The most common cause of hair loss in men.

Male pattern baldness — clinically called androgenetic alopecia — is the gradual miniaturisation of hair follicles in genetically susceptible areas of the scalp, driven by the hormone dihydrotestosterone (DHT). Affected follicles produce progressively finer, shorter hair until they stop producing visible hair altogether.

The pattern is consistent: recession at the temples, then thinning at the crown, with the bridge of hair between front and crown thinning last. The back and sides of the head are typically spared because those follicles are genetically resistant to DHT — which is why they make excellent donor hair for transplantation.

Onset can begin as early as the late teens; most men who'll experience pattern loss will see it begin by their mid-thirties. Progression rate varies enormously — some men lose slowly over decades, some lose rapidly over a few years.