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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 15 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.
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 (of, say, the pattern in diagram 3) 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 of men and women 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.
There have been occasional studies documenting patterns of chest hair in men and occurrence of these patterns. A study of 1100 men aged 17 to 71 defined and documented ten patterns of chest hair in Caucasoid men. In this study 6 percent of the men were found to have no chest hair. The largest group, 56 percent, displayed pattern four as shown in the accompanying figure. The remaining 38 percent of the men displayed a lesser quantity of chest hair. Seven percent displayed pattern one, 13 percent displayed pattern two and 18 percent displayed various other patterns.
The same study documented the chest hair patterns of 60 African-American men aged 20–40. For these men 22 percent were found to have no chest hair. The largest group, 37 percent displayed pattern four and the remaining 41 percent had a lesser quantity of chest hair. Eight percent displayed pattern one, 12 percent pattern two and 11 percent displayed various other patterns.
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