Chest hair is hair that grows on the chest in the region between the neck and the abdomen. Chest hair develops during and after puberty along with other types of androgenic hair. Development and growth Although vellus hair is already present in the area in childhood, chest hair is the terminal hair that develops as an effect of rising levels of androgens (primarily testosterone and its derivatives) due to puberty. Different from the head hair, it is therefore a secondary sexual characteristic. Men tend to be covered with far more terminal hair, particularly on the chest, the abdomen, and the face. The development of chest hair begins normally during late puberty, usually between the ages of 12 and 18. It can also start later, between the age of 20 and 30, so that many men in their twenties have not yet reached their full chest hair development. The growth continues subsequently. Patterns and characteristics The individual occurrence and characteristics of chest hair depend on the genetic disposition, the hormonal status and the age of the person. The genes primarily determine the amount, patterns and thickness of chest hair. Some men are very hairy, while others have no chest hair at all. All ranges and patterns of hair growth are normal. The areas where terminal hair may grow are the periareolar areas (nipples), the centre and sides of the chest and the clavicle collarbone. The direction of growth of hair can make for interesting patterns, akin to depictions of mathematical vector fields. Typical males will exhibit a node on the upper sternum, the hair above which points up and the hair below which points down. Some individuals have spirals on their upper pectoral regions (several inches from the nipple towards the neck) which run clockwise on the left breast and counter-clockwise on the right. Considering an individual occurrence of chest hair as abnormal is usually not due to medical indications but primarily to cultural and social attitudes. An excessive growth of terminal hair on the body is called hypertrichosis. This medical term has to be distinguished from hirsutism that just affects women. These women can develop terminal hair on the chest following the male pattern as a symptom of an endocrine disease. Setty patterns There have been occasional studies documenting patterns of chest hair in men and occurrence of these patterns. A study of 1,400 white men aged 17 to 71