Baldness happens primarily because a hormone called DHT (dihydrotestosterone) shrinks your hair follicles over time until they can no longer produce visible hair. This process, called miniaturization, is driven by your genetics and affects roughly 30% of men by age 30, 50% by age 50, and continues climbing with each decade. But genetics aren’t the whole story. Stress, nutrition, autoimmune conditions, and hormonal changes all play roles in different types of hair loss.
How DHT Shrinks Hair Follicles
Every hair on your head cycles through three phases: a growth phase lasting 3 to 10 years, a brief transition phase of 2 to 3 weeks, and a resting phase of 3 to 4 months before the hair falls out and a new one begins growing. This cycle is what makes normal hair shedding (50 to 100 hairs a day) perfectly routine.
The trouble starts when your body converts testosterone into DHT, a more potent form of the hormone. DHT binds to receptors on hair follicles, particularly those on top of the head and along the temples, and triggers them to shrink. With each growth cycle, the follicle produces a thinner, shorter, lighter hair. Eventually the follicle miniaturizes so much that it stops producing anything visible at all. The growth phase also gets shorter, meaning each hair spends less time growing and more time resting or shedding. This is why balding areas often first look “thin” before they look truly bare.
Importantly, the follicles on the sides and back of your head are largely resistant to DHT. That’s why classic male pattern baldness creates a horseshoe shape of remaining hair, and it’s also why hair transplant surgeons harvest follicles from the back of the scalp: those follicles keep their DHT resistance even after being moved.
Genetics Are the Biggest Factor
Baldness runs in families, but the inheritance pattern is more complex than a single gene. The androgen receptor gene, located on the X chromosome (which men inherit from their mothers), plays a major role. A specific variant of this gene was found in 98% of young bald men in one study, compared to only 77% of men with no hair loss. Shorter repeat sequences within the gene were also more common in bald men, suggesting these markers sit very close to the functional variant that contributes to pattern baldness.
Because the androgen receptor gene is on the X chromosome, the old saying that baldness comes from your mother’s side has some truth. But it’s not the complete picture. Baldness is polygenic, meaning dozens of genes across multiple chromosomes influence whether and how fast you lose hair. Your father’s side of the family matters too. If both your parents carry relevant variants, your chances go up significantly. This is why two brothers can have very different hair loss timelines despite sharing the same parents.
Why Women Lose Hair Differently
Women experience pattern hair loss too, though it looks different. Instead of a receding hairline or a bald crown, women typically see diffuse thinning that starts along the part line and spreads across the top of the scalp. The front hairline usually stays intact, and hair around the sides and back maintains normal density. This pattern was formally classified in 1977 using a three-stage scale that’s still the standard in hair loss diagnosis.
In the earliest stage, thinning is most visible along the part and around the crown. By the second stage, the part widens noticeably and hair feels thinner overall. In the most advanced stage, the crown may be completely bare while the rest of the scalp looks relatively normal. Hormonal shifts during menopause are a common trigger, as declining estrogen allows the effects of androgens like DHT to become more pronounced. Women with pattern hair loss also tend to have a negative “pull test,” meaning hair isn’t easily dislodged from the follicle. This helps distinguish it from other conditions where hair falls out more actively.
Stress Can Push Hair Into a Resting Phase
Severe or prolonged stress causes a different kind of hair loss called telogen effluvium, where large numbers of follicles simultaneously enter their resting phase and shed 2 to 3 months later. Research from Harvard’s Stem Cell Institute revealed how this works at a cellular level. The stress hormone cortisol acts on a cluster of cells beneath the hair follicle called the dermal papilla. Under chronic stress, cortisol prevents these cells from releasing a signaling molecule that normally activates hair follicle stem cells. Without that signal, the stem cells stay dormant far longer than usual, and new hairs simply don’t start growing.
The good news is that stress-related hair loss is typically reversible. Once the stressor resolves and cortisol levels return to normal, stem cells can reactivate. But when stress is chronic, it essentially amplifies a built-in biological braking system that already exists between your adrenal glands and your hair follicles, making regrowth progressively harder the longer the stress continues.
When the Immune System Attacks Hair
Alopecia areata is an autoimmune condition where the immune system mistakenly identifies hair follicles as foreign invaders and attacks them. This produces distinctive round, smooth patches of hair loss that can appear anywhere on the scalp or body, sometimes seemingly overnight. Unlike pattern baldness, alopecia areata can strike at any age and affects men and women equally.
In some cases, the patches regrow on their own within months. In others, the condition progresses to total scalp hair loss or even complete body hair loss. The unpredictability is one of its most frustrating features. Because the follicles aren’t destroyed (just suppressed by immune activity), regrowth is always possible, though recurrence is common.
Nutritional Deficiencies That Thin Hair
Your follicles need a steady supply of nutrients to sustain years of continuous growth. Iron is one of the most common culprits in unexplained hair thinning. Standard lab work often labels ferritin (your body’s iron storage protein) as “normal” at 15 to 30 ng/mL, but hair specialists use higher thresholds. Levels below 30 ng/mL are highly likely to contribute to hair loss, and optimal hair growth generally requires levels above 70 ng/mL. This gap between “medically normal” and “good enough for hair” means many people with thinning hair are told their bloodwork looks fine when their follicles are actually starving for iron.
Vitamin D deficiency is also consistently linked to hair loss, though researchers haven’t pinpointed an exact threshold. People experiencing hair loss tend to have lower vitamin D levels than those without it. Since vitamin D receptors are present on hair follicles and play a role in the growth cycle, keeping your levels adequate is a low-risk measure worth discussing with a healthcare provider if you’re noticing thinning.
What Actually Works for Treatment
Two treatments have the strongest clinical track records. Minoxidil, available as a topical liquid, foam, or oral pill, works for roughly two-thirds of people who use it. It doesn’t block DHT. Instead, it extends the growth phase and increases blood flow to follicles. The catch is that it only works as long as you keep using it. Stop, and the hair it maintained will gradually thin again.
Finasteride works for the majority of men who take it by blocking the enzyme that converts testosterone into DHT, reducing DHT levels in the scalp. It’s taken as a daily pill and, like minoxidil, requires ongoing use to maintain results. Some men use both together for a stronger effect. Neither treatment can resurrect follicles that have completely miniaturized, which is why starting earlier tends to produce better outcomes.
Is Baldness an Evolutionary Advantage?
This is one of the more puzzling questions in human biology. If baldness were strongly disadvantageous, you’d expect natural selection to have weeded it out long ago. Some researchers have proposed that a bald scalp may have signaled maturity or social dominance in early human groups, potentially conferring a mating or leadership advantage. Others have suggested it could enhance vitamin D production through increased sun exposure on the scalp.
A large-scale DNA study analyzing recent human evolution found evidence that variants associated with a lower chance of male pattern baldness have actually been under positive selection in West Eurasian populations, meaning natural selection may be slowly reducing baldness over time. But the researchers noted that the DNA alone doesn’t explain why these traits were useful. For now, the honest answer is that baldness likely persists because it doesn’t significantly reduce reproductive success. Most men who carry baldness genes have already had children before significant hair loss becomes visible, so the trait faces little selective pressure to disappear.