NRSC 4132 Blog

Forget everything you thought you knew about the differences between men and women. The latest research has shown that the differences between the sexes run much deeper than body structures and the equipment they have downstairs, to the point that even the way they think might be distinctly masculine or feminine and attempting to understand the thinking of the other sex may be a larger gap to cross than previously thought. Now there's much promise that recent scientific developments may help create understanding of atypical sexual activities, preferences, and anatomies. Margaret M. McCarthy and Arthur P. Arnold of the University of Maryland and UCLA, respectively, have culled the research of recent years to update the last century's model of sexual development in their May 2011 Nature Neuroscience published review, Reframing Sexual Differentiation of the Brain.
Things used to be simpler in the past when an animal simply had either testosterone or estrogen produced in the gonads and then those molecules would go off into tissues and structures all over the body and exert influences over long periods of time. Now, McCarthy and Arnold tell us that that's only part of the story. Recent research has shown that estradiol, the major masculinizing and defeminizing metabolite of testosterone, can be synthesized on demand in neurons and have rapid effects, that genetically castrated rats still behave like males, and that synaptic connection strategies differ between the sexes. The authors are presenting an entirely new, more nuanced model of sexual differentiation that takes into account the multiple parallel signaling mechanisms recently discovered and the role of environment.
Their model is helping to explain why men are from Mars and women are from Venus. It turns out that there are many differences in brain structure between the sexes. Males tend to have larger spinal cord nuclei and medial preoptic areas of the hypothalamus than women, which is to be expected since these areas control penis muscles and masculine sexual behavior, respectively. What's more surprising is that females grow more new cells in the amygdala soon after birth, and males get more in the hippocampus, areas more typically related to fear and memory than sexual behaviors. Would that be something that would affect the way their brains function, making them think differently? Well, if not that then certainly the recent discovery that male dendrites are longer and more branched than females in the ventromedial nucleus of the hypothalamus, along with differences in spine density as much as 2-3 times between the sexes in diverse areas may clear some things up. With such varied connection strategies, it's no wonder that the sexes often can't follow each other's thoughts; the ideas may end up in completely different parts of the brain depending on whether the listener has a male or female brain. Indeed, proportionally larger brain areas may contribute a larger slice of the overall cognitive set, and so it may make sense that women and men will have different strategies for solving problems.
Another extension of the authors' work worthy of note is that their model makes room for not just dimorphism of sexual behavior and physiology, but polymorphism. Cloning has made possible the creation of XX genome rats with testes and XY genome rats with ovaries by moving the testes-determining gene, Sry, off of the Y chromosome to an autosome. Many other things stayed the same though, regardless of gonadal sex, such as habit formation patterns, alcohol preference, and aggression. This means that the things that make a person male or female are likely largely independent of what sex organs they have. Certainly there's room in the human genome, particularly atypical genomes, for combinations of genetically influenced traits all across the spectrum. Hopefully this enlightenment can lead to more understanding and tolerance in a world know for its brutally against those with sexual differences.
These more nuanced new ways of thinking have a certain charm in their complexity. I remember being taught as a child that all things male came from testosterone, all things female from estrogen, and then being appalled to hear in my teens that women had testosterone too and that it was a good thing. It's pleasant to imagine now describing to my children this more complex understanding and watching their minds engage and take in the realization of the depth of the mystery of the world.