At one time or another, dandruff has been blamed on dry skin, oily skin, shampooing too often or not often enough, a poor diet, stress, and the use of too many fancy styling products. Although some of these factors may exacerbate or contribute to scalp flaking, the real culprit may be a fat-eating, yeast-like fungus called malassezia, formerly known as pityrosporum.

Malassezia lives on the scalps of most healthy adults without causing problems. But sometimes it grows out of control, feeding on the oils secreted by your hair follicles and causing irritation that leads to increased cell turnover.

All skin cells die and are replaced by new cells. Normally, it takes about a month for new cells to move from the lowest layer of your skin, where they form, to the outermost layer, where they die and scale off in flakes. Because cells renew themselves slowly, this process usually isn't noticeable.

But on scalps where malassezia thrives, the whole process can take as little as 11 days. The result is a large number of dead skin cells. As the cells fall off, they tend to clump together with oil from your hair and scalp, making them appear white, flaky and all too visible.

Scientists used to think that a type of malassezia fittingly called M. furfur was responsible for dandruff. But new studies show that two other forms of the fungus — M. restricta and M. globosa — are to blame. Exactly what causes an overgrowth of these organisms isn't known, although increased oil production, hormonal fluctuations, stress, illness, neurologic disorders such as Parkinson's disease, a suppressed immune system, infrequent shampooing, extra sensitivity to the malassezia fungus and even heredity may contribute to the development of dandruff.