According to a new study, bald men tend to have an abnormal amount of a protein called "prostaglandin D2" on their scalps. This protein appears to block hair growth.
Researchers at the University of Pennsylvania tested the theory in mice and found those given a high level of the protein went completely bald.
Researchers say the discovery could lead to new treatments for hair loss in men. The findings appear in the journal "Science Translational Medicine".