Decreased number of hairs per unit area of skin of the scalp. The phenotype is Sparse scalp hair (Human Phenotype Ontology id HP_0002209). Also known as: Decreased number of scalp follicles, Hypotrichosis on scalp, Reduced amount of scalp hair, Reduced/lack of hair on scalp, Reduction in the number of scalp follicles, Scalp hypotrichosis, Thinning scalp hair.