Most adult men will have armpit hair. Armpit hair spreads in two directions - upwards towards the biceps (“arm side”) and downwards towards the torso (“torso side”). For some men, the downward spread is not as far as the upward spread. These variances among men leads to three main types of armpit hair growth – no hair in the armpits, hair that has about the same spread on both sides of the crease of the armpit, and hair that is more concentrated under the arm side than the torso side. Armpit hair is thought to wick sweat and moisture away from the sweat glands and also to reduce friction of the arm against the torso.[i]
Armpit hair typically begins growing in the middle stages of puberty. A staging of puberty based on armpit hair growth is called the Wolfsdorf Staging and is shown below:
We conducted a survey looking asking men about the growth of armpit hair. The majority of men grow armpit hair although a fair number do not (especially in certain ethnic groups). Although we don't necessarily think about hair patterns, some men grow more hair on the arm side rather than the torso side while others grow hair fairly equally on both the arm and torso side of the armpit. This varied by ethnicity as seen below.
[i] https://en.wikipedia.org/wiki/Underarm_hair (2/4/17)
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