Sex Differences: Do females and males have different brains? Addendum


This'll be a short one.
Source: Miles Wolstenholme

Curtain Open

Not all the studies I read end up in my posts. They might not fit the general flow, have nothing new to offer, or excised for other reasons.

Here are two studies I could not bare to leave unmentioned, and for which I wrote this addendum. One of them (the second one) is one of my favorite papers in the field of sex differences—although I don't explore my favorite side of it here, the theoretical side, because I would end up just copying the whole thing. Rather, I explore just that part of it that has to do with the scientific study its author conducted.

So let's get to it.

422 trios of British twins


The Jedward twins, with their song Lipstick, were my prediction to win the 2011 Eurovision Song Contest. They failed to measure up to the prediction.
Source: Robyn Gallagher

Studies on rodents (mice, rats, gerbils) had revealed that females that have two male intrauterine(=inside the uterus) twins exhibit more masculinized behavior, whereas males with two female twins exhibit more feminized or less masculinized behavior. That's because when a female grows up inside an environment with two other males, she has "higher levels of testosterone in blood and amniotic fluid"[1].

If human twins could be studied, a presence of similar effects might further prove the influence of hormones on behavior.

So three (non-twin) researchers got the idea to pore through previous studies done on twins and see if the data mirrors the rodents'. They found studies totaling 422 trios of British twins that had scores available on tests such as disinhibition, thrill and adventure seeking, experience seeking, and boredom susceptibility.

It's important to emphasize that this is a re-analysis of previously reported data. This means that, barring backward causation, the data could not be influenced by the biases of the authors.

The results both confirmed and deviated from that of the rodents. They deviated in that:

In contrast to the results for females, male cotwins of female twins did not show a consistent pattern of results.[1]

So males growing up in the uterus with two sisters were not affected significantly.

How about a female sharing a belly sack with two males?

Results were significant for measures of disinhibition, experience seeking, and overall sensation seeking.[1]

It's possible, however, that some other factor may have had this influence on the females. For example:

female cotwins of males may be more likely to be disinhibited or demonstrate increased risk taking by virtue of time spent interacting with their male cotwins.[1]

How can we know the effect is due to hormones rather than time spent playing with brothers? The authors call for further research, which is the equivalent of ending a movie with a question mark, something I generally detest (in movies). You're free to go with the rodents on this one, perhaps citing the principle of parsimony. You will not be alone: at least one population researcher did just that.

If the theory ain't broken, don't fix it


Are human yawns socially constructed, as opposed to those of other animals? Or does one theory fit all?
Source: Joseph Ducreux & Daisuke Tashiro & Rachel C from Scotland, modified

The parsimony-lover begins thusly:

When I was in graduate school, I was scarred for life by the slash of Occam's razor. I adopted parsimony as my mandate; I wrote the shortest dissertation in the history of my department; I am one of the shortest persons to hold this high office; [...] I try never to invent a theory when a good one exists. [...] I have been deeply impressed by what animal models in biological research have taught us about human biological functioning. I like the theory of biological evolution because it is panspecies and includes human beings. If there is an established theory about gender in other species, we ought to see how it works on humans.[2]

The established theory he's talking about is sex dimorphism, which in broad outline states that "sex dimorphism in behavior is controlled by hormones"[2]. All mammals, from rats to apes, have similar primary sex hormones, and they guide the development of both physical and behavioral sex-dimorphisms. "Forty years of animal experiments have firmly established this model".[2]

What are the chances that the universal theory of sex dimorphism that ties all other vertebrates, let's us off the hook and excepts us?

Not one to ask the question without attempting to answer it, the author, Richard Udry, did an experiment, which again was mostly based on prepublished and publicly available data. The prepublished research was conducted by the Kaiser Research Foundation and the University of California-Berkeley School of Public Health, and it involved taking blood samples from women during each trimester of pregnancy, and these women and their children were followed up with measurements and interviews at varying ages of their offspring, ending at 17 years old. Udry additionally reinterviewed ~350 female offspring when they were 27 to 30 years, and got blood samples from 250 of them.

Then he let gendered behavior be determined by the subjects themselves. So if most women answered that they liked caring for babies, this was rated as a feminine behavior, and those females that did not follow the majority in liking baby care were rated low on the feminine spectrum.

What counted as female behavior didn't matter. What mattered was if the blood tests covaried with the behavior to a large degree. If the idea that gender is 100% socially constructed was true, then women liking baby care would not be predicted by their hormones. What the research found was that feminine traits follow the hormones by 25%. The parsimonious primate model:

predicts one-fourth of the variance in Women's GENDERED behavior[2].

He's not shouting: GENDERED was simply the name Udry gave to a superfactor—a superfactor being the combination of all gender components in the study, not a gender studies superhero.

That leaves at least—in fact at most—75% for social influences. It seems that 25% of our gendered behavior is biological, and the rest is exaggerated by social factors.

Again, to make the results a bit clearer: some women were on the positive end of "baby care" (and many other measured factors), some on the negative end, and many in-between. Social constructionism predicts that the distribution is random. Biological influencism (?) predicts that female hormones partly explain why some women are closer to one end and some to the other. It's the latter theory that was borne out in the experiment, since hormones can account for and predict 25% of the distribution.

Curtain Close

So there you have it. The paper by Udry, called The Nature of Gender, can be read here, and I recommend it if you have some free time. It's not long. It's not complicated. I find it entertaining. He explains his sex/gender hypothesis using very illustrative bell curves and arguments. The research I've read so far on the topic of sex differences supports his ideas: it does seem to me that a healthy part of our behavior is biologically influenced, and the majority is socially constructed. That means that a society that treats the sexes like there's 0% biological differences between them will actually oppress them, which is a thought again explored in Udry's paper. But he does add that sometimes oppression is called for: maybe our biologically primed behavior is evil, and it ought to be oppressed. An example (a crude one) could be males that are more prone to violence. But still, knowing this is the case will help us achieve our goals, maybe by directing the need for violence toward socially accepted avenues, like sports.

So that's it. Though it feels to me like we only got started. I'm not used to this kind of short post!

I'll leave you with a pertinent quote from two female sex differences researchers:

even if differential treatment of the sexes did not occur, behavioral differences would remain, nevertheless, between males and females. [3, p. 238]


1. Resnick, S. M., Gottesman, I. I., & McGue, M. (1993). Sensation seeking in opposite-sex twins: An effect of prenatal hormones? Behavior Genetics, 23, 323-329.

2. Udry, J. R. (1994). The nature of gender. Demography, 31, 561-573.

3. Reinisch, J. M., & Sanders, S. A. (1992). Prenatal hormonal contributions to sex differences in human cognitive and personality development. In A. A. Gerall, H. Moltz, & I. L. Ward (Eds.), Sexual differentiation: Handbook of behavioral neurobiology (Vol. 11). New York: Plenum.

Earlier Sex Differences episodes:

5: Sex Differences: Do females and males have different brains? Pt 3

4: Sex Differences: Do females and males have different brains? Pt 2

3: Sex Differences: Do females and males have different brains?

2: Sex Differences: Check out the gonads on that one!

1: Sex Differences: Does the Chromosome Maketh the Man?

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I just love this post. Entertaining and informative--also had me arguing throughout--especially the last quote: "even if differential treatment of the sexes did not occur". The absurd proposition of the quote stunned me. As soon as there are perceived differences between any two things, there is a difference in treatment. Even in the case of identical twins, there are differences in treatment. Until AI does rule the universe, people will bring subjective perceptions to every situation. This can be based on something as random as birth order, or something as substantial as perceived similarity to a birth parent. So...perceived gender differences will always affect socialization. This is a never-ending loop.

I remember taking an aptitude/preference test (to help me choose a career path) as a very young woman. The testing took about three days--I stretched it out because it was a tedious procedure. At the end, I was told by the administrator that the result showed (among other things) that I had both female and male preferences. Well, that was helpful.

Whatever science tells us about gender determination in general, this is of little relevance to the individual. I think the results only tend to reinforce policies that satisfy someone's agenda.

We should forget about gender. Just offer kids--people--all options and try (impossible) to keep our expectations out of it. Life and circumstances will take over in ways we cannot anticipate.

You're right in that keeping our expectations (broadly, our values) out of it is impossible. All we can do is educate ourselves as much as possible, and hope that this will direct our actions more in the right direction.

I briefly tackled some of the other points you raised in previous posts of mine, including this one, in which I give a quote and then comment on it:

Claims that feminist aims would be invalidated were science to establish that cognitive sex differences are caused partly or totally by biological factors [...] reflect a failure to understand the scientific literature.[4, p. 245]

There's a lot of reasons behind that statement, but one of the main ones is that differences between individuals are much greater than differences between the group labelled "male" vs the group labelled "female". If you're looking to hire someone, or admit someone to a course, or offer them a teaching position, knowing their sex does nothing to help your hiring algorithm.[4, p. 245]

And afterwards I offer an example (having to do with female periods) about how ignoring the science can actually hurt feminist aims.

So I agree that the majority of our behavior is the result of our socialization, but, like the quote you disliked at the end, the science tells us that even without it, some male/female differences would remain. It's easier to see this if we look at other animals. I don't see why we should be so different from all our animal relatives in that we, as opposed to them, are 100% the result of socialization.

But, again, in real life, as I explain in other posts, individual differences matter much more than average group differences. Just because John knows Jane is a female, doesn't tell him almost anything about her specifically, except maybe her genitals, and even that's not 100%.

Oh dear, I have to do my research to get back and offer what I hope will be a valid rebuttal. I read your blog on hormones/gender determination. This is where I think I'll have to begin. Hormones are certainly powerful factors, but what determines those? Are there epigenetic influences that help to regulate those hormones? Can we ever truly separate hormone secretion from social influences? Two articles I found in a cursory Google search hint at the direction of my reading: Epigenetics meets endocrinology and Prenatal endocrine influences on sexual orientation and on sexually differentiated childhood behavior. I think, in order to have a purely objective discussion, I'll have to leave gender out of it. This is a charged subject. Better to look generally at how social/external/epigenetic factors can influence hormone secretion. Right now, I have nothing meaningful to say, but I hope that will change in the future.
Good, provocative blog--obviously got me working :)

What you're saying is already meaningful!

Great article (post is good as well :) )

I simply don't understand how something natural became a taboo.

Having different physiology/ psychology/ goals in life/ point of view is not bad. It's simply different. Framing something fundamentally different into the same mould is absolutely wrong.

This is awesome as usual. Where I come from, we take whatever we perceive as what biology had offered and we make it a rule. There are no exceptions but I have always known that black and white are hardly existent in nature and whenever we think we have seen either of those, we must look again and even then, we must make allowances that we have not seen correctly. Your sex difference series had been educative and has confirmed some of my deep thoughts on gender and sexuality. As usual, I have learned a few things on writing style. So thank you.

You’re the man.

Great information, organization, articulation.

I dunno where I get my beliefs from, a combination of life experiences, reading etc. But I generally had the same idea (hypothesis?) that biology makes up the initial structure but the social aspects -upbringing, experiences , influences, etc.- at an earlier age (and potentially after and continuously) affect who a person is overall, not just in the sex/gender realm.

Yeah. I guess, thinking about it, it's not such a radical hypothesis to have. The difficult part is offering arguments for how that may be the case, and exploring the ramifications. In many animals biological differences are exaggerated by various behaviors, for example a male bird adding a fake feather to his tail to exaggerate the tail-size that is already, on average, bigger than a female's. So I think to some important degree our socializing men and women toward greater extremes is an attempt to accentuate those already-present biological differences, and, ultimately, to attract mates.

The problem (re: socializing) in humans (which is also a great “blessing”) is the ability to think, speak and form an opinion. Because one can influence their fellow human with more than just the physical senses but all the psychological.

Where everything gets skewed beyond a relatively uniform specieal (of the species) normalcy.

Like genes, like memes! One physical, the other mental.

There definitely are gender differences, which may correlate with neural connectivity and structural differences between male and female brains. Man can observe superficial phenotypic difference between male and female of our species - males are bigger, more muscular, less risk averse, etc. - that are clearly biological. Furthermore, cursory glance at human employment distribution within a society suggests the male-female dimorphism. Some fields women do not pursue, regardless of employment availability; others are dominated by women. Male and female of the species throughout the animal kingdom are specialized to serve distinct purposes. To ignore reality is, as you write, to foster and perpetuate tyranny born of ignorance.

Well we all have different brains if that's what you

From a purely biological perspective, it is quite obvious to the blind that we're very different so having disdistinct anatomies isn't a bad thing. Males and females SHOULD be different

We do indeed all have different brains. That's a far more important fact to realize, and research, than the fact that there are (much smaller) average differences between the sexes.

I don't know about the 'should' part. We are probably as attracted to the opposite sex for our similarities as we are for our differences.

a great article well articulate thanks for the enlightenment

I resteem you.i helpful

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Αύριο ποστάρω! ... Been busy lately!

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