; it is fun then.
As a child I was hunting, using woodworking tools, making forts, swamping, adventuring in Nature and other "boyish" stuff.
I beat my principal arm wrestling in elementary school. I was unusually strong.
I didn't care for dolls, tea set or dress up. I did like stuffed animals.
As my childhood girl friends came of age I was left behind.
I enjoy cooking, sewing, art, growing flowers and using make-up which is the extent of my girly side.
I was NOT attracted to Ken Doll types when I came of age. I had the hots for Steven Tyler who, curiously, some describe as androgynous.
I've never suffered dislike of myself for not exactly fitting in.
I am mostly quiet, but can be a bit of a rebel, for good cause.
I probably would have never gotten married if I hadn't "settled" for someone in my thirties which ended up in divorce because I hadn't done my homework (he was abusive). TTT females have a higher rate of being single according to a UK study. I remain single after almost 20 years. I have good male friends.

April Gawboy

I stumbled into this blog just today, frankly after searching transgender to better understand that community (since two of my children have very close trans friends). As a biology student I already knew about the effect a bull calf has on a heifer in utero. The heifer is born a free-martin. Sterile due to the effects of exposure to her brother's "T". Her reproductive tract is altered, and she may have a smaller udder as well. In some rare instances a female calf having a male co-twin can breed; but not always.
I have to admit I mulled a bit on how that might apply to me as a girl with a boy co-twin. It seems in humans, the woman is still fertile, as evidenced by my own four children, and the fact I never came across anything about fraternal female humans being sterile.I figured the whole hormone transfer thing didn't apply to me.
After reading this blog and some of the comments I definitely had an Aha moment! I always connected more with males than females. I never liked dresses or girly things. A cognitive test showed strong spatial skills which is generally linked to a males stronger parietal lobe, and I pursued a career in drafting (always having been nuts about architectural design). I was bored when my first job was nothing more than sewer and water work infrastructure, and pursued the other skill I had up my sleeve: Pet Grooming. That I stuck with for 20+ years.
There are many female aspects to me, but probably more male. I've been a "member of the itty-bitty titty committee" for most of my life. (Only in my menopause has the chest filled out somewhat more, as my mother's did. She had to get a breast reduction. I am far from having to do that.) My fat index, still, is way low. I believe my body type and my fun with sports competition allowed me to be the star swimmer that I was in Junior High (excelling at breaststroke). I was teased a lot for being flat, I remember that. Most boys were uninterested in me--but a few of the more interesting ones were, so it was all good. I started experimenting with make-up in about 10th grade. It really kicked up the male response to me. Being hetero-sexual I liked that. To date, socially, I have men friends who value me for me, and no close women friends. I am working on the women friendship thing, although not interested in gossip, fashion, customs, etc. I do love to garden and sew. My hobbies are what connect me with the women, my understanding of men connect me with men.
This is getting long, but I learned something about myself today! Very revealing! Like Jeana suggested--let's get a FB Group started for us!!!


HI all, how do we keep this conversation going? Someone mentioned a FB group. I would love to chat more about it.


This is extremely interesting!
I'm gay and my twin sister is straight.

We lived in the same house for ~20 years of our lives and spent a lot of our childhood playing together but also separately (I had a male friend group and my sister had female friend group).I've always wondered why I'm a lot less aggressive than other boys and why my sister is a lot more 'boyish' than her female friends (she hates dresses and other girly habits).I wonder why we both don't fit the stereotypical gender behaviour but I ended up being gay and her straight.

Jeana Pethel

I would like to add that when my hormones at puberty kicked in, as mentioned in the article, I became a girly girl on the outside and very interested in boys and that continued through my life. I believe the other dynamics in my life are a direct result of the TTT. Such an interesting discussion.


Female here with a male twin. I definitely fit the stereotype of being more “masculine” and aggressive. I used to get in fights, play lots of sports well, and I don’t care about makeup, fashion, or cooking. I’m definitely straight, but even guys I date point out that I seem kind of masculine in my personality. I always attributed my masculine characteristics to growing up with a brother and having mostly male friends in my early youth. Interesting to think that it could have been partially due to prenatal influence. My brother also fits this— he is more easygoing and less aggressive in general.


I'm a female with a twin brother. I have been described as having more masculine personality traits. I would certainly consider myself having androgynous personality. I have hypothesized that I was exposed to higher levels of testosterone in utero and that may explain it. This is an interesting article and has provided me with some base terminology for further research. Thanks so much!

Doe Cartwright

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


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

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?


And, yes, my relative digit lengths are "masculine."


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

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

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


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

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.