By Lise Eliot, PhD

What is the difference between men's and women's brains? This question has far-reaching implications for both health and disease and has been debated for centuries. Fortunately, we are now in an era of large-scale, high-resolution MRI studies that make the answer within reach.

I have been studying brain sex differences for about 15 years, beginning with work on my book "Pink Brain, Blue Brain: How Small Differences Grow into Troublesome Gaps." I've found that despite the popular view that "men are from Mars, women from Venus," we are all very much earthlings when it comes to the structure and function of the human brain. Based on the largest, most recent studies and meta-analyses, it is clear that men and women have all the same brain structures, in much the same proportions. In addition, functional MRI studies have found that males and females carry out all of our language, memory, spatial and emotional tasks using very similar neural processing. Unlike a true sex organ, such as the ovaries or testes, our brains are basically unisex — more like our hearts, livers, kidneys and lungs.

Of course, men tend to be taller and heavier than women, and this is the basis of the one, truly reliable brain sex difference: men's brains average 11 percent larger than women's (just as their hearts, livers, etc. are all some 10 to 20 percent larger). We also know that larger brains, regardless of sex, have subtle architectural differences, including a 3 percent higher ratio of white-to-gray matter. White matter comprises the communication "highways" of the brains. Just as larger cities need bigger freeways, larger brains need slightly more white matter than smaller brains to get their information across longer distances. But when men and women in research studies are matched for brain volume, there is no difference in their white-to-gray matter ratio, which is strictly a function of brain size, not sex. Nor are there any known consequences of a 3 percent variation in this ratio for individuals' thinking or emotional abilities.

Unfortunately, this is not the message most people find when searching for information about brain gender difference. Instead, we hear that men are "hardwired" for aggression, competition and tech aptitude; whereas, women are said to be "hardwired" for empathy, communication and nurturing skills. It's true that adult men and women differ, on average, in these and other features of behavior, but the catch is that none of this appears to be baked into our brains at birth.

Rather, most evidence suggests that these differences emerge and are amplified through a lifetime of social learning. Whether it is mastering a language, solving math problems, hitting a baseball or playing the flute, everything we do with our complex human brains is learned through thousands of hours of practice and immersion with peers and other role models. Especially today, when we can see the fluidity with which trans- and other gender-nonconforming children navigate the space between male and female identities, it has become increasingly clear that gender is a psychological construct, not a fixed biological trait. Each of us has the potential to express a wide range of masculine or feminine behaviors thanks to our largely unisex brains.