March 15, 2012

New study indicates that female chromosomes may make males more masculine

I am always taking biological research on sex and gender with a grain salt, as many of the studies are both reductionistic (in the sense of reducing amazingly complex phenomena to one or two variables) or plain out sexist.

I continue to read them, though, as many of them give additional input and additional perspectives that may be valuable in our journey towards a better understanding of transgender conditions.

Here is one such study.

Hormones in the womb

Remember that the ruling paradigm on the biological side of sex and gender research is that the sex of a boy or a girl is determined by pre-natal hormones, i.e. the hormones the fetus is exposed to before it is born.

"The predominant idea is that the difference between male and female behaviours is down to hormones," says Emilie Rissman at the University of Virginia in Charlottesville. Male fetuses are exposed to testosterone from 4 weeks old, while females are not.

In this scenario the role of genes is reduced to being triggers deciding which hormones are to be produced and in what quantity. Because of this, sex variation -- including transsexuality -- may be understood as the result of genes not producing the normal amount of the  expected hormones. 

Rissman's team wanted to find out of if genes -- or, to be more precise, sex chromosomes -- could influence sex-specific behavior directly, i.e. beyond what could be expected from the pre-natal flow of hormones. 

Remember that the XY sex chromosome combination will normally make a boy, while the XX variant normally leads to the fetus becoming a girl.

XX does not a girl make

What they seem to have found is very unexpected: An extra set of female (sic!) genes appears to make males more -- not less -- masculine. And yes, the effect is not the result of the production of pre-natal hormones.

The New Scientist puts it this way:

"To find out if sex chromosomes play a role in sex-specific behaviours beyond dictating which hormones are present, Rissman's team took advantage of a mutation in mice that causes the sex-determining region of the male Y chromosome to jump to a non-sex chromosome. The mice are male but have two X chromosomes.

While these XX male mice had the same level of testosterone as normal XY mice, they displayed more masculine sexual behaviours - mounting females more often and ejaculating more frequently.

To confirm that the differences were a result of a hidden factor on the X chromosome and not the lack of a Y chromosome, the team compared XY male mice with XXY male mice, which carry an extra X chromosome. Sure enough, the XXY mice also showed more male sexual behaviours."

(See my presentation of the debate on genes, chromosomes, hormones and sex identity for more information on the roles of chromosomes.)

Femininity and masculinity in mice

What these researchers have found is differences in behavior, which is not necessarily the same as "femininity" and "masculinity" in a human setting. Nor does the behavior of XXY male mice say anything about how these mice feel about their sex identity. I would guess they do not reflect much on gender issues at all.

Moreover, this definitely says nothing about the sex identity of XXY human male bodied persons. I remind you of the recent comment made by Tina, an Indian male to female crossdreamer, who has reason to believe he has the Klinefelter XXY condition.

In transgender circles it is well known that XXY male bodied persons can be transgender. Chloe Alison Prince, the founder of the PINK essence transgender site, is -- for instance --  an XXY transwoman (photo here).

The complexity is much bigger than previously expected

So what does this research mean for crossdreamers?

Well, there is nothing in this search report that can be used to explain various transgender conditions directly. In fact, according to this and related research XX  and XXY male mice are not transgender. If anything, they are more masculine than their traditional XY brethren.

This tendency has apparently also been reported among human XXY males. In other words: Among XXY human male bodied persons the condition can lead to (1) more male typical sexual behavior in some cases and  (2) a female sex identity in others. This tells me that there is no one to one relationship between men having two X chromosomes and being masculine in the human sense of the word.

What it does tell us, though, is that the biological system underpinning the sexual behavior in mice -- including genetic, hormonal and environmental factors -- is amazingly more complex than our high school text books tell us. This complexity leaves room for a lot of behavioral variation in mice.

In humans, of course, cultural and social factors are significantly more important in determining behavior than in mice. Even the most biologically oriented scientists  admits that only 50 percent of what we do can be attributed to nature (as opposed to nurture), meaning that you should expect much more variation among men and women than among mice.

That is, of course, exactly what we find. That being said: It is unreasonable to think that biology is of no importance when it comes to the development of our identity as men or women, which again leads me to conclude that we should continue to follow natural science studies of sex and gender with interest.


"X-chromosome dosage affects male sexual behavior" by Paul J. Bonthuis,  Kimberly H. Cox,
and Emilie F. Rissman. From Hormones and Behaviour, DOI: 10.1016/j.yhbeh.2012.02.003.

"Sex differences in the brain and behavior are primarily attributed to dichotomous androgen exposure between males and females during neonatal development, as well as adult responses to gonadal hormones. Here we tested an alternative hypothesis and asked if sex chromosome complement influences male copulatory behavior, a standard behavior for studies of sexual differentiation. We used two mouse models with non-canonical associations between chromosomal and gonadal sex. In both models, we found evidence for sex chromosome complement as an important factor regulating sex differences in the expression of masculine sexual behavior. Counter intuitively, males with two X-chromosomes were faster to ejaculate and display more ejaculations than males with a single X. Moreover, mice of both sexes with two X-chromosomes displayed increased frequencies of mounts and thrusts. We speculate that expression levels of a yet to be discovered gene(s) on the X-chromosome may affect sexual behavior in mice and perhaps in other mammals."

See also "What Brain Science Says about Male to Female Transsexuals".


  1. "That is, of course, exactly what we find. That being said: It is unreasonable to think that biology is of no importance when it comes to the development of our identity as men or women, which again leads me to conclude that we should continue to follow natural science studies of sex and gender with interest."

    I totally agree with this. Nature is so fascinating... It will take us forever to fully understand their amazing mechanisms...

    Great job, Jack! Super interesting post (as usual).

  2. Having read the paper in full and understanding the experiment that went on, I really, really disagree with the way both New Scientist and you have represented this finding. What the research shows really, really shouldn't be reduced down to "more female chromosome makes men more male", but rather, that there's probably a gene on the female chromosome that activates downstream genes to cause more sexual desire/mounting. That's really not that surprising, given the amount of crosstalk between the sex chromosomes and the way chromosomes work - which is that there are many genes on any given one, such that no single chromosome has only a single function. Both sexes use the X chromosome for much, much more than just sex determination, so there's every chance they simply hit on a sex desire gene, since they did no gene finding for this at all.

    It's useful to stop thinking in the binary when talking about entire chromosomes. Many nonsexual and male genes exist on the X-chromosome. The real difference is whether a fellow X or a Y is signalling it (for reference, the Y chromosome doesn't have half as much in the way of useful genetic material, and has genes mostly involved in signaling genes on other chromosomes). The authors of the paper even suggested the reason for the extra X's effect may be that a gene that originally be male and signaled by the presence of a Y chromosome, and it's simply that the extra X means two doses of the gene activated by the Y. That's what I mean by "chromosomes aren't binary, genes are.

    There's no way of knowing whether the trait was male or female associated (if there was actually significant proof, then the paper would be published in something higher than Hormones and Behavior, but that's my pompous academic side talking). The real take home message probably should be that X-chromosome genes are not necessarily female, and their expression depends on the context - in this case, when a Y is present. There's thousands of genes on the X-chromosome, and since both sexes have one, it's not surprising that an extra dose can cause more "maleness".

    Side note: I kinda found the research a bit ridiculous. Really, castrating all the specimens and injecting testosterone to make them sexually active? If anything, this may just demonstrate that testosterone activates some gene on the X chromosome that triggers sexual desire, and that's really not at all surprising and links everything back to testosterone... And their presentation of the difference in mounting frequency, their main point, was that XXY > XX > XY > X0. This really makes me wonder about how people are interpreting this. All I'm getting is that the more X/this particular X gene, the more mounting sex when exposed to testosterone, because normal females mount even more than males when given T. Y just makes you a little more receptive to T, somehow... And T makes you more male.

  3. @Iris

    You really have a good point and everything you said makes sense. I think many of us will totally agree with you. But you have kind of put subtitles to what is obvious for most of us.

    I think Jack's point wasn't "more female chromosome makes men more male" but that the study of natural sciences is needed for a better understanding of sex and gender and that is wrong to think that biology doesn't play an important role in human sexuality. At least, I think that is the utility of the research for us.

  4. "What the research shows really, really shouldn't be reduced down to "more female chromosome makes men more male", but rather, that there's probably a gene on the female chromosome that activates downstream genes to cause more sexual desire/mounting. "

    You are right of course, and as Ariadna points out, my point was to use the research to demonstrate the complexity of the system called the human mind and body.

    Sometimes I am just too tempted to write an eye catching headline....

  5. Is there any truth that crossdreamers or people who think they are a woman trapped in a man's body need to have brain surgery because there is something wrong in there? I'm not sure, you can correct me if I'm wrong though.

  6. @brain injury

    Your comment borders on link spam, but your question is relevant so I will let it pass.

    No one has, to my knowledge, proposed brain surgery as a possible cure for transsexualism.

    We know far too little about the brain to be able to pinpoint a sex identity region that could somehow be brought in line with the biological sex.

    Even though I personally believe there is some kind of biological core to the transsexual condition, I doubt very much that it can be reduced to the effect of one and only one part of the brain.

    There is an ethical issue here as well. At the moment the body is realigned with the mind through sex reassignment surgery. In other words: It is the personality of the person that is given precedence, not the biological sex.

    Some would probably argue that changing the brain would be a violation of that person's personality, and a kind of cultural and political suppression.


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