Most baldness is genetic. Around 30% to 50% of men show visible hair loss by age 50, and fewer than 15% reach age 70 with little or no thinning. The underlying cause in the vast majority of cases is a inherited sensitivity to a hormone called DHT, but stress, nutrition, autoimmune conditions, and lifestyle factors can all play a role.
How DHT Shrinks Hair Follicles
The primary driver of common baldness, known medically as androgenetic alopecia, is dihydrotestosterone (DHT). Your body converts testosterone into DHT using an enzyme called 5-alpha reductase. DHT itself isn’t the problem. The problem is what happens when DHT binds to receptors inside genetically susceptible hair follicles.
When DHT locks onto these receptors, it triggers signaling that kills tiny blood vessels feeding the follicle’s base. Without adequate blood supply, the follicle gradually shrinks. Each hair cycle produces a thinner, shorter strand until eventually the hair is too fine and short to even reach the skin’s surface. This is why balding areas often still have a light fuzz rather than being truly bare. The follicles aren’t dead yet, but they’re producing hair that’s essentially invisible.
DHT also disrupts the timing of hair growth. A healthy scalp hair grows for two to eight years during its active phase, then rests for two to three months before falling out and restarting. In balding follicles, the growth phase gets progressively shorter while the resting phase stretches longer. The result is hair that never reaches its former length and takes longer to replace after it falls.
The Genetics Behind It
The single most important gene associated with baldness is the androgen receptor (AR) gene, located on the X chromosome. Since men inherit their X chromosome from their mother, this is why people often say baldness comes from the maternal side. That’s partially true, but it’s not the whole picture.
Genome-wide studies have identified at least eight independent genetic regions linked to baldness across multiple chromosomes, not just the X. Genes on chromosomes 2, 3, 5, and 12 all contribute, with one of the strongest associations found in a gene called WNT10A on chromosome 2. Other genes involved include those that regulate hair follicle development and the enzyme aromatase, which converts testosterone to estrogen and can influence how much DHT reaches your follicles. The takeaway: baldness doesn’t follow a simple one-gene inheritance pattern. Both sides of your family tree matter.
Why Women Lose Hair Differently
Women experience the same basic mechanism of DHT-driven follicle shrinkage, but the pattern looks different. Instead of a receding hairline and bald crown, women typically notice diffuse thinning across the top of the scalp while keeping their frontal hairline. This is partly because women have lower overall levels of DHT, and their follicles may have different receptor sensitivity depending on scalp location.
Hormonal shifts after menopause often accelerate the process. As estrogen levels drop, the relative influence of androgens like DHT increases, and hair that had been stable for decades can start thinning noticeably within a few years.
Stress and the Cortisol Connection
Stress-related hair loss, called telogen effluvium, works through a completely different pathway than genetic baldness. When your body is under sustained stress, it produces high levels of cortisol. Research published in Nature showed that cortisol acts on the dermal papilla cells at the base of hair follicles, suppressing a protein called Gas6 that normally signals hair follicle stem cells to wake up and start growing.
Without that activation signal, stem cells stay dormant and follicles remain stuck in their resting phase far longer than usual. The hair that would normally cycle out and regrow simply doesn’t get replaced on schedule. The good news is that this type of hair loss is usually reversible. Once cortisol levels normalize, the stem cells can reactivate. In experiments, restoring Gas6 expression was enough to restart hair growth even when cortisol levels were still elevated.
The classic trigger is a major stressor: surgery, severe illness, childbirth, or prolonged emotional distress. Hair typically falls out two to three months after the event, which often confuses people because the stressor has already passed by the time they notice thinning.
When the Immune System Attacks Follicles
Alopecia areata is an autoimmune condition where the body’s own immune cells mistakenly target hair follicles. The attack is led primarily by cytotoxic T cells that recognize proteins associated with melanocytes, the pigment-producing cells in the follicle. These T cells infiltrate the follicle wall, triggering cell death and rapid hair loss.
Natural killer cells also participate, targeting follicles that display certain stress markers on their surface. The result is distinctive round, smooth patches of hair loss that can appear suddenly. In some cases, the condition progresses to total scalp hair loss or even loss of all body hair. Unlike genetic baldness, alopecia areata can strike at any age and affects men and women equally.
Nutritional Deficiencies That Thin Hair
Iron and zinc deficiencies are the two most common nutritional causes of hair thinning. Hair follicles are among the most rapidly dividing cells in the body, and they need a steady supply of both minerals to function. Clinicians who specialize in hair loss now suggest that a ferritin level (your body’s stored iron) below 60 ng/mL can contribute to shedding, which is well above the threshold many standard lab reports flag as “low.” You can have iron levels that look normal on a routine blood test and still not have enough to support healthy hair growth.
Zinc plays a role in cell division within the follicle. Levels below 700 µg/L are associated with increased shedding. Both deficiencies are more common in women, particularly those with heavy periods or plant-based diets, and correcting them often improves hair density over several months.
Smoking and Follicle Damage
Smoking accelerates hair loss through several overlapping mechanisms. Nicotine constricts the blood vessels that supply the dermal papilla at the base of each follicle, reducing oxygen and nutrient delivery. It also accumulates DNA damage in follicle cells, promotes local inflammation, and triggers fibrosis (scarring) around the follicle. A systematic review found that these effects mirror the way smoking ages skin: the same inflammatory and vascular damage that causes wrinkles also degrades the environment your hair needs to grow.
How Hair Loss Treatments Work
The two most established treatments for genetic hair loss target opposite ends of the same problem. One approach blocks DHT production. Finasteride inhibits the enzyme that converts testosterone to DHT, reducing DHT levels and slowing follicle miniaturization. The other approach, minoxidil (available over the counter), works by widening blood vessels around the follicle and extending the growth phase of the hair cycle. Neither regrows a full head of hair on a slick-bald scalp, but both can slow progression and partially reverse thinning, particularly when started early.
For alopecia areata, a newer class of treatments called JAK inhibitors has changed the outlook significantly. These medications, including baricitinib and ritlecitinib, work by calming the overactive immune response that attacks follicles. They reduce T cell activation and help restore the follicle’s normal immune-protected status. For people with severe alopecia areata who previously had no effective options, these drugs can produce substantial regrowth.
Why Humans Evolved to Go Bald
One lingering question is whether baldness serves some evolutionary purpose or is just a genetic accident. Researchers have explored several hypotheses: that a bald scalp might signal social maturity, improve vitamin D synthesis through sun exposure, or simply hitchhike along with genes selected for other traits. A large genetic study comparing African and European populations found that the differences in baldness patterns between populations are governed more by neutral genetic drift than by natural selection. In other words, baldness doesn’t appear to have been actively favored or eliminated by evolution. The genes most strongly linked to it, particularly those near the androgen receptor, may have been selected for their effects on other traits entirely, with hair loss as an incidental side effect.