Hair loss is incredibly common. According to the American Hair Loss Association, two thirds of American men will experience noticeable hair thinning or loss by age 35. At 50 years old, that figure rises to 85 percent. And though it's less often spoken about, plenty of women lose hair, too: about 40 percent of people experiencing hair loss in the United States are women.

Despite its ubiquity, hair loss is not as well understood as you might think -- especially among people who are not men, who were the subjects of the majority of past research. Experts discuss the biology of balding: who experiences it, why it happens, and what we can do about it.

Even millennia ago, there were hints that balding had something to do with male sexual development; ancient Greek philosophers and physicians noted in their medical writings that eunuchs don't go bald.

Scientists now understand that male pattern baldness -- by far the most common cause of balding in men -- happens thanks to androgens, or male sex hormones. Unfortunately, research owes that knowledge to a cruel period of American history. In 1942, anatomist James Hamilton studied people with mental disabilities who were castrated as part of the eugenics movement. Hamilton discovered that men castrated before puberty never developed male pattern baldness, except if they were treated with testosterone.

Hamilton's studies also produced a classification scheme for male pattern baldness, which is still used today to score degrees of androgenetic alopecia (pattern baldness).

Scientists think that male pattern baldness has to do with a hormone called dihydrotestosterone or DHT (about 10 percent of the testosterone in the body is converted to this more potent form daily). Oversensitive follicles on the scalp react to DHT by shrinking down and spending less time in the growth phase of the hair cycle, says Andrew Messenger, a retired dermatologist at the University of Sheffield in the U.K. This leads hair to thin and fall out.