Body hair serves many purposes, and it evolved long before humans walked the earth. Hair comes from the same keratin structures that allow birds, reptiles and mammals to grow claws, and scientists suspect that early hair structures developed to protect the body from extremes in temperature and humidity.
Androgen, a group of male hormones such as testosterone, fuel the growth of hair on the back. While both men and women have androgen, men have higher levels.
Sensitivity levels to androgen vary in each individual hair follicle. This can cause hair to grow in patches. A man with more hair on his abdomen may have minimal hair on his back, or a man may have portions on his back--even individual hair follicles--that grow voraciously.
Back hair regulates the temperature of the body. The coiled structure of hair helps to move heat into and out of the body. It traps heat close to the body when it is cold, and cools and shades the skin when it is hot.
Increased hair provides clues to the opposite sex by functioning as a secondary sex characteristic. Men do not grow back hair until they experience puberty, so back hair can function as a sign of sexual maturity.
Medication and Illness
Certain medications, medical treatments or illnesses may affect the hair growth on the back. Chemotherapy, blood thinners and anti-depressants can slow or stop hair growth, and medications such as prednisone can cause unusual hair growth as a side effect.