In humans, there is one gene that impacts observable variation more than any other, by far. It is equally common everywhere, so it doesn’t cause differences between populations, just within populations. Shared across mammals, it represents the oldest genetic difference in our lineage and one of the oldest in the entire biosphere. Unlike typical DNA variants, it is associated with a major rearrangement of genome’s basic structure. This gene is called SRY, and it is responsible for the sex chromosomes, X and Y.
The Y chromosome is basically a shriveled version of the X chromosome, and it usually contains SRY. A body with SRY usually develops male traits like testicles and facial hair. A person with testicles and facial hair usually identifies as male gender.
The answer is complicated. There are a lot of baseless stereotypes out there. But while myths about race tend to lack any biological justification at all, there are genuine sex differences grounded in biology. Sex hormones permeate the body and influence the development of tissues from head to toe.
But, remember the bicycles in Chapter 3. The impact of a genetic difference depends on the environment, which can change.
A century or two ago, it was almost unheard of for women to be successful doctors, scientists, engineers, or judges. Society didn’t permit it. Many assumed that women were innately incapable of such tasks. These assumptions were wrong.
Still today we see average differences between men and women. Some of these differences are influenced by SRY or other sex-linked genes. But, it would be foolish to assume that we have now removed all barriers to equality and levelled the playing field. The sensible prediction is that gaps between the sexes will continue to shrink. Still, some differences may be relatively immune to environmental adjustments. These could include differences in athletic achievement as well as brain differences, like what kind of person you are sexually attracted to. The real question is, are these differences large and robust enough to matter in any practical sense? Should we expect men and women to perform differently on average in academic or professional realms, solely because of biology? The answer is no.
Here’s why. Perhaps the strongest behavioral effect of SRY not directly tied to reproduction is its influence on aggression. Violent behavior is strongly correlated with SRY, much more than so-called “warrior gene” MAOA, or any other gene that isn’t sex-linked. Now, imagine a society where most leadership positions were held by women.
From that vantagepoint, one could readily argue that men were not naturally suited to hold power, because SRY condemns them to angry irrationality. This argument would seem valid, but it would be wrong. We all know that men are fully capable of controlling their behavior. SRY is not an excuse to attack people. Even our legal system agrees.
The effects of SRY on violence can be culturally and willfully overcome. Every law-abiding carrier of SRY knows this, and it should also be obvious from the vastly different crime rates seen within the same society but at different points in history. And if that major genetic effect can be toppled, what does that say for any weaker trends in behavior? Surely they are even less robust.
And now, to wrap things up.
Reflect back across all four chapters. The guide has explained how genetic differences between human groups, be they races or genders, are not as stark as they are often portrayed. Many people find this conclusion hard to accept. There seems to be a strong human drive to create strict categories. However, a scientific mindset seeks to avoid such biases. Just think about the popularity of astrology. It’s a pseudoscience. Hopefully you already know that a person’s Zodiac sign is not a major determinant of their personality or potential. Don’t then fall into the trap of thinking that race or sex is a good predictor of your brain’s abilities.
Here’s another example. Which square below is darker? It probably looks like the left square is. But in fact, they are exactly the same. Whether it’s shades of geometry or shades of humanity, our minds really like to magnify differences or event invent them out of thin air.
We don’t yet fully understand every way that DNA shapes human lives. For complex traits like behavior, it is especially challenging. Suppose you hear about a study testing for a genetic difference in some ability between two groups of humans. The data will invariably be confounded by factors that can’t be controlled. We can’t do proper experiments on humans like we can with flowers, after all. So always ask yourself: what are the caveats?
🕺🐂🐏 🆚 🐑🐄💃
Vive la différence
The Y chromosome is basically a shriveled version of the X chromosome, and it usually contains SRY. A body with SRY usually develops male traits like testicles and facial hair. A person with testicles and facial hair usually identifies as male gender.
How it usually works:
🇾=SRY=🥎🥎🍆=🕺
🚫🇾=🚫SRY=🌷=💃
Notice all of the usuallys. That’s because none of these steps are guaranteed. Sometimes SRY switches from the Y to the X. Sometimes bodies with SRY develop female features, or intersex features than are not unambiguously male or female. Sometimes a person’s gender doesn’t match what you might guess from looking at their body.How it sometimes works:
🚫🇾=SRY=🥎🥎🍆 =🕺
=De la Chapelle syndrome
🇾=🚫SRY=🌷=💃
=Swyer syndrome
🇾=SRY=🌷=💃
=Androgen insensitivity syndrome
🇾=SRY=🥎🥎,🚫🍆=🕺
=Aphallia
🇾=SRY=🥎🥎🍆 =💃
=Trans woman
🚫🇾=🚫SRY=🌷=🕺
=Trans man
etc.
It gets even more complicated, because some people have combinations of chromosomes other than the typical XX or XY. Since gender isn’t perfectly coupled to sex, any of these could occur in any gender:👧:
🇽🇽or🇽🇾or🇽or🇽🇽🇾or🇽🇾🇾or🇽🇽🇽or🇽🇽🇾🇾
👦:
🇽🇽or🇽🇾or🇽or🇽🇽🇾or🇽🇾🇾or🇽🇽🇽or🇽🇽🇾🇾
🧒:
🇽🇽or🇽🇾or🇽or🇽🇽🇾or🇽🇾🇾or🇽🇽🇽or🇽🇽🇾🇾
And there is nothing special about the Y chromosome or SRY across the tree of life. Most species have completely unrelated sex-determining mechanisms, or don’t even have two primary sexes at all.Sexual systems
🐱🦟: 🇽 🇾 ♂ | 🇽 🇽 ♀
🐦🦋: 🇿 🇿 ♂ | 🇿 🇼 ♀
🟩 (moss): 🇻 ♂ | 🇺 ♀
🐝: 🇦 ♂ | 🇦 🇦 ♀
🐊:🔥 ♀ | ⛅ ♂ | ❄️ ♀
🐌: ⚥
🍌: 🚫
Sex and gender aren’t always cut and dry. But in the big picture, most people are male or female, and there seem to be really big differences between these two categories. Not only in which organs are present, but differences in size, strength, personality, behavior, etc. Of course these are merely trends, not absolutes. Clearly the world’s weakest man is no match for the world’s strongest woman. But are the average differences biologically real?🕺 🆚 💃
Genetic or synthetic?
The answer is complicated. There are a lot of baseless stereotypes out there. But while myths about race tend to lack any biological justification at all, there are genuine sex differences grounded in biology. Sex hormones permeate the body and influence the development of tissues from head to toe.
❤️🧠🦴👁️💪💀👄
Some organs affected by sex hormones
But, remember the bicycles in Chapter 3. The impact of a genetic difference depends on the environment, which can change.
🚳 🆚 🚲
Whether you can ride depends on the bike not your genes
A century or two ago, it was almost unheard of for women to be successful doctors, scientists, engineers, or judges. Society didn’t permit it. Many assumed that women were innately incapable of such tasks. These assumptions were wrong.
👩🏼⚕️👩🏾🔬👩🏽🔧👩🏻⚖️
You didn’t see this in the olden days
Still today we see average differences between men and women. Some of these differences are influenced by SRY or other sex-linked genes. But, it would be foolish to assume that we have now removed all barriers to equality and levelled the playing field. The sensible prediction is that gaps between the sexes will continue to shrink. Still, some differences may be relatively immune to environmental adjustments. These could include differences in athletic achievement as well as brain differences, like what kind of person you are sexually attracted to. The real question is, are these differences large and robust enough to matter in any practical sense? Should we expect men and women to perform differently on average in academic or professional realms, solely because of biology? The answer is no.
👩🏽🏭👨🏼🏭👨🏾🏭👩🏼🏭👨🏾🏭👨🏻🏭👨🏿🏭👨🏻🏭👨🏿🏭👨🏽🏭👨🏼🏭👨🏻🏭👩🏿🏭👨🏽🏭
Don’t blame biology if your organization is dominated by one gender
Here’s why. Perhaps the strongest behavioral effect of SRY not directly tied to reproduction is its influence on aggression. Violent behavior is strongly correlated with SRY, much more than so-called “warrior gene” MAOA, or any other gene that isn’t sex-linked. Now, imagine a society where most leadership positions were held by women.
👩🏿💼👮🏾♀️👩🏼⚖️👸🏽👩🏻🎓👩🏽⚖️👸🏿🦸🏾♀️👮🏼♀️👩🏻🏫
What if “The Man” weren’t The Man?
From that vantagepoint, one could readily argue that men were not naturally suited to hold power, because SRY condemns them to angry irrationality. This argument would seem valid, but it would be wrong. We all know that men are fully capable of controlling their behavior. SRY is not an excuse to attack people. Even our legal system agrees.
🔪💣🔫🧨 💥☠️
SRY didn’t make you do it
The effects of SRY on violence can be culturally and willfully overcome. Every law-abiding carrier of SRY knows this, and it should also be obvious from the vastly different crime rates seen within the same society but at different points in history. And if that major genetic effect can be toppled, what does that say for any weaker trends in behavior? Surely they are even less robust.
♂🧠⚖️♀🧠
Hard to find any meaningful imbalance in brain function between the sexes
And now, to wrap things up.
🔚
Coming soon
Reflect back across all four chapters. The guide has explained how genetic differences between human groups, be they races or genders, are not as stark as they are often portrayed. Many people find this conclusion hard to accept. There seems to be a strong human drive to create strict categories. However, a scientific mindset seeks to avoid such biases. Just think about the popularity of astrology. It’s a pseudoscience. Hopefully you already know that a person’s Zodiac sign is not a major determinant of their personality or potential. Don’t then fall into the trap of thinking that race or sex is a good predictor of your brain’s abilities.
♈♉♊♋♌♍♎♏♐♑♒♓
What’s your sign? Humans love to categorize each other for no good reason
Here’s another example. Which square below is darker? It probably looks like the left square is. But in fact, they are exactly the same. Whether it’s shades of geometry or shades of humanity, our minds really like to magnify differences or event invent them out of thin air.
🦵🏽 🆚 🦵🏽
Technically there’s a real biological difference, but so what?
We don’t yet fully understand every way that DNA shapes human lives. For complex traits like behavior, it is especially challenging. Suppose you hear about a study testing for a genetic difference in some ability between two groups of humans. The data will invariably be confounded by factors that can’t be controlled. We can’t do proper experiments on humans like we can with flowers, after all. So always ask yourself: what are the caveats?
👩🏽🔬🔎🗒️🌸🌺🌼normal science (legal)
🦹🏻♂️🧪⚡👶🏽👶🏿👶🏻mad science (illegal)
This brings up a final, more philosophical point. With incomplete data, we make educated guesses. Precisely how different are two particular groups in some particular aspect? Whenever there is scientific uncertainty, it’s best err on the side of caution. Overestimation of differences between human groups has led to some of the most horrific tragedies of history. Meanwhile, any negative consequences of underestimating these differences are slim. If you are driving through the fog and the GPS estimates that the edge of the cliff is either 50 or 60 feet away, both of those estimates are equally consistent with the data. But where do you stop the car?🌫️🌫️🌫️🌫️🌫️🌫️🌫️🌫️🌫️🌫️🌫️🌫️🌫️🌫️🌫️
🌫️🌫️🌫️🌫️🌫️🌫️🌫️🚗🌫️🌫️🌫️🌫️🌫️🌫️🌫️
Human evolutionary genetics is an enormous field, and this guide is just the tip of the iceberg. Hopefully you’ve learned something. More importantly, hopefully you are inspired to learn more. Check out the References and Acknowledgments section for where to go next.