Male pattern baldness is driven by a hormone called DHT (dihydrotestosterone) that gradually shrinks hair follicles on the top and front of your scalp until they can no longer produce visible hair. About two-thirds of men experience noticeable hair loss by age 35, and roughly 85% have significant thinning by 50. The process is slow, usually unfolding over years or decades, but it follows a surprisingly predictable biological sequence.
The Role of DHT
Testosterone circulates through your bloodstream and, in certain tissues, gets converted into a more potent form called DHT by an enzyme known as 5-alpha reductase. DHT is useful elsewhere in the body, but in the scalp it binds to androgen receptors on hair follicles and triggers a process called miniaturization. Each time a follicle completes a growth cycle under the influence of DHT, it comes back slightly smaller, producing a thinner, shorter, lighter hair than before.
What makes this confusing is that DHT doesn’t affect all hair equally. The follicles on the sides and back of your head are genetically resistant to it, which is why even men with advanced baldness keep that horseshoe ring of hair. The follicles on the crown and along the hairline are the vulnerable ones, and their sensitivity to DHT is largely determined by your genetics.
Why Your Hair Cycle Gets Shorter
Every hair on your head goes through a repeating cycle: a growth phase (anagen), a brief transition, and a resting phase (telogen). In a healthy scalp, the growth phase lasts two to six years, which is why head hair can grow so long. The resting phase lasts a few months before the hair falls out and a new one begins growing in its place.
In male pattern baldness, DHT progressively shortens the growth phase with each cycle. A follicle that once spent five years growing hair might spend three, then one, then just a few weeks. The resting phase, meanwhile, stays the same length or actually gets longer. A substage of the resting phase called kenogen, where the follicle sits completely empty with no hair at all, becomes extended. This means more follicles are sitting idle at any given time, which is why thinning becomes visible before full baldness sets in. You’re not necessarily losing more hair per day. The hair just isn’t growing back as quickly or as fully.
Eventually, after enough shortened cycles, the follicle miniaturizes to the point where it only produces a tiny, nearly invisible “vellus” hair, the kind of fine peach fuzz you see on a child’s arm. The follicle still technically exists, but it’s no longer contributing to visible hair coverage.
Genetics Are Polygenic, Not Just Maternal
The popular belief that baldness comes from your mother’s side has a grain of truth but misses the bigger picture. The androgen receptor gene (AR gene) sits on the X chromosome, which men inherit from their mothers. Variants in this gene appear to increase androgen receptor activity in the scalp, making follicles more responsive to DHT. These genetic changes are most common in men whose hair loss starts early, sometimes before age 21.
But the AR gene is only one piece. Researchers have identified dozens of other genetic regions on non-sex chromosomes that contribute to baldness risk. That means your father’s hair, your paternal grandfather’s hair, and broader family patterns all matter. Male pattern baldness is polygenic, meaning many genes contribute small effects that add up. Looking at one relative won’t reliably predict your outcome.
The Norwood Scale: How Baldness Progresses
Dermatologists classify male pattern baldness using the Norwood scale, a seven-stage system that maps how hair loss typically advances. Stage 1 means no significant loss. In the early stages (2 and 3), you’ll notice the hairline receding at the temples, forming an M or V shape. By stages 4 and 5, the thinning crown expands and begins merging with the receding hairline. Stages 6 and 7 represent extensive loss across the top of the scalp, leaving only the resistant band of hair around the sides and back.
There’s also a less common variation called Norwood class A, where the hairline recedes uniformly from front to back without creating a separate bald spot on the crown. Instead of the typical “island” of hair between a receding hairline and a thinning crown, the loss moves as a single front. Not everyone follows the standard pattern exactly, but the Norwood scale gives a useful framework for tracking progression and discussing treatment options.
How Early It Can Start
Around 25% of men with male pattern baldness begin losing hair before age 21. That catches many people off guard because baldness is culturally associated with middle age. In reality, the process often begins in the late teens or early twenties with subtle temple recession that’s easy to dismiss. By 35, two-thirds of men have some degree of visible loss. The earlier it starts, the more advanced it tends to become over a lifetime, though the rate of progression varies widely from person to person.
How Treatments Target the Process
The two most established treatments work by interrupting the DHT pathway at different points. One approach blocks the enzyme that converts testosterone into DHT, reducing DHT levels in the scalp. This slows miniaturization and, in many men, allows partially shrunken follicles to recover somewhat, producing thicker hair again. The other main approach is a topical treatment that stimulates blood flow to follicles and extends the growth phase, partially counteracting the cycle shortening caused by DHT.
Both approaches work best when started early, before follicles have fully miniaturized. Once a follicle has been dormant long enough, it becomes much harder to revive. This is why men who begin treatment at Norwood stage 2 or 3 tend to see better results than those who wait until stage 5 or 6. Neither treatment is a cure. They slow or partially reverse the process for as long as you use them, and hair loss typically resumes if you stop.
Hair transplant surgery takes a different approach entirely. It relocates DHT-resistant follicles from the back and sides of the scalp to thinning areas. Because those follicles are genetically programmed to resist miniaturization, they continue growing normally in their new location. The transplanted hair is permanent, but it doesn’t stop the surrounding native hair from continuing to thin, which is why many transplant patients also use medical treatments to maintain overall density.