If you look at a single strand of your hair and examine it closely, does it look more thick or thin? What you're looking at is the hair shaft. Thinner or thicker hair shafts are influenced by your genetics. An AncestryDNA^® + Traits test can reveal what your genes suggest about the strand thickness your type of hair has.

Human Hair Strand Thickness

Hair thickness is not necessarily tied to how much hair you have on your head—the number of hair strands. When scientists talk about "hair strand thickness," what they’re referring to specifically is the diameter of each individual strand, not an overall hair growth pattern.

In other words, having lots of hair doesn’t always mean you have thick hair. You could still have thin, fine 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 hair strands could be very thin. Or you could have little hair, but the hair strands themselves could be very thick.

Genetics of Human Hair Thickness

Genetics strongly influence human hair strand thickness. An AncestryDNA test looks at over 750 DNA markers associated with hair strand thickness. Hair strand thickness is an "additive" trait. This means that the hair thickness you have depends on how many thick hair gene variants you inherit.

Among the DNA markers that an Ancestry test looks at for hair strand thickness are those in the EDAR gene. The EDAR is a gene on chromosome 2 that plays a key role in the development of hair and skin before you're born. Researchers have pinpointed a variant in this gene that's linked to thicker hair strands.

The EDAR gene carries instructions for making a protein that affects hair strand 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 strand" version of the EDAR gene. Meanwhile, it’s almost nonexistent in people of African and European descent.

Other Factors That Impact Hair Strand Thickness

Genes aren't the only thing that affects your hair thickness. Your age, environment, and health can also have a big impact on how your hair looks.

While age isn’t perfectly correlated with hair thickness, hair growth does slow down and hair strands do get thinner as you age. Having thinner, finer strands of hair can also make it feel like a hair loss—there just isn’t as much hair to put into a hairstyle like a ponytail.

Your sex hormones can also affect your hair shaft thickness. Men tend to have thicker hair strands than women, for example.

Fun Facts About Hair Thickness

Some people believe shaving a baby's head will make the hair that grows back thicker. But changing low-density or thin hair this way is a myth. Shaving your baby's hair won't affect hair thickness because that’s mostly determined by genetics.

Cutting an adult's hair doesn't affect its thickness either. But a haircut can help get rid of the frizz of split ends, which can make hair strands look thinner.

References

"Aging changes in hair and nails." MedlinePlus. Accessed August 4, 2023. https://medlineplus.gov/ency/article/004005.htm.

Fujimoto A, Jun Ohashi J, 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. September 2008. https://pubmed.ncbi.nlm.nih.gov/18704500/.

Fujimoto, Akihiro, Ryosuke Kimura, et al. "A scan for genetic determinants of human hair morphology: EDAR is associated with Asian hair thickness." Human Molecular Genetics. March 2008. https://pubmed.ncbi.nlm.nih.gov/18065779/.

Kaiser, Jocelyn. "A thick head of hair is in the genes." Science. October 31, 2007. http://www.sciencemag.org/news/2007/11/thick-head-hair-genes.

Leo, R. Alan. "Why ‘good hair’ matters." Harvard Medical School. February 14, 2013. https://hms.harvard.edu/news/why-good-hair-matters.