Sex Differences: Do females and males have different brains? Addendum
This'll be a short one.
Source: Miles Wolstenholme
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".
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.
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.
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.
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.
The established theory he's talking about is sex dimorphism, which in broad outline states that "sex dimorphism in behavior is controlled by hormones". 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".
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.
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.
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. https://link.springer.com/article/10.1007%2FBF01067432
2. Udry, J. R. (1994). The nature of gender. Demography, 31, 561-573. http://people.virginia.edu/~ser6f/udry.pdf
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. https://www.springer.com/gp/book/9780306439834
Earlier Sex Differences episodes: