Baldness is overwhelmingly driven by genetics and hormones. By age 35, two-thirds of American men experience some degree of noticeable hair loss, and by 50, roughly 85% have significantly thinning hair. While those numbers point to male pattern baldness as the most common cause, several other factors can thin or eliminate hair in both men and women.
How Genetics Set the Stage
The single biggest factor in baldness is a gene called AR, which tells the body how to build androgen receptors. These receptors sit on hair follicles and respond to hormones called androgens. In people with certain variations of the AR gene, those receptors are more easily activated than normal. When they’re overstimulated, hair follicles gradually shrink, producing thinner and shorter strands with each growth cycle until some follicles stop producing visible hair altogether.
This gene sits on the X chromosome, which is why the old saying about inheriting baldness from your mother’s side has some truth to it. But baldness isn’t controlled by a single gene. Researchers have identified dozens of genetic regions that contribute, and you can inherit risk factors from either parent. The pattern is complex enough that two brothers can have very different hairlines.
What DHT Does to Hair Follicles
The hormone most directly responsible for pattern baldness is dihydrotestosterone, commonly known as DHT. Your body converts a portion of testosterone into DHT using an enzyme in the skin. DHT is far more potent than regular testosterone at binding to androgen receptors on hair follicles.
When DHT locks onto a follicle with oversensitive receptors, it triggers a process called miniaturization. The follicle’s growth phase gets shorter, so each new hair comes in thinner and lighter than the one before. Eventually the growth phase becomes so brief that the hair barely breaks through the skin. The follicle doesn’t technically die in most cases; it just produces hair too fine to see. This is why balding areas often still have a layer of nearly invisible “peach fuzz.”
Animal studies confirm this mechanism: mice exposed to DHT developed early hair regression, miniaturization, and density loss. When researchers blocked the androgen receptor with a drug, those effects were partly reversed, reinforcing that the DHT-receptor interaction is the core engine of pattern baldness.
Why Baldness Follows Specific Patterns
Not all hair follicles respond to DHT equally. Follicles on the top and front of the scalp are far more sensitive to androgens than those on the sides and back. That’s why the classic horseshoe shape of advanced baldness spares the lower ring of hair. Dermatologists use the Norwood scale to classify this progression into seven stages, from no visible loss (stage 1) to the most advanced form, where only a band of hair around the sides and back remains.
Some men follow a less common pattern where the hairline recedes uniformly from front to back without developing a separate bald spot on the crown. Both patterns are genetically determined, and the specific variant you get depends on which combination of genes you carry.
How Baldness Differs in Women
Women experience pattern hair loss too, but it looks quite different. Instead of a receding hairline or a bald crown, women typically see diffuse thinning that starts along the part line and spreads evenly across the top of the scalp. The front hairline usually stays intact, and complete baldness is rare. Dermatologists classify this using the Ludwig scale, which has three stages of increasing severity.
Hormonal shifts play a major role. Estrogen helps protect hair follicles from androgens, so hair loss in women often accelerates after menopause, when estrogen levels drop. Women with polycystic ovary syndrome or other conditions that raise androgen levels can experience thinning earlier.
Autoimmune Hair Loss
Not all baldness is hormonal. Alopecia areata is an autoimmune condition where the immune system attacks hair follicles directly. Specific immune cells infiltrate the scalp and destroy the protective environment around follicles, causing hair to fall out in smooth, round patches. In severe cases, it can progress to total scalp hair loss or even loss of all body hair.
The attack creates a self-reinforcing cycle: damaged follicles release signals that attract more immune cells, which cause more damage. This is fundamentally different from pattern baldness because the follicles aren’t shrinking from hormonal exposure. They’re being targeted as if they were foreign invaders. Alopecia areata can strike at any age and affects men and women equally. Hair sometimes regrows on its own, but the condition can be unpredictable.
Stress, Illness, and Temporary Shedding
A type of hair loss called telogen effluvium happens when a physical or emotional shock pushes a large number of hair follicles into their resting phase all at once. Normally, only about 10% of your hair is in the resting phase at any given time. During telogen effluvium, up to 70% of actively growing hairs can prematurely shift into the shedding phase.
Common triggers include high fever, childbirth, severe infections, major surgery, thyroid disorders, significant psychological stress, and crash diets low in protein. The shedding usually doesn’t begin until two to three months after the triggering event, which is why people often don’t connect the cause. The good news is that telogen effluvium is almost always temporary. Once the underlying trigger resolves, hair typically returns to its normal density over six to nine months.
The Role of Nutrition
Your hair follicles are among the most metabolically active cells in your body, and they need a steady supply of nutrients to function. Iron is one of the most common nutritional gaps linked to hair thinning. While most labs consider ferritin (the body’s stored iron) levels of 15 to 30 ng/mL as normal, hair specialists use a higher threshold. Levels below 30 ng/mL are highly likely to contribute to hair loss, and optimal hair growth requires levels above 70 ng/mL. That gap between “normal” lab results and what your hair actually needs explains why some people with thinning hair are told their bloodwork looks fine.
Vitamin D deficiency shows a similar pattern. Studies consistently find that people experiencing hair loss have lower vitamin D levels than those with full heads of hair. Protein, zinc, and B vitamins also support the hair growth cycle, and prolonged deficiency in any of them can cause noticeable thinning.
Why Evolution Didn’t Eliminate Baldness
Given how common baldness is, one natural question is why evolution hasn’t selected against it. One theory, proposed by researchers Muscarella and Cunningham, suggests baldness may have actually been selected for through sexual selection. As men age and lose hair, the visual signal of a bald or balding head may have communicated social maturity, lower aggression, and a shift toward nurturing behavior. In social groups where older, more experienced males provided stability, that signal could have been an advantage rather than a drawback.
It’s also worth noting that pattern baldness typically progresses after peak reproductive years have already begun, which means it faces relatively weak evolutionary pressure. Traits that don’t significantly reduce your chances of reproducing can persist in a population indefinitely, even if they offer no direct survival benefit.