■ Do women think differently than men? From the battle of the sexes to clashes between cultures, it’s always been a politically explosive topic. Only very recently, however, have we begun to understand that there are important sex distinctions in the brain. Investigating these differences could lead to more effective wellness care for women, as well as an increase in the diagnosis, treatment, and prevention of disease. Dr. Jill M. Goldstein of Harvard Medical School explains, “Only over the last 15 years or so, was it ‘acceptable’ to discuss the importance of understanding sex differences in the brain in humans, with respect to understanding sex differences in multiple diseases. This is unfortunate, given that sex differences are pervasive in clinical medicine.”
__ For many years, feminists and social scientists have quibbled over whether the biological sciences placed too much importance on identifying differences between the sexes. Some have suggested that the biological sciences adhere too rigidly to polarized notions of sex. They note that studying the brain doesn’t fully account for the social and environmental factors that affect the way men and women adapt to the world.
__ Today, the two camps are closer to finding common ground, by focusing on understanding the brain rather than fueling cognitive competition between the sexes. Doctors can treat patients more effectively by recognizing factors that biologically distinguish most women from most men. Dr. Goldstein clarifies, “Sex differences exist in almost every chronic disease. For example, cardiovascular disease, Alzheimer’s, autoimmune disorders such as rheumatoid arthritis and multiple sclerosis, depression, anxiety disorders, autism, schizophrenia…the list goes on. It is important for us to understand the sex differences in the physiology of the healthy brain in order for us to compare those differences with what we see in men and women who have chronic diseases or who are at risk.”
What is a Female Brain?
■ An important sex divergence typically occurs in brain development around the eighth week of gestation. In a male embryo, the testes are formed and the XY genotype initiates testosterone production. The female brain will exhibit distinguishable sex differences by birth, which might include size, amount of white or gray matter present, or the way the neurological pathways are arranged. At birth, the female brain is slightly smaller than a male brain, with fewer brain cells and less brain tissue—which doesn’t imply anything about male or female intelligence. We might consider the female brain to be a tighter package, as it boasts more neural connections and has the potential for faster transfer of information between brain hemispheres.
__ Of course, discussions of sex are often burdened with gender semantics, so it’s important to note that presence or lack of a Y chromosome doesn’t necessarily determine a person’s gender, biologically or socially. A female embryo subjected to high amounts of testosterone might exhibit more “masculinization” than a typical female embryo, and vice versa for male embryos. The concept of a “female” brain is based on physical characteristics largely present in the brain structures of a majority of individuals with XX genotypes.
__ But the variations between female and male brains are not limited to structural differences. From a behavioral point of view, there are many acknowledged diversities between men and women. Dr. Simon Baron-Cohen of the University of Cambridge attempts to account for these sex distinctions in behavior by referring to three defined brains types: the female brain, which is more empathizing; the male brain, which is more systematizing; and the balanced brain, which falls somewhere between the two. According to a 2003 article in The Guardian, Baron-Cohen further explains his classifications: “The empathizer intuitively figures out how people are feeling, and how to treat people with care and sensitivity… The systemizer intuitively figures out how things work, or what underlying rules are controlling a system.” Though he believes that sex largely determines brain type, he also states that “Not all men have the male brain, and not all women have the female brain.”
__ It’s worth pointing out that discussions of sexual dimorphism in the brain do not set out to prove that one sex is somehow superior to the other when it comes to intelligence, simply to illustrate how the brains of men and women have different weaknesses and strengths that we’re only just beginning to consider.
■ In 350 BC, when Aristotle mused, “Woman is more compassionate than man,” the scientific world had not yet discovered that the brain is the central organ of our nervous system. Yet the ancient philosopher’s observations were closer to some modern scientific notions about women’s behavioral patterns than you might expect. (Aristotle also believed that one of the brain’s functions was to cool the blood, and that the heart was the home of the intellect.) Modern studies of children just hours after birth suggest that some traditional notions about gender could be rooted in brain differences—regardless of social programming.
__ In “Sex Differences in Human Neonatal Social Perception,” a paper published in Infant Behavior and Development in 2001, Dr. Simon Baron-Cohen and his Cambridge team studied 102 babies within 36 hours of birth. Suspended above each child’s crib was a human face and a mechanical mobile. The researchers timed how long the babies spent looking at either the face or the man-made object. To eliminate any potential biases, only the babies’ eyes were filmed during the study, and the mobile was made to be approximately the same size and coloring of the face, with an added protrusion to account for the dimension of the nose. When the study was concluded, a majority of the girls recorded more time spent looking at the human face, while a majority of the boys spent more time looking at the mechanical object. In an earlier study, the Cambridge researchers recorded evidence of sexual dimorphism by studying eye contact in babies at 12 months old. __ __ They found that 12-month-old girls made more eye contact with their parents than boys did.
All of this could explain why boys at play might be more likely to build villages out of Legos, while girls might create narratives with their dolls. Which isn’t to say these results support gender stereotyping, because they don’t—remember, Baron-Cohen’s theory doesn’t necessarily assume that because you have an XX genotype you will automatically follow female-brain behavioral patterns. But the results of his experiment do illustrate that there are clear sex differences recordable from the very earliest stages of human life.
__ Our brain’s potential for empathy, however, is subject to neuroplasticity and can change. During pregnancy, for example, women’s brains go through many changes. Not only do they physically shrink due to the fetal absorption of Omega-3 fatty acids, but they have to deal with a major influx of hormones like cortisol and progesterone in the brain, and shifts in most hormone levels throughout the body. A 2009 study in Hormone Behavior, led by R. M. Pearson of the University of Bristol, illustrated that women exhibited an enhanced ability to decode faces in the late stages of pregnancy. This indicates that a woman is preparing to communicate more effectively with her newborn, sensing precisely what her baby needs by assessing its face.
__ Male and female brains have other differences in emotional wiring. Women are more vulnerable when it comes to depression. In a 2008 issue of Neuroimage, Hristina Jovanovic, a doctoral student at Karolinska Institutet, and Anna-Lena Norstrom, who led the study, uncovered a potential reason for this difference, rooted in sexual bimorphism: Using a PET scan, these researchers illustrated greater numbers of seratonin receptors in female brains. They also believe that women have less of a specific protein that transports seratonin. This may begin to explain differences in depression between men and women, and why women often respond differently to antidepressant medications.
__ Even so, there are shades of variation among female brains: Women with severe premenstrual symptoms did not respond as well to hormone fluctuations during menstrual cycles as women free of premenstrual symptoms.
Sex Hormones and Stress
■ Hormones are not only the actors behind the scenes during pregnancy, but they’re a huge part of sexual dimorphism in the human brain. Androgens and estrogens determine brain sex during gestation, but some scientists believe testosterone also impacts such cognitive functions as memory, attention and spatial skills. Being a male with a typical male brain doesn’t guarantee you’ll also have a better memory, but it may make it more likely. The same goes for women and empathy, as well as nurturing, which can be tied to a rush of oxytocin in the brain during pregnancy and childbirth. This rush is what prepares the female brain to be able to care for a newborn.
__ A recent study published in the Journal of Neuroscience illustrates that women have a different set of innate sensibilities thanks to their hormones: Women’s brains, it seems, are better at managing stress. These findings will likely have long-term implications in women’s health. “Brain regions that are involved in regulating stress also are involved in regulating other functional systems, such as heart regulation (which can affect cardiovascular disease), memory (which can affect aging of the brain and memory decline), and mood and fear (which will affect depression and anxiety disorders),” says the study’s author, Dr. Jill Goldstein of Harvard Medical School.
__ To test the impact of stress, these scientists monitored the brain activity of normal, healthy women and men using functional MRI. Researchers showed stress-triggering images to the study’s participants and measured their responses to these images. Women were scanned both at the beginning of their menstrual cycle and during ovulation. The results showed that women’s brain activity was similar to men during their menstrual cycle, but during ovulation their response to stress was much lower than their male counterparts. “We were studying the regulation of the stress response (or the ‘fight-flight response’) in the brain by hormones,” explains Dr. Goldstein. “We believe this tells us that there are different pathways through which men and women regulate stressful stimuli in the environment in order to function in the everyday world. Thus, even though the male and female brains were activating differently under stress, that does not necessarily mean that the resulting behavior or functioning is different. The male and female brains may respond differently to produce the same feeling state or performance level.”
__ Understanding how hormones regulate the body and uncovering the biological factors behind sex differences in the brain could help scientists understand how sex differences impact reactions to heart disease, memory decline, or depression and anxiety disorders. Goldstein hopes “that this new knowledge will ultimately be used to develop sex-specific treatments for these disorders and even prevention strategies. We believe that an understanding of sex differences in diseases will thus have major public health implications, in terms of new scientific knowledge, new treatments and prevention, and thus implications for the economics of health and disease.”
■ If stress levels vary in the female brain between ovulation and menstruation, it’s logical to wonder exactly what that means for aging women’s brains. “Unfortunately, there is very little work on understanding sex differences in aging,” Dr. Goldstein says. This lack of knowledge about the aging brain leaves a huge gap in what we know—and what we need to know—in relationship to treating disorders associated with aging women, along with keeping them healthy. Dr. Goldstein believes this under-researched area has “tremendous implications for public health, given the fact that we are living longer, and, as baby boomers age, the majority of our population will be seniors.”
__ A 2008 study entitled “Gene expression changes in the course of normal brain aging are sexually dimorphic,” led by Nicole C. Berchtold, indicates that “marked differences emerged between men and women in the pattern of gene change occurring with aging.” These results suggest that “the male brain gene expression generally remains stable after age 80, whereas in the female brain genes continue to undergo age-related change into the eighth and ninth decades of life.”
__ Studies need to show how these bimorphic genetic changes apply to specific diseases like Alzheimer’s or dementia. While many studies about sex differences and aging are inconclusive, particularly regarding Alzheimer’s, Dr. Goldstein notes, “The transition from perimenopause to menopause is a critical period in which to study changes in brain function across a number of domains, such as the stress response and its implications for depression and cardiovascular disease…and the aging of the memory circuitry and cognitive decline with implications for the fact that there is higher risk of Alzheimer’s among women than men (even after adjusting for the fact that women live longer).”
Towards Future Understanding of Sex Differences
■ While men and women may never fully understand one another, we are getting closer to learning what’s happening on a neural level in our brains. Though they are fundamentally similar, it’s clear is that there are many bimorphic differences between men’s and women’s brains—from how we react to stress, to why our emotions affect us the way we do, to the reason girls are more likely to correctly identify the emotion on a person’s face. None of this research discounts the impact of environmental and social factors on an individual, and of course, not all individuals will act the same way, behaviorally or biologically. Questions will still arise regarding the politics of sex and gender, and intersex individuals. The challenge will be to find the common ground on which to celebrate our differences, and to use our newfound understanding of the intricacies of brain sex to help determine how best to care for patients, to treat disease, to maintain wellness through to the end of our lives, and maybe even to understand why we act the way we do. bw