Are Brains Male or Female?

New research shows most brains to be a gender “mosaic”

Posted May 10, 2016

Between books claiming that men and women come from different planets and presidential candidates trying to tell people what gender they have to be, you might think that the human brain reveals some pretty robust sex differences.  It is the brain and not the genitals, after all, where gender identity must reside.

A recent study, published in the prestigious journal Proceedings from the National Academy of Science, sought to provide a more definitive answer to this longstanding question.  For years, researchers who have conducted neuroimaging studies of the brain have reported that this region or that region tends to be bigger in males or females.  The problem, however, has been that the studies are often inconsistent, the magnitude of these differences tend to be rather small, and one would very often be incorrect in trying to guess someone’s sex on the basis of that single brain region.  Such a pattern is also true when it comes to personality traits.  While on average certain traits might tend to be more or less prominent in females versus males, the differences are fairly small in magnitude.  Furthermore, when one examines the full profile of traits within an individual person, it turns out that most of us possess a mix of traits, some of which are typically, or stereotypically, male and some of which are more typically female. 

The unique aspect of this study was to do a similar analysis for brain regions.   First, they took the areas that were shown to be most sexually dimorphic (that is, showed the biggest differences between males and females).  These were things like the right cerebellum or left thalamus, that tend to be larger in males.  Using multiple brain databases, they then looked at each brain across those regions to see whether or not people who tended to have, for example, a more “male looking” region X also tended to have a more male looking region Y and Z, versus a pattern in which an individual brain was more mixed up in terms of having a combination of male and female looking brain regions.

The answer was that it was quite rare for someone’s brain to look classically male and female across the board (less than 5% followed this pattern) and much more common for a brain to have some regions that were more female appearing and others more male appearing, based on a region’s size or strength of connection to other brain areas.  It was even difficult to find much evidence for some kind of quantitative male-female continuum that an individual’s brain could be placed upon.

The authors recommended that we shift our conceptualization of the human brain away from the idea that it falls into a binary category of male versus female and towards an appreciation of a brain “mosaic” when it comes to sex differences. 

This study, despite its interesting topic and being published in a prominent journal, did not get a whole lot of media attention, perhaps because it isn’t the easiest study to read and understand.   Will it likely lead to an end of the black & white thinking that often figures so prominently in discussions of sex and gender?  Probably not, but that might just be my left inferior frontal gyrus talking. 

@copyright by David Rettew, MD

David Rettew is author of Child Temperament: New Thinking About the Boundary Between Traits and Illness and a child psychiatrist in the psychiatry and pediatrics departments at the University of Vermont College of Medicine.

Follow him at @PediPsych and like PediPsych on Facebook.