The Causes of Hair Growth

Hair is a tricky one. I hypothesize that there are three causes to hair growth and all three are in play simultaneously. Fact: hair grows at more areas of the face and body on men than on women. This would still be true if we lived in a society where women did not shave their shins, forearms, armpits, etc.  Based on this fact, one can conclude that either testosterone or the lack of estrogen is responsible for certain patches of hair on the body (e.g. sternum, upper lip, chin, etc.). A very popular belief today is that more facial/body hair = more masculine/testosterone. Based on this belief, people also believe that Asian men must be the least masculine and have the least testosterone of all ethnicities.

Table of total plasma testosterone by ethnic group using data from over 150 peer-reviewed medical journal articles.

This table shows that East Asians actually have very high average total plasma testosterone (5,673 ρg/mL). Thus, the popular belief that Asian men have low testosterone is not actually supported by evidence. Furthermore, the table shows that there is no correlation between testosterone level and hairiness. So what causes hair?

I hypothesize three causes—all in play simultaneously:

1.      Hair grows at areas that “expect” to encounter friction. (Genetic)

This is partly based on the fact that we have armpit and groin hair. Humans have been walking for hundreds of thousands of years and the genes we inherit today still expect us to do so. When walking, the legs move to and fro which creates friction between the thighs and the genitals. Also, the arms sway to and fro and encounter friction with the torso. Thus, patches of hair preemptively grow at these areas. Less obvious are the eyelashes. This developed as a result of friction between the eyelids upon blinking.

“Expected” friction is also how I explain scalp hair. Imagine a life in the rough outdoors, like a forest, as humans have lived for thousands of years. We would be brushing our heads against foliage from time to time. And any time we lay down and roll around, the head would encounter friction. As a result of hundreds of thousands of years of evolution, we’ve developed scalp hair. Today, though, in our sheltered lifestyles, the head is no longer brushing against many things. Head hair still grows as a vestigial structure, though.

Why isn’t the entire head simply covered in hair? Because only the top half of the head encounters friction. Imagine the head leaning and tilting in every angle possible. The bottom half of the head remains untouched.


2.      Hair grows at areas that frequently encounter friction. (Nurture)

This is based on areas of the body that encounter a lot of friction in modern society, but not necessarily in caveman days. This is adaptation in action. For instance, men may develop nipple hair because the nipples chafe against shirts—a modern invention. It is the nipples that develop hair because they stick out the most and thus encounter the most friction. I believe that wiping your butt with toilet paper is frequent friction that will eventually lead to the growth of butt-crack hairs. Some people have hair on their big toes. This may come from friction between the big toe and the strap of sandals or the toe box of (loose) shoes. In my personal experience, I began to develop hair atop my knees when I started a new workout routine that involved getting on the knees a lot.

It seems that higher testosterone level will also increase the likelihood that hair will grow from friction. That is, a man would have to wipe his butt, say, 1,000 times for one hair to grow, whereas a woman would have to do it 10,000 times.


3.      Hair grows from muscle disuse. This is greatly amplified by testosterone.

Disuse of any muscle—face or body—will result in fat. For adult males but not females, it’s fat and hair. By “disuse” I mean that there is muscle that is not being used at its full capacity. “Underused muscle” may be more accurate. This third cause of hair growth is based on personal observation of thousands upon thousands of male faces and bodies. I noticed a correlation between body fat and body hair.

Pictured above is what the internet likes to call a “neckbeard”—a derogatory term for a fat, lazy man. I believe the internet came close to my hypothesis in that they stereotyped fat, lazy men as having excess facial hair around the jaw and neck area. My hypothesis is that if someone frequently used a muscle then hair would cease to grow atop that muscle. In the case of the full beard, the muscle is the masseter, which is responsible for biting, chewing, and closing the jaw. Disuse of the masseter muscle results in hair atop the masseter and the surrounding area. Frequent masseter usage makes the lower corners of the face low in fat and nearly hairless. Most Asian men do not grow a full beard, as opposed to men of just about every other ethnicity. I hypothesize that this is due to generations of high-frequency usage of the masseter muscles. This may been from hundreds of years of a grazing on a low-calorie food for sustenance, as opposed to Europeans consuming a more caloric food that required less chewing.

There are some muscles of the face that the majority of men disuse. These muscles are the orbicularis oris (upper and lower lip), mentalis, digastric, and mylohyoid muscles, and the result of their disuse is a moustache and a goatee.

Why these particular muscles? These muscles control movements of the lips and tongue—the organs responsible for speech. As you probably know, men speak far less often than women, which means that men underuse the speech muscles. The orbicularis oris controls lip movements. The mentalis is also known as the pouting muscle. The digastric muscle controls tongue movements. The mylohyoid muscle is responsible for high-pitched speech and singing. If a man underuses the masseter muscle, along with the muscles responsible for speech, then he would develop a full beard.

In the torso, there are two areas of muscle that most men seem to underuse. The first area is the lower abdomen (above the groin)—the underlying muscles are the abdominals. This is where fat is “stubborn” and hard to lose. The other area is the sternum. This hair seems to develop if the pectoral muscles are developed, but become underused over time. If this disuse of muscle persists for a long time, then body hair continues to spread.

Here’s an example of hair that resulted from disused (fat) masseter, chest, and abdominal muscles. If he had exercised these muscles with higher frequency then they would have less fat and, I hypothesize, less body hair. In the bodybuilding community, a body with big muscles and body fat is known as “bear mode” because of the resemblance to a bear in size and hairiness. There is a very strong correlation between fat (from muscle disuse) and body hair.

The function of the extensor muscles is to extend the fingers and dorsiflexion of the wrist. Disuse of the extensor muscles produces hair atop the forearm. Frequent use of the extensor muscles results in shrunken hair follicles atop the forearm. These muscles are used when typing on a keyboard, playing the piano, or performing lateral raises. Dexterity requires frequent use of these muscles. Women in their natural state tend to be more dexterous than men. This is my explanation for why women generally don’t develop as much forearm hair as men. I go into detail on dexterity in “The Fundamental Difference between Men and Women, Part 2: Face and Body.”

Most humans disuse the anterior muscles of the foreleg—we seldom lift our toes upward and flex our ankles upward. This is known as dorsiflexion, and the muscles responsible for this are located in the shins. Just like the forearms, underuse of the foreleg muscles leads to hair growth on the shins. I believe this is the natural state for men. Just like the forearms, I believe women have a “dexterity” tendency with the forelegs too, which would lead to hairless limbs. In today’s society of shoes, however, this natural tendency is not realized. Instead, it is people who flex their ankles upwards a lot (e.g. jump-ropers) that display a lack of shin hairs.

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