Why Can't All Men Grow Full Beards?
When it comes to growing a full, thick, luscious beard, testosterone doesn't play as large of a role as you might think. In fact, the major factor in determining whether or not a man will grow a full beard is genetics. Population-wide studies have shown that testosterone levels in men with full beards and men who don't grow full beards are roughly the same. The reason some blonde men have red-haired beards is that they carry the redhead trait, and are expressing that gene via facial hair.
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from Stuff Mom Never Told You - HowStuffWorks
Key Facts In This Video
The word for the ability to grow a beard is pogonotrophy. (0:06)
Research shows that testosterone levels in men with full beards and men who don’t grow full beards are roughly the same. (0:50)
Men's facial hair growth is more mediated by genetics than testosterone. (1:19)