Male pattern baldness is caused by a hormone called DHT (dihydrotestosterone) shrinking genetically sensitive hair follicles on the scalp. About two-thirds of white men show meaningful hair loss by their late 50s, and the process can start as early as the late teens. The underlying mechanism is straightforward: your body converts testosterone into a more potent form, and if your follicles are genetically programmed to respond to it, they gradually shrink until they stop producing visible hair.
How DHT Shrinks Hair Follicles
Testosterone on its own isn’t the main culprit. An enzyme called 5-alpha reductase, concentrated in the outer root sheath of hair follicles, converts testosterone into DHT. DHT binds roughly five times more strongly to androgen receptors than regular testosterone does, and those receptors sit exclusively in the dermal papilla cells at the base of each follicle. When DHT locks onto these receptors, it triggers a process called miniaturization: the follicle physically shrinks with each growth cycle.
A healthy hair follicle cycles through a growth phase lasting two to six years, a short transition period, and a resting phase before the hair falls out and regrows. In DHT-affected follicles, each successive growth phase gets shorter. The hair that emerges becomes thinner, lighter, and shorter until eventually the follicle produces only a fine, nearly invisible vellus hair, or stops producing hair altogether. Studies comparing balding and non-balding scalps consistently find higher concentrations of both the 5-alpha reductase enzyme and DHT-receptor complexes in areas experiencing hair loss.
Genetics Determine Which Follicles Are Vulnerable
Not every man with normal testosterone levels goes bald, and that’s because genetics control which follicles are sensitive to DHT. The popular belief that baldness comes only from your mother’s side is a myth. While the androgen receptor gene does sit on the X chromosome (inherited from your mother), dozens of other genes contribute to your risk, and they come from both parents. A history of baldness on either side of the family increases your chances.
This genetic component explains the distinctive pattern of male pattern baldness. Follicles on the temples and crown carry more androgen receptors than those on the sides and back of the head. That’s why hair loss follows a predictable shape, starting with recession at the temples and thinning at the crown, while the horseshoe-shaped fringe remains. A man with fewer androgen receptors in those areas, or lower 5-alpha reductase activity, can have identical testosterone levels and keep a full head of hair.
The Role of Scalp Inflammation
Although male pattern baldness is officially classified as non-inflammatory hair loss, research tells a more complicated story. Biopsies of balding scalps reveal immune cell infiltration in roughly 50% of samples, including activated T cells and degranulated mast cells clustered around the follicles. This low-grade microinflammation appears to damage the stem cell region of the follicle (called the bulge), which is responsible for regenerating new hair with each cycle.
The practical significance of this inflammation is measurable. In one study, only 55% of men with visible microinflammation responded to topical minoxidil treatment, compared to 77% of men without inflammation. This suggests that DHT alone doesn’t fully explain hair loss in every case. The immune environment around follicles, including shifts in specific T cell populations and the possible collapse of the hair follicle’s natural immune protection, plays a meaningful supporting role in driving miniaturization and fibrosis around the follicle.
Scalp Tension as a Contributing Factor
One intriguing line of research points to mechanical tension in the scalp as a factor that amplifies DHT’s effects. The top of the head is anchored to a tough sheet of connective tissue called the galea aponeurotica, and the muscles of the forehead and back of the skull pull on it constantly. Researchers found a highly significant correlation (r = -0.885) between the pattern of elastic deformation across the scalp and the areas where hair loss occurs.
The proposed mechanism ties back to DHT. Hair follicle cells contain a protein called Hic-5, an androgen receptor co-activator that responds to mechanical stretching. When the scalp is under chronic tension from the underlying muscles, this protein may amplify the follicle’s sensitivity to DHT. The areas under the most mechanical stress, the temples and vertex, are exactly where hair loss begins. This doesn’t replace the hormonal explanation but may help explain why the pattern is so consistent and why psychological stress (which increases involuntary muscle tension in the forehead) sometimes seems to worsen hair loss.
Who Is Most Affected
Male pattern baldness affects all ethnicities, but at different rates. Data from nearly 189,000 men in the UK Biobank found that about 68% of white men showed some degree of balding, with 18.3% reaching the most severe category. South Asian men showed nearly identical rates. Black men were significantly less likely to experience hair loss: 54% showed minimal or no balding, roughly four times the rate of minimal balding seen in white men. Chinese men fell somewhere in between.
Age is the strongest predictor after genetics. Hair loss is progressive, meaning it worsens over time if untreated. Most men notice the first signs in their 20s or 30s, typically as subtle temple recession or a widening part. By 50, the majority of white and South Asian men have noticeable thinning.
How Hair Loss Is Staged
Doctors use the Norwood-Hamilton scale to classify the severity of male pattern baldness into seven types. Type I represents a full or nearly full hairline with no significant recession. Type II involves minor recession at the temples, no more than about 2 cm behind where the hairline naturally sits. Type III marks the point where recession becomes cosmetically noticeable, extending back to or beyond the midpoint of the scalp.
Types IV and V involve progressively deeper recession combined with thinning at the crown. By Type V, the strip of hair separating the frontal and crown bald areas has narrowed significantly. Types VI and VII represent the final stages, where only the characteristic horseshoe band of hair on the sides and back remains. Each type also has an “A” variant that accounts for differences in how hair loss presents, since not every man follows the textbook pattern exactly.
What the Scalp DHT Connection Means
One detail that matters for understanding treatment is the relationship between DHT in your blood and DHT in your scalp. These aren’t the same thing. In a controlled study, a standard dose of finasteride (a common hair loss medication) reduced blood DHT levels by about 71%, but scalp DHT dropped by only 62 to 69%. Even at higher doses, scalp DHT levels never dropped as far as blood levels did. This gap exists because DHT is produced locally within the follicle itself, not just delivered through the bloodstream.
This local production is why male pattern baldness doesn’t correlate neatly with blood testosterone levels. Two men with identical hormone panels can have very different hair loss outcomes depending on how much 5-alpha reductase their scalp follicles express and how many androgen receptors those follicles carry. It also explains why treatments that block DHT production work better than those targeting testosterone directly: the problem is what happens inside the follicle, not what’s circulating in your blood.