This is actually not the case: head hair and arm hair grow at about the same rate, around half an inch (1.25 cm) each month on average. Hair continues to grow throughout a person's life (until hair follicles eventually die), but eventually reaches equilibrium when the rate of loss is equal to the rate of growth.

Head hair tends to grow to much greater lengths because the hair growth cycle on the scalp – termed the anagen phase – can endure for several years in a healthy follicle (anywhere from two to six years or longer), meaning hairs are allowed more opportunities to lengthen before falling out and beginning to regrow.

By contrast, the anagen phase for arm hair tends to last under a month (between two and four weeks); thus, hairs remain in growing mode for much shorter lengths of time, and have significantly less opportunity to grow long before shedding starts and the new crop starts from scratch.