Scientists haven’t found any major differences between women’s and men’s brains
Are there innate differences between female and male brains?
People have searched for sex differences in human brains since at least the 19th century, when scientist Samuel George Morton poured seeds and lead shot into human skulls to measure their volumes.
Gustave Le Bon found men's brains are usually larger than women's, which prompted Alexander Bains and George Romanes to argue this size difference makes men smarter. But John Stuart Mill pointed out, by this criterion, elephants and whales should be smarter than people.
So focus shifted to the relative sizes of brain regions. Phrenologists suggested 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, neuroanatomists argued instead the parietal lobe is more important for intelligence and men's are actually larger.
In the 20th and 21st centuries, researchers looked for distinctively female or male characteristics in smaller brain subdivisions. As a behavioral neurobiologist and 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 male rodents and humans.
But the goal for many researchers was to identify brain causes of supposed sex differences in thinking – not just reproductive physiology – and so attention turned to the large human cerebrum, which is responsible for intelligence.
Within the cerebrum, no region has received more attention in both race and sex difference research 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 the whole corpus callosum is proportionally larger in women on average while others found only certain parts are bigger. This difference drew popular attention and was suggested to cause cognitive sex differences.
But smaller brains have a proportionally larger corpus callosum regardless of the owner's sex, and studies of this structure's size differences have been inconsistent. The story is similar for 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
A chart showing how measurements that often differ between sexes, like height, substantially overlap. (Ari Berkowitz, CC BY)
Even when a brain region shows a sex difference on average, there is typically considerable overlap between the male and female distributions. If a trait's measurement is in the overlapping region, one cannot predict the person's sex with confidence. For example, think about height. I am 5'7". Does that tell you my sex? And brain regions typically show much smaller average sex differences than height does.
Neuroscientist 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 "female" or "male" for all structures. Everyone else was a mosaic.
When brain sex differences do occur, what causes them?
A 1959 study 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) permanently "organizes" the brain. Many later studies showed this to be essentially correct, though oversimplified for nonhumans.
Researchers cannot ethically alter human prenatal hormone levels, so they rely on "accidental experiments" in which prenatal hormone levels or responses to them were unusual, such as with intersex people. But hormonal and environmental effects are entangled in these studies, and findings of brain sex differences have been inconsistent, leaving scientists without clear conclusions for humans.
Genes cause some brain sex differences
While prenatal hormones probably cause most brain sex differences in nonhumans, there are some cases where the cause is directly genetic.
This was dramatically shown by a zebra finch with a strange anomaly – 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 have also been found in mice.
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 – alas, more slowly – 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 – 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 "the Knowledge" – the complex routes, roads and landmarks of their city. Researchers discovered this learning physically altered a driver's hippocampus, a brain region critical for navigation. London taxi drivers' posterior hippocampi were found to be larger than nondrivers by millimeters – more than 1,000 times the size of synapses.
So it's 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.
- Changes in the brain during sex are slightly different in women than ... ›
- How Different Are Men's and Women's Brains? - Big Think ›
- Why women have nearly twice as many adverse drug reactions - Big Think ›
What would happen if you tripled the US population? Join Matthew Yglesias and Charles Duhigg at 1pm ET on Monday, September 28.
The idea of 'absolute time' is an illusion. Physics and subjective experience reveal why.
- Since Einstein posited his theory of general relativity, we've understood that gravity has the power to warp space and time.
- This "time dilation" effect occurs even at small levels.
- Outside of physics, we experience distortions in how we perceive time — sometimes to a startling extent.
Physics without time<p>In his book "The Order of Time," Italian theoretical physicist Carlo Rovelli suggests that our perception of time — our sense that time is forever flowing forward — could be a highly subjective projection. After all, when you look at reality on the smallest scale (using equations of quantum gravity, at least), time vanishes.</p><p style="margin-left: 20px;">"If I observe the microscopic state of things," writes Rovelli, "then the difference between past and future vanishes … in the elementary grammar of things, there is no distinction between 'cause' and 'effect.'"</p><p>So, why do we perceive time as flowing <em>forward</em>? Rovelli notes that, although time disappears on extremely small scales, we still obviously perceive events occur sequentially in reality. In other words, we observe entropy: Order changing into disorder; an egg cracking and getting scrambled.</p><p>Rovelli says key aspects of time are described by the second law of thermodynamics, which states that heat always passes from hot to cold. This is a one-way street. For example, an ice cube melts into a hot cup of tea, never the reverse. Rovelli suggests a similar phenomenon might explain why we're only able to perceive the past and not the future.</p><p style="margin-left: 20px;">"Any time the future is definitely distinguishable from the past, there is something like heat involved," Rovelli wrote for the <a href="https://www.ft.com/content/ce6ef7b8-429a-11e8-93cf-67ac3a6482fd" target="_blank"><em>Financial Times</em></a>. "Thermodynamics traces the direction of time to something called the 'low entropy of the past', a still mysterious phenomenon on which discussions rage."</p>
The strange subjectivity of time<p>Time moves differently atop a mountain than it does on a beach. But you don't need to travel any distance at all to experience strange distortions in your perception of time. In moments of life-or-death fear, for example, your brain would release large amounts of adrenaline, which would speed up your internal clock, causing you to perceive the outside world as moving slowly.<br></p><p>Another common distortion occurs when we focus our attention in particular ways.</p><p style="margin-left: 20px;">"If you're thinking about how time is <em>currently</em> passing by, the biggest factor influencing your time perception is attention," Aaron Sackett, associate professor of marketing at the University of St. Thomas, told <em><a href="https://gizmodo.com/why-does-time-slow-down-and-speed-up-1840133782" target="_blank">Gizmodo</a></em>.<em> "</em>The more attention you give to the passage of time, the slower it tends to go. As you become distracted from time's passing—perhaps by something interesting happening nearby, or a good daydreaming session—you're more likely to lose track of time, giving you the feeling that it's slipping by more quickly than before. "Time flies when you're having fun," they say, but really, it's more like "time flies when you're thinking about other things." That's why time will also often fly by when you're definitely <em>not</em> having fun—like when you're having a heated argument or are terrified about an upcoming presentation."</p><p>One of the most mysterious ways people experience time-perception distortions is through psychedelic drugs. In an interview with <a href="https://www.theguardian.com/books/2018/apr/14/carlo-rovelli-exploding-commonsense-notions-order-of-time-interview" target="_blank"><em>The Guardian</em></a>, Rovelli described a time he experimented with LSD.</p><p style="margin-left: 20px;">"It was an extraordinarily strong experience that touched me also intellectually," he said. "Among the strange phenomena was the sense of time stopping. Things were happening in my mind but the clock was not going ahead; the flow of time was not passing any more. It was a total subversion of the structure of reality."<br></p><p>It seems few scientists or philosophers believe time is completely an illusion.</p><p style="margin-left: 20px;">"What we call <em>time</em> is a rich, stratified concept; it has many layers," Rovelli told <em><a href="https://physicstoday.scitation.org/do/10.1063/PT.6.4.20190219a/full/" target="_blank">Physics Today</a>.</em> "Some of time's layers apply only at limited scales within limited domains. This does not make them illusions."</p>What <em>is</em> an illusion is the idea that time flows at an absolute rate. The river of time might be flowing forever forward, but it moves at different speeds, between people, and even within your own mind.
Dominique Crenn, the only female chef in America with three Michelin stars, joins Big Think Live.
Having been exposed to mavericks in the French culinary world at a young age, three-star Michelin chef Dominique Crenn made it her mission to cook in a way that is not only delicious and elegant, but also expressive, memorable, and true to her experience.
Controversial physics theory says reality around us behaves like a computer neural network.
- Physicist proposes that the universe behaves like an artificial neural network.
- The scientist's new paper seeks to reconcile classical physics and quantum mechanics.
- The theory claims that natural selection produces both atoms and "observers".
Vanchurin interview:<span style="display:block;position:relative;padding-top:56.25%;" class="rm-shortcode" data-rm-shortcode-id="539759cbfd8fcd5b6ebf14a3b597b3f9"><iframe type="lazy-iframe" data-runner-src="https://www.youtube.com/embed/bmyRy2-UhEE?rel=0" width="100%" height="auto" frameborder="0" scrolling="no" style="position:absolute;top:0;left:0;width:100%;height:100%;"></iframe></span>
Vanchurin on “Hidden Phenomena”:<span style="display:block;position:relative;padding-top:56.25%;" class="rm-shortcode" data-rm-shortcode-id="18886ffd5e5840bb19d4494212f88d82"><iframe type="lazy-iframe" data-runner-src="https://www.youtube.com/embed/2NDVdNwsHCo?rel=0" width="100%" height="auto" frameborder="0" scrolling="no" style="position:absolute;top:0;left:0;width:100%;height:100%;"></iframe></span>Vitaly Vanchurin speaking at the 6th International FQXi Conference, "Mind Matters: Intelligence and Agency in the Physical World." The Foundational Questions...
43% of people think they can get a sense of someone's personality by their picture.
If you've used a dating app, you'll know the importance of choosing good profile pics.
Quarantine rule breakers in 17th-century Italy partied all night – and some clergy condemned the feasting
17th-century outbreaks of plague in Italy reveal both tensions between religious and public health authorities.