* The primary determinant of whether someone will have back hair.
* Genes control the amount of body hair a person has, including back hair.
Hormones
* Testosterone, a hormone produced in greater amounts in males than females, stimulates hair growth.
* In both sexes, higher testosterone levels are associated with more back hair.
* Changes in hormone levels, such as those that occur during puberty or menopause, can also affect back hair growth.
Age
* Back hair tends to increase with age; older individuals usually have more back hair than younger ones.
Other factors
* Certain medical conditions, such as hirsutism and polycystic ovary syndrome (PCOS), can cause excessive hair growth, including on the back.
* Certain medications, such as anabolic steroids, can also increase back hair growth.
* Obesity is sometimes seen with increased body hair; losing weight may decrease back hair.