“Therefore, children have less oily scalps, teenagers often have very oily hair, and, as one matures, the scalp tends to decrease in oil production, and hair is generally less oily.” In that same vein, menstruation, pregnancy, and menopause can cause ample oiliness, as can increased stress levels (remember, cortisol is a hormone) and hormonal disorders, explains Howard Sobel, MD, board-certified dermatologist and clinical attending cosmetic dermatologic surgeon at Lenox Hill Hospital in New York City. Other factors that can cause oily hair include going too long between washes, which causes buildup dirt, oil, and various styling products, using hair care products that are too heavy for your hair type, high humidity levels, sweating, and even certain scalp conditions.