Tuesday, May 14, 2024

Brain Bias, Male and Female



Image by The Conversation

“On the basis of the information available it seems unrealistic to deny any longer the existence of male and female brain differences.”  

                                                --Richard Restak, The Brain: The Last Frontier (1979)


In the late 1970s, neuroscience investigator Richard Restak was excoriated as a sexist for suggesting—based on evidence, that is, expert opinion—that the brains of human males and females are different.  To hold this opinion is to risk accusations of neurosexism. However, for “gender justice” advocates, female (but not male) brain differences are now insisted on.  This advocacy is mobilized in order to distinguish uniquely women’s issues as the platform for rights and protections, including diversity, health, social, and psychological.

Ongoing studies show a mosaic of male and female features in virtually every brain; there is no pure male or female exhibit.  Yet certain structures and chemistry occur more commonly in each gender.  Male and female brains show bias in a continuum across a number of aspects, from drug processing to reasoning style, mental states to mental disorders. 

One of the reasons brain-sex dimorphism is controversial, Restak explains, is that sex identity and behavior in our species aren’t neatly compartmentalized as in other research subjects like mice and monkeys.  Many question whether animal studies are analogous to studies of brains in primates.  In humans, genotypic sex, phenotypic sex, sex attraction, and gender identity are not reliably aligned.  Sex organs and hormone effects are visible evidence markers between the sexes.   However, these are aligned by genes and their expression throughout the body, like testosterone (male) and estrogen (female) hormones, influencing thinking and behavior in the brain.  These include connections happening prenatally, before exposure to cultural or environmental experience.  The female brain is the default through the X chromosome, meaning that every brain begins as female but only half develop as male.

Stepping outside the research lab, what is the first thing you notice when meeting a new person?  The only biological difference between people isn’t race, class, or age.  It is gender.  We depend on gender knowledge to adjust our communication style to suit male or female.  Gender is embedded in our DNA as chromosomes XY or XX. No matter how gender is expressed or repressed within cultural norms (epigenetics), these genes wire our secondary physical expression as breast size, genitalia, height, weight, muscle size, hip width, sex drive, voice pitch, facial features, and pubic hair.  Unsurprisingly, gender also sets up the male and female brain in distinctive ways.  Whatever gender persona you might decide to exude, your DNA doesn’t migrate between the two gender codes, XX and XY. 

Sex hormones are important to the way people look, feel, and behave.   In men, the Y chromosome carries a protein promoting testes formation, testosterone production, and creation of the male brain.  While this protein makes males more prone to retardation and learning / speech disorders, dyslexia, and autism, males also make up the majority of geniuses at the opposite end of the intelligence scale.  Female fetuses are better able than male to recover from prebirth brain damage. 

Estrogen in women encodes language as spoken sounds (phonemes) as well as the visual coding of written language, and also plays a role in long-term memory (so women are slower to forgive and forget than men).  Testosterone predisposes males to risk danger and aggression, so that most murderers are male.  The male hormone sets desire in men, but testosterone also drives sexual desire in women, even with far less of it in the female brain mix.   

Whatever the differences, Restak concludes, “it helps to keep in mind that such differences do not imply that one sex is superior to the other” (Mysteries of the Mind, 2000, p. 64).   Only that each has a likely inbuilt bias to prefer one type of thinking or acting over others—and therefore, to practice and excel at that behavior.   An article on The Conversation site ventures that  On the other extreme, we are dismissed by women’s health advocates, who believe research has overlooked women’s brains – and that neuroscientists should intensify our search for sex differences to better treat female-dominant disorders, such as depression and Alzheimer’s disease” (April 22, 2021).

For example, women perform better than men on verbal tasks (estrogen-promoted), as well as intuitive reasoning, motor skills, and scanning environments for select features (finding all the green chairs in a mostly blue auditorium, or the best fruits on the tree, or a child on a bustling playground).  The hippocampus, the human memory center, is larger in females, with a higher density of neural connections.  It facilitates memory for people and reading their emotions.

Men excel at spatial tasks, including rotation of objects in mental space, and do better at math, logical reasoning, and motor skills directed at distant targets (aiming and tracking).  Men in mazes navigate by dead reckoning, while women rely on landmarks sequenced in memory. This is why men generally ask for directions much later than women. This is the root of the difference between hunters and gatherers, and why hunter-gatherer societies were based on gendered division of labor.

Another sex-brain disposition is in the anatomical balance of gray and white matter—women have more grey (the core of nerve cell bodies) while men have more white matter (nerve fibers for signaling around the nervous system) involved in connecting brain and body.  Female brains show increased coordination between regions, whereas males have a more separated left-right structure and connect back to front versus women’s hemispheric coordination left to right.  Men and women process neurochemicals differently using different receptors, as for example serotonin synthesis, seen in dominant primates, is over 50% higher in males.

Experience and attitude influence brain dynamics and the development of structure and function.  Lived experience, for example, education, can establish new circuits and outputs, facilitating the hardwiring of the brain. Upbringing and culture can activate or repress brain functions and their genetics, creating new nerve cells and connections, the essence of neuroplasticity.  Adaptability through specific action and memory is our species’ main strength: the ability to connect our biology to our cultural learning by brain growth. The human ability is to adapt to new circumstances over generations, as well as from moment to moment to meet needs and build opportunities. 

The question now arises: How do we know these abilities and their sex DNA are not simply cultural rather than hormonal, as in biodeterminism? For example, Jews make up 2.4% of the US population, but 35% of US Nobel prize winners. Is this nature, or nurture?  The simple response is that culture tends to follow rather than determine biology, reinforcing rather than forcing brain bias. In the Jewish case, this bias is an environment of reward for literacy, encouraging questioning to find answers.  Culture is built on the existing body and brain, but then determines how they work within the process of cultural values and conditioning—for instance, in the way we think about and recognize gender.  This bio-cultural feedback loop is our uniquely human heritage.


Thanks to Dr. Herb Adler for consulting on this topic.