“We are just starting to understand and study the ways in which gender identity, rather than sex, may cause the brain to differ in males and females,” says Nancy Forger, professor and director of the Neuroscience Institute at Georgia State University.
Though the terms “sex” and “gender” are often used interchangeably by the average person, for neuroscientists, they mean different things, Forger says.
“Sex is based on biological factors such as sex chromosomes and gonads [reproductive organs],” she says, “whereas gender has a social component and involves expectations and behaviors based on an individual’s perceived sex.”
These behaviors and expectations around gender identity can be seen in “epigenetic marks” in the brain, which drive biological functions and features as diverse as memory, development, and disease susceptibility. Forger explains that epigenetic marks help determine which genes are expressed and are sometimes passed on from cell to cell as they divide. One generation can also pass them down to the next, she says.
“While we are accustomed to thinking about differences between the brains of males and females, we are much less used to thinking about the biological implications of gender identity,” she says.
“There is now sufficient evidence to suggest that an epigenetic imprint for gender is a logical conclusion. It would be strange if this were not the case, because all environmental influences of any importance can epigenetically change the brain.”
Forger, with doctoral student Laura Cortes and postdoctoral researcher Carla Daniela Cisternas, reviewed previous studies of epigenetics and sexual differentiation in rodents, along with new studies that have linked gendered experiences among humans and changes in the brain.
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In one example involving rats, the authors cite a study by University of Wisconsin researchers who gave female rat pups extra attention designed to simulate the increased licking that mother rats normally perform on their male offspring. That treatment led to detectable changes in the brains of the female rats that received extra stimulation as compared to those who got the normal level of attention for female pups.
Among the studies involving humans, researchers considered the example of Chinese society during the Great Chinese Famine from 1959-1961, when many families preferred to spend their limited resources on boys, leading to higher rates of disability and illiteracy among female survivors in adulthood. This demonstrates, they say, that early life stress can be a gendered experience as it changes the neural epigenome.
“Given our lifetimes of layered gendered experiences, and their inevitable, iterative interactions with sex, it may never be possible to completely disentangle the effects of sex and gender on the human brain,” Forger says.
“We can start, however, by including gender in our thinking any time a difference between the brain functioning of men and women is reported.”
The paper appears in Frontiers in Neuroscience. A National Science Foundation Graduate Research Fellowship and a Georgia State Brains & Behavior Seed grant supported the research.