Septal deformity is of two kinds, which may occur independently, or together: 1) anterior cartilage deformity of the quadrilateral septal cartilage, caused by direct trauma or pressure at any age; and 2) combined septal deformity involving all the septal components, caused by compression across the maxilla from pressures occurring during pregnancy or parturition. This is part of a facial deformity. The incidence of septal deformity was investigated in 2,380 Caucasian infants at birth, 2,112 adult skulls of five ethnic groups (European, Indian [Asian], Chinese, African and Australian Aborignal), 918 mammals (266 higher and lower apes, 457 other placental mammals and 185 marsupials). The method of nasal testing of infants by passage of special testing struts (6 by 2 mm) is described. Forty-two percent of septa of infants were straight, 27% deviated and 31% kinked. A similar pattern was found in adult skulls, namely 21% straight, 37% deviated and 42% kinked. Anterior cartilage deformity occurred in about 4% of births. The maxillary molding theory of transmitted pressures during pregnancy or partitution, causing septal deformity, is described. The findings show that varying degrees of septal deformity occur at a constant rate at birth and in the adult. These may vary slightly for each ethnic type. Birth molding pressures are a major cause of dental malocclusion. The shape and strength of the skull and the erect posture appear to be major factors, for septal deformity did not occur in the lower animals, but occurred in 37% of the higher apes and also in a skull of a hominid 1,750,000 years old. This concept enables easy recognition at birth, and the carrying out of a rational method of treatment by manipulation and rapid maxillary expansion.