So focus shifted to the relative sizes of brain regions.ย Phrenologists suggestedย the part of the cerebrum above the eyes, called the frontal lobe, is most important for intelligence and is proportionally larger in men, while the parietal lobe, just behind the frontal lobe, is proportionally larger in women. Later,ย neuroanatomists arguedย instead the parietal lobe is more important for intelligence and menโs are actually larger.
In the 20th and 21st centuries, researchers looked for distinctively female or male characteristics in smaller brain subdivisions. As aย behavioral neurobiologistย andย author, I think this search is misguided because human brains are so varied.
Anatomical brain differences
The largest and most consistent brain sex difference has been found in the hypothalamus, a small structure that regulates reproductive physiology and behavior. At least one hypothalamic subdivision is larger in maleย rodentsย andย humans.
But the goal for many researchers was to identify brain causes of supposed sex differences in thinking โ not just reproductive physiology โ and so attention turned to the large human cerebrum, which is responsible for intelligence.
Within the cerebrum, no region has received more attention inย both race and sex difference researchย than the corpus callosum, a thick band of nerve fibers that carries signals between the two cerebral hemispheres.
In the 20th and 21st centuries, some researchers found theย whole corpus callosum is proportionallyย larger in womenย on average while others foundย only certain partsย are bigger. This difference drewย popularย attentionย and was suggested toย cause cognitive sex differences.
Butย smaller brains have a proportionally larger corpus callosumย regardless of the ownerโs sex, and studies ofย this structureโs size differences have been inconsistent. The story is similar forย other cerebral measures, which is why trying to explain supposed cognitive sex differences through brain anatomy has not been very fruitful.
Female and male traits typically overlap
Even when a brain region shows a sex difference on average, there is typically considerable overlap between the male and female distributions. If a traitโs measurement is in the overlapping region, one cannot predict the personโs sex with confidence. For example, think about height. I am 5โ7″. Does that tell you my sex? And brain regions typically show much smaller average sex differences than height does.
Neuroscientistย Daphna Joel and her colleagues examined MRIs of over 1,400 brains, measuring the 10 human brain regions with the largest average sex differences. They assessed whether each measurement in each person was toward the female end of the spectrum, toward the male end or intermediate. They found that only 3% to 6% of people were consistently โfemaleโ or โmaleโ for all structures. Everyone else was a mosaic.
Prenatal hormones
When brain sex differences do occur, what causes them?
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Aย 1959 studyย first demonstrated that an injection of testosterone into a pregnant rodent causes her female offspring to display male sexual behaviors as adults. The authors inferred that prenatal testosterone (normally secreted by the fetal testes)ย permanently โorganizesโ the brain. Manyย later studies showed this to be essentially correct,ย though oversimplifiedย for nonhumans.
Researchers cannot ethically alter human prenatal hormone levels, so they rely on โaccidental experimentsโ in whichย prenatal hormone levels or responses to them were unusual, such as withย intersex people. But hormonal and environmental effects are entangled in these studies, and findings of brain sex differences have been inconsistent,ย leaving scientists without clear conclusions for humans.
Genes cause some brain sex differences
This was dramatically shown by a zebra finch with a strange anomalyย โ it was male on its right side and female on its left. A singing-related brain structure was enlarged (as in typical males) only on the right, though the two sides experienced the same hormonal environment. Thus, its brain asymmetry was not caused by hormones, but by genes directly. Since then, direct effects of genes on brain sex differences haveย also been found in mice.While prenatal hormones probably cause most brain sex differences in nonhumans, there are some cases where the cause is directly genetic.
Learning changes the brain
Many people assume human brain sex differences are innate, but this assumption is misguided.
Humans learn quickly in childhood and continue learning โ alas, more slowly โ as adults. From remembering facts or conversations to improving musical or athletic skills, learning alters connections between nerve cells called synapses. These changes are numerous and frequent but typically microscopic โ less than one hundredth of the width of a human hair.
Studies of an unusual profession, however, show learning can change adult brains dramatically. London taxi drivers are required to memorize โthe Knowledgeโ โ the complex routes, roads and landmarks of their city. Researchers discovered this learningย physically altered a driverโs hippocampus, a brain region critical for navigation.ย London taxi driversโ posterior hippocampiย were found to be larger than nondrivers by millimeters โ more than 1,000 times the size of synapses.
So itโs not realistic to assume any human brain sex differences are innate. They may also result from learning. People live in a fundamentally gendered culture, in which parenting, education, expectations and opportunities differ based on sex, from birth through adulthood, which inevitably changes the brain.
Ultimately, any sex differences in brain structures are most likely due to a complex and interacting combination of genes, hormones and learning.
Ari Berkowitz, Ph.D., is a Presidential Professor of Biology and Director of the Cellular & Behavioral Neurobiology Graduate Program at the University of Oklahoma. His research focuses on how the spinal cord selects among and generates leg movements. He is author of Governing Behavior: How Nerve Cell Dictatorships and Democracies Control Everything We Do. Find Ari at his website.ย
A version of this article was originally published at the Conversation and has been republished here with permission. The Conversation can be found on Twitter @ConversationUS
