100 Years of Research Results: No Difference Found Between Male and Female Brain

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People have been studying the difference between male and female brains since at least the 19th century. At this point, Samuel George Morton decided to put seeds inside to measure the size of these two separate brains and place them in the skull.

When Gustave Le Bon revealed that the male brain was larger, Alexander Bains and George Romanes began to investigate whether this difference made men more intelligent. But John Stuart Mill said that by this logic, elephants and whales should be smarter than humans.

So attention was turned to the size of the segments in the brain. Phrenologists show that the frontal lobe above the eyes, which they show as the most important region for intelligence, in men; stated that the sidelobe just behind the frontal lobe is larger in women. Later, neuroanatomists suggested that they saw the lateral lobe more important for intelligence and that the male was larger.

In the 20th and 21st centuries, researchers have studied and continue to examine whether smaller parts of the brain have sexual characteristics. As a behavioral neuroscientist and author, I think this research is heading in the wrong direction due to the diversity of the human brain.

Structural differences of the brain

The largest and most common sexual difference in his brain was found in the small hypothalamus, which regulates behavior and reproductive physiology. In male rodents and humans, at least one hypothalamic section is larger than that of females.

But the goal of most researchers was to find out whether the so-called sexual differences in the brain affect not only reproductive physiology but also thinking. Thus, the new target became the human brain responsible for intelligence.

Studies on racial and sexual differences in the brain have so far focused mostly on the corpus callosum. This section, which consists of a thick layer of nerve fibers, provides communication between the right and left brain.

Some researchers in the 20th and 21st centuries argued that the entire corpus callosum was larger in women, while others argued that only some parts were the case. These results had great repercussions and pointed to the presence of cognitive sex differences.

However, smaller brains already have larger corpus callosum regardless of gender. Also, studies on the size differences of this section were inconsistent. The result was the same in the measurements of other parts of the brain. Therefore, searching for cognitive sexual differences based on the anatomy of the brain has not been very fruitful.

Female and male characteristics are generally the same

Although any part of the brain is sexually different, the parts where the parts are separated in men and women are generally the same. If the measurement of a feature is made in that same part, we cannot accurately predict the gender of the person.

Take the length, for example. Does my 1.70 tell you my gender? Parts of the brain are generally much less sexually different from the height.

Neurologist Daphna Joel and her colleagues studied MRIs of more than 1,400 brains and measured 10 brain regions with the most sexual differences.

While making the measurements, they looked at whether the brains of the people were on the female or male side of the measure, or in the middle. Only three to six percent of people could be counted as “women” or “men” for all brain parts. Everyone else showed a mixed character.

Prenatal hormones

If sexual differences occur in the brain, what causes it?

A 1959 study found that when testosterone was injected into a pregnant rodent, the female displayed male characteristics in adulthood.

Researchers realized from this that prenatal hormones (normally hidden by the testicles of the fetus) permanently affect the brain. Many subsequent studies have revealed that although this is true, it remains too simple for non-human creatures.

Researchers do not play with people’s prenatal hormones because it is unethical. So they rely on “accidental experiments”. In these experiments, prenatal hormones or reactions to them are different, such as intersex people.

But in these studies, hormonal and environmental effects are very interrelated. Also, studies on sex differences in the brain are inconsistent. So researchers could not come to a definitive conclusion.

Genes can cause sex differences in the brain

Although prenatal hormones usually cause sex differences in the brain in non-human creatures, in some cases it can be directly caused by genes.

This assumption was strengthened by the appearance of an interesting disorder in the zebra finch. The left side of the bird was male while the right side was female. The part of the brain associated with the crowing was only large on the right side (as in men), but both sides showed the same hormone properties.

In other words, the asymmetry of the brain was caused by genes, not hormones. Thereafter, the effects of genes on sexual differences in the brain were also observed in mice.

You may be interested: Scientists have identified a bee with a male on the left and a female on the right.

Learning changes the brain

Although sexual differences are assumed to be innate in the human brain, this assumption is somewhat misunderstood.

People learn faster in childhood and slower in adulthood. From remembering speech and events to improving your music or sports skills, all kinds of learning change the communication between nerve cells called synapses. Although these changes are numerous and frequent, they are generally microscopic, that is, less than one-hundredth of the width of a human hair.

Learning can drastically alter adults’ brains, according to a study of a profession that may surprise you. The taxi drivers in London were told to memorize the knowledge, which shows the city’s complex routes, roads, and landmarks.

As a result of this memorization, the researchers noticed that the drivers’ hippocampus underwent a physical change. The hippocampus is an important part of the brain for navigation. It was observed that the taxi drivers in London had a hippocampus several millimeters (1,000 times the synapse) larger than non-drivers after the experiment.

As a result, it would not be correct to say that any sexual change in the brain is congenital, it may also result from learning. We live in a very gendered society. Parents’ attitudes, education, expectations, and opportunities, starting from birth through adulthood, are shaped by gender. This, of course, causes changes in the brain.

Ultimately, sexual differences in brain structure are likely a complex and common result of genes, hormones, and learning.

Ari Berkowitz, Professor of Biology, Prime Ministry; Executive, Cellular and Behavioral Neuroscience Undergraduate Program, University of Oklahoma