DOES SIZE MATTER?
Neuroscientists Sandra Witelson, Larry Cahill and
Melissa Hines find sex differences in the brain.
Images courtesy of Melissa Hines, Maria Lioudyno,
and Ron Scheffler
Recent research may also provide some clues to a long-standing paradox. Men’s brains are bigger than women’s—about 100 grams heavier on average—even when their larger proportional size is factored into the equation. Brain volume is correlated with intelligence, according to a 2005 study (pdf), yet both sexes score about the same on IQ tests.
Even though women’s brains are smaller, they have some distinct advantages that may level the playing field. Women have more gray matter—areas of neuron cell bodies. Studies consistently show that women surpass men on verbal and memory tasks, and the superior temporal cortex, one of the brain areas responsible for language, is 29 percent larger in females. Blood flow is about 15 percent higher in the female brain, which offsets the cognitive losses of aging, and women’s nerve cells are also more tightly packed together, which suggests that the neurons may function more efficiently.
A postmortem analysis of human brain tissue, for example, conducted by Witelson and her colleagues at the Michael G. DeGroote School of Medicine at McMaster, revealed that women’s neurons were 11 percent denser than men’s in the prefrontal cortex and in a region of the temporal cortex that is involved with language processing, comprehension, and memory. “Little girls develop language skills earlier than boys, and they’re better able to make fine phonetic discriminations between speech sounds,” says Witelson. “These microscopic anatomical differences may have something to do with it.”
This may be part of the reason why women do better in school—it’s not merely because they’re conditioned to be people pleasers or to sit in a chair without squirming. According to behavioral studies, even in kindergarten and first grade, girls are more articulate than boys, their handwriting is more legible, and they’re quicker at answering questions, says Louann Brizendine, a neuropsychiatrist at the University of California at San Francisco (UCSF) and author of The Female Brain. Over time, this pattern appears to become more pronounced: Females now outnumber males in college, and more women go on to graduate.
The male brain, by contrast, is filled with more white matter, which consists of longer neuron fibers coated with a fatty myelin sheath that communicate with more distant regions of the brain. White matter also contains fibers from inhibitory neurons that block the dispersal of information in the cortex, which enhances local processing. The bulkier white matter may be what gives men their ability to focus intently on work and tune out distractions, as well as their clear-cut superiority when it comes to spatial reasoning, says Ruben Gur, a psychologist and director of the Brain Behavior Laboratory at the University of Pennsylvania in Philadelphia. Little wonder, then, that men still predominate in fields like architecture, organic chemistry, physics, and neurosurgery. There are only a handful of female chess grandmasters, and girls are less likely than boys to sit transfixed for hours playing video games.
Men are faster at mentally rotating an object—imagining what something looks like from a different perspective—and have an internal compass that enables them to squint up at the sun to figure out cardinal directions. Women, on the other hand, rely on landmarks—bear right at the church, turn left after the fire station—to find their way around.
Because it takes hundreds of thousands of years for the genetic evolution of natural selection to occur, these sex differences in navigation strategies probably have their roots in the Stone Age. Women most likely watched the kids and foraged close to home, using familiar sights to find their way back. Men, meanwhile, were hunters and ventured into unknown territory scavenging for food. In all likelihood, they figured out where they were by estimating the distance they had already traveled and their orientation in space—what neurobiologist Larry Cahill calls “dead reckoning.”
Over the millennia, men and women apparently evolved different neural pathways to get around, and men mastered the use of geometric cues to navigate unfamiliar terrain. In a 2000 German study, for example, men sped through a three-dimensional virtual-reality maze much faster than women, averaging two minutes and 22 seconds compared with three minutes and 16 seconds for women.
Brain imaging techniques revealed that men found their way out of the maze using the left hippocampus, a memory storage region that also governs spatial mapping in the physical environment. Women employed their right parietal and prefrontal cortices, which are linked to visual identification and reasoning. The women’s use of the prefrontal cortex, say researchers, suggests that they relied on landmarks and pictured the objects in their minds, while the men used both landmarks and geometric cues, like shapes and angles, to escape the maze.
These spatial skills may have enabled men to navigate distant lands across unmarked desert sands and vast uncharted oceans. In one famous 1916 incident, New Zealand native Frank Worsley, a member of the ill-fated Antarctic expedition led by explorer Ernest Shackleton, relied almost solely on his internal compass to save the lives of 28 men marooned on an island in the Antarctic.
Worsley set off from the remote island in a 22-foot lifeboat across the South Atlantic Ocean, eventually traveling 800 miles to one of the South Sandwich Islands near the tip of South America, where there was a whaling station where they could get help. Worsley had only been able to take sightings of the sun four times during the stormy 17-day trek, and the rest had been based on dead reckoning.
ANGER MANAGEMENT
The way males and females handle their anger or emotionally upsetting situations—women may feel sick to their stomach, while men tend to act out—may also stem from fundamental differences in how their brains have evolved.
A 2002 study using MRI scans showed that brain areas keeping aggression and impulsive behavior in check were relatively larger in women than in men. Female brains had a significantly greater volume than males of orbital frontal cortex, the seat of cool-headed decision making behind the forehead, in proportion to the amygdala, a more primitive, almond-shaped structure deep inside the brain that pumps us up by stimulating the fight-or-flight reaction and getting the adrenaline flowing.
