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If you grab a single strand of your hair and examine it closely to see if it looks more thick or thin, what you're looking at is the hair shaft. Are thinner or thicker hair shafts in your genetics? An AncestryDNA® test can reveal what your EDAR gene can tell you about your hair strand thickness.
Human Hair Thickness
Hair thickness is not necessarily tied to how much hair you have on your head. When scientists talk about “hair strand thickness,” what they’re referring to specifically is the diameter of each individual strand.
In other words, having lots of hair doesn’t always mean you have thick hair. And having thick hair doesn’t necessarily mean you have a lot of it.
You could have a lot of hair, but the individual strands could be very thin. Or you could have little hair, but the hair strands themselves could be very thick.
And while age isn’t perfectly correlated with hair thickness, your hair strands do get thinner as you age.
Genetics of Human Hair Thickness
Genetics strongly influence human hair thickness. Although several genes help determine hair thickness and texture, an AncestryDNA test focuses on the EDAR gene.
Hair thickness is an “additive" trait. This means that if you inherited two copies of the “thick hair" version of the gene (one from each parent), you'll likely have even thicker hair strands than if you only inherited the "thick hair" version from one parent.
What Science Says About Hair Thickness
The EDAR gene carries instructions for making a protein that affects hair thickness. A gene variation of EDAR that arose about 30,000 years ago seems to give some people thicker strands of hair.
More than 90% of Han Chinese, 70% of Japanese and Thai people, and 60% to 90% of Native Americans carry the “thick hair" version of the gene. Meanwhile it's almost nonexistent in people of African and European descent.
Both age and sex can also affect your hair shaft thickness. Men tend to have thicker hair strands than women, and as mentioned everyone's strands get thinner as they age.
Fun Facts About Hair Thickness
Some people believe shaving a baby's head will make the hair that grows back thicker. But this is a myth. Shaving your baby's hair won't affect hair thickness because it's mostly determined by genetics.
Cutting an adult's hair doesn't affect its thickness either. But it can help get rid of split ends, which can make hair strands look thinner.
What can impact your hair's thickness, as mentioned, is your heritage. But how much? If you look at a cross-section of a strand of hair from people of East Asian heritage, it's about 30% larger than hair strands of African people and 50% bigger than those of Europeans.
Fujimoto A, Ohashi J, Nishida N, et al. A replication study confirmed the EDAR gene to be a major contributor to population differentiation regarding head hair thickness in Asia. Human Genetics. 2008;124(2):179–185. doi:10.1007/s00439-008-0537-1.
Fujimoto A, Kimura R, Ohashi J, et al. A scan for genetic determinants of human hair morphology: EDAR is associated with Asian hair thickness. Human Molecular Genetics.2007;17(6):835–843. doi:10.1093/hmg/ddm355.
A thick head of hair is in the genes. Science. October 31, 2007.
Why ‘‘good hair’’ matters. Harvard Medical School. February 14, 2013. https://hms.harvard.edu/news/why-good-hair-matters