Rakugami, kuzume: When you're happy, your hair grows; / when sad, your fingernails
-- Japanese proverb
There would be no haircuts were the hair not wont to grow. The head-top crop is characterized by this fact, which makes the hair of human beings distinct from that of other animals. Were we to allow our head hair to grow unimpeded at its normal rate of about 1.3 cm per month, it could eventually reach extravagant lengths of up to 3.7 meters or so. Most of us cut our hair before it reaches this point, for hair this long is cumbersome and even dangerous. Long hair gets caught on things and gives an enemy something to grab onto.
It is thought that prehistoric humans must have learned to cut their hair early on, probably by using sharpened stones as blades. Those whose religions prohibit them from haircuts, such as the Sikhs and the Rastafarians, devise ways to bind the hair so it doesn't get in the way. Women of medieval Japan were obscured, weighted down, and virtually immobilized not only by their many layers of silken kimono, but by the heavy curtains of long hair with which they were cloaked.