Researchers are finding brains of women and men display distinctive differences that are shaped by the interplay of heredity, experience, and biochemistry. Science writer Robert Lee Hotz explains on Lunch Break. Photo: Getty Images.
So many things come down to connections—especially the ones in your brain.
Women and men display distinctive differences in how nerve fibers connect various regions of their brains, according to a half-dozen recent studies that highlight gender variation in the brain’s wiring diagram. There are trillions of these critical connections, and they are shaped by the interplay of heredity, experience and biochemistry.
No one knows how gender variations in brain wiring might translate into thought and behavior—whether they might influence the way men and women generally perceive reality, process information, form judgments and behave socially—but they are sparking controversy.
“It certainly is incendiary,” said Paul Thompson, a professor of neurology and director of the University of Southern California’s Imaging Genetics Center. He is directing an effort to assemble a database of 26,000 brain scans from 20 countries to cross-check neuroimaging findings. “People who look at findings about sex differences are excited or enraged,” he said.
Combined brain scans of 949 subjects, ages 8 to 22, show how neural connections differ by gender. Male brains, top, have more connections within hemispheres (blue lines). Female brains, bottom, have more between hemispheres (orange lines). Proceedings of The National Academy of Sciences/University of Pennsylvania
Researchers are looking at the variations to explain the different ways men and women respond to health issues ranging from autism, which is more common among men, and multiple sclerosis, which is more common among women, to strokes, aging and depression. “We have to find the differences first before we can try to understand them,” said Neda Jahanshad, a neurologist at USC who led the research while at the University of California, Los Angeles.
Dr. Jahanshad and her UCLA collaborators conducted a 2011 brain-imaging study of healthy twins, including 147 women and 87 men, to trace connections in the brain. She discovered “significant” sex differences in areas of the brain’s frontal lobe, which is associated with self-control, speech and decision-making.
In the most comprehensive study so far, scientists led by biomedical analyst Ragini Verma at the University of Pennsylvania found the myriad connections between important parts of the brain developed differently in girls and boys as they grow, resulting in different patterns of brain connections among young women and young men.
The team imaged the brains of 949 healthy young people, 521 females and 428 males, ranging in age from 8 to 22. Like Dr. Jahanshad’s team, Dr. Verma employed a technique called Diffusion Tensor Imaging to trace how water molecules align along the brain’s white-matter nerve fibers, which form the physical scaffolding of thought. The study was reported earlier this month in the journal the Proceedings of the National Academy of Sciences.
Pairs of scan images show gender differences in brain wiring in childhood (1), adolescence (2) and young adulthood (3). Male brains are on left, female on right.University of Pennsylvania, Proceedings of The National Academy of Sciences.
The neural patterns emerged only when combining results from hundreds of people, experts said. In any one person, gender patterns may be subsumed by the individual variations in brain shape and structure that help make every person unique.
Dr. Verma’s maps of neural circuitry document the brain at moments when it is in a fury of creation. Starting in infancy, the brain normally produces neurons at a rate of half a million a minute, and reaches out to make connections two million times a second. By age 5, brain size on average has grown to about 90% of adult size. By age 20, the average brain is packed with about 109,000 miles of white matter tissue fibers, according to a 2003 Danish study reported in the Journal of Comparative Neurology.
Spurred by the effects of diet, experience and biochemistry, neurons and synapses are ruthlessly pruned, starting in childhood. The winnowing continues in fits and starts throughout adolescence, then picks up again in middle age. “In childhood, we did not see much difference” between male and female, Dr. Verma said. “Most of the changes we see start happening in adolescence. That is when most of the male-female differences come about.”
Broadly speaking, women in their 20s had more connections between the two brain hemispheres while men of the same age had more connective fibers within each hemisphere. “Women are mostly better connected left-to-right and right-to-left across the two brain hemispheres,” Dr. Verma said. “Men are better connected within each hemisphere and from back-to-front.”
That suggests women might be better wired for multitasking and analytical thought, which require coordination of activity in both hemispheres. Men, in turn, may be better wired for more-focused tasks that require attention to one thing a time. But the researchers cautioned such conclusions are speculative.
Experts also cautioned that subtle gender differences in connections can be thrown off by normal disparities in brain size between men and women and in the density of brain tissue. Other factors, such as whether one is left- or right-handed, also affect brain structure.
Also affecting results are differences in how computer calculations are carried out from one lab to the next. “With neuroimaging, there are so many ways to process the data that when you do process things differently and get the same result, it is fantastic,” Dr. Jahanshad said.
Write to Robert Lee Hotz at firstname.lastname@example.org