After living with a flaky scalp for as long as I could remember, I recently figured out what it was—seborrheic dermatitis ("sebo" for short) many thanks to Alan Dattner, M.D.'s, OG skin care bookRadiant Skin. It was the only beauty book I could find with extensive information—an entire chapter!—dedicated to the condition. Also known as "adult cradle cap," sebo is a type of eczema triggered by stress, hormone imbalance, chemical irritants in shampoos, and extremely dry or cold weather. According to Cybele Fishman, M.D., sebo could also be a sign that your skin's microbiome is out of balance.