There is no single cause of baldness, which is one of the reasons that treatment can be difficult.
Doctors, as a general rule, focus on a hormone called dihydrotestosterone or DHT.
This article looks at the hormone's effects and other causes of hair loss.
If a person is partially or totally bald, many of his or her hair-producing follicles have stopped doing their jobs.
The follicles may have shrunk, become dormant or died.
If very fine hairs that are thin and light in color are still present, the follicles have shrunk.
If no hairs are being produced, the follicles may be dormant or dead.
If the lack of production followed a period in which thin, lighter hairs were produced, then the follicles are probably dead.
Becoming dormant or resting is a natural stage that the follicles go through on a regular basis.
They are productive for several months at a time.
They enter a short transitional phase and then a resting phase, which normally lasts for several weeks.
In a condition called Telogen effluvium, there is sudden hair loss, caused by a large number of follicles entering the resting stage all at once.
The hairs seem to fall out by the handful, but the condition is not usually a cause of baldness.
Telogen effluvium often resolves on its own, without any sort of treatment.
However, it may be a symptom of an underlying medical condition or nutritional deficiency.
If that is the case, treating the medical problem or correcting the nutritional deficiency is necessary to reactivate the hair-producing follicles.
DHT is one of the things that cause the follicles to shrink and can eventually kill them.
The effect that the hormone has is considered genetic, because a person's follicles are not necessarily sensitive to it.
DHT is often cited as the cause of baldness in men and women when there is a "pattern".
The pattern in men includes hair loss on the top or crown of the head only, at first.
Gradually, the hair may be lost all over the top of the head, but retained on areas below the tops of the ears.
This is the kind of hair loss that Julius Caesar is believed to have had and may, according to legend, be the reason that he began wearing a laurel wreath on his head.
In women, the hairs are lost around the central portion of the scalp, usually from front to back.
DHT may be a partial cause of baldness in women, but there are complicating factors.
Lack of estrogen following menopause is believed to play a role, as well.
There are other causes of hair loss, including drug treatments, such as chemotherapy in cancer, and exposure to toxins, such as mercury.
These cases correct themselves, once the treatment is halted or exposure to the toxin ends.
Although there are many over-the-counter treatments for hair loss, many of them are ineffective.
In order to work, the treatment must address more than one cause of baldness.
So, if you want results, look for one that does.