The male hormone testosterone has many different effects upon a man's body and brain. Before a male baby is born, testosterone secretions cause his internal and external reproductive organs to develop. Once he reaches puberty, a man begins secreting constant levels of testosterone that affect his physical development and produce characteristic traits of adult males. Several of these effects are facial.
Testosterone causes body and facial hair to proliferate, resulting in the characteristically darker, thicker hair of the adult male as compared to women and boys. Facial hair, which typically covers the cheeks, chin, upper lip and neck of an adult man, is one of the results of testosterone-driven hair growth, explains Dr. Lauralee Sherwood in her book "Human Physiology." The purpose of facial hair is unknown, but it doesn't normally occur in women, and disappears in men who develop testosterone deficiencies. Testosterone-dependent facial hair is much darker, thicker and more wiry than normal head hair, and has a flattened shape, giving it more curl than the hair a man grows on his head.
While boys may eagerly anticipate many of the effects of testosterone associated with puberty, they typically don't look forward to its effect upon skin quality. Testosterone increases oil secretion all over the body, but this effect is particularly emphasized on the skin of the face. One of the purposes of increased testosterone-driven oil secretion is to increase a man's body odor, which helps make him attractive to women, as body oil contains characteristic scent molecules to which women respond sexually. Dr. Gary Thibodeau, in his book "Anatomy and Physiology," notes that the transition from childhood to adult skin can be particularly rough, with oil secretions increasing tremendously before leveling off in post-pubescent males.
Testosterone also increases facial acne. Some of this effect is due to increased facial oil; Dr. Thibodeau notes that when oil clogs facial pores--particularly if bacteria become trapped in the pores--acne results. Testosterone also makes skin more prone to acne to begin with, predisposing it to problems in the presence of increased facial oil. Adult men typically don't have the same degree of difficulty with acne that teenage boys do, since testosterone concentrations level off toward the end of puberty and the body adapts. Men who supplement with testosterone-like substances, including many anabolic steroids, can experience increased facial acne, however, as a side effect of increased hormone levels.