Hair growth and shedding are mainly regulated by the hair follicle cycle. Less hair may be caused by congenital genetics or may be a normal phenomenon caused by increasing age. In addition, bad habits, endocrine abnormalities, scalp diseases, etc., can affect the growth of hair and hair volume. A, non-disease factors 1, genetic: if the parents have less hair, the children’s hair will also be less, belonging to the common causes of less hair; 2, bad habits: excessive life stress, mental anxiety, improper hair dyeing and perming, easily lead to hair loss, so that the hair is less. In addition, with age, especially after entering the old age stage, hair reduction belongs to the normal physiological phenomenon; 3, drug factors: chemotherapy drugs, anticoagulants, contraceptives, etc., can lead to temporary or permanent hair loss, so that the hair is less. Second, disease factors 1, endocrine abnormalities: such as pituitary, thyroid, parathyroid or gonadal abnormalities may affect hormone secretion, thus changing the development of hair follicles and hair growth and loss cycle, also more common in women during pregnancy, childbirth or menopause; 2, scalp infection: infectious diseases of the scalp, such as folliculitis, boils, ringworm, inflammatory reactions destroy hair follicles, resulting in hair loss; 3, scalp injury : including mechanical trauma, burns, radiation treatment, etc., resulting in the destruction of hair follicles and the appearance of low hair volume; 4, autoimmune factors: the human immune system undergoes dysfunction, generating specific immune reactions against its own cells, thus destroying the structure of hair follicles and producing hair loss and reduced hair volume; 5, congenital diseases: congenital hairlessness or hypotrichosis, manifested by the absence or scarcity of hair at birth or shortly after birth. Some patients with androgenetic alopecia and baldness have a family history related to congenital genetic factors.