There are many causes of hair loss but by far the most common in men and women is genetic or pattern hair loss. Male pattern hair loss in men is extremely common in the United States; it affects over 50 million men and over 35 million women. There are other causes of hair loss as well; they're just not quite as common. There is telogen effluvium which is actually just the shedding of hair and and that often can happen with periods of stress; it can happen after general anesthesia, it can happen with a traumatic event, and it can happen after crash dieting as well. There are other medical conditions that cause hair loss like thyroid disease and anemia. And there's something called alopecia areata, where people lose generally little patches of baldness and that's an autoimmune process that affects about 2% of the population. But again, to reiterate, the most common form of hair loss is genetic, it's inherited, it's male and female pattern hair loss.