Having hair is a trait shared among mammals. For humans, the hair on the scalp is particularly salient because it can signal differences in sex, religion, and culture according to how it is styled and treated. The shape and color of hair varies both among populations but also across different body areas and at macro and micro levels. In this research we studied  variation in human scalp hair using light microscopy and transmission electron microscopy (TEM). We compiled a dataset of measurements taken from cross-sections of hairs, which had been collected by Mildred Trotter in the early to mid-twentieth century, and compared the measurements we were able to collect using higher magnification and imaging software. The data previously collected was found to be reliable and that data has been compiled into an accessible database on scholarsphere (https://scholarsphere.psu.edu/concern/generic_works/08k71ng97d) for use by other researchers. This database is especially useful as it contains information on hair samples from numerous populations from around the world and the hairs were free of modern chemical treatments often used to straighten or color hair today.

Using hair samples with associated genetic ancestry data, we were able to study the variation in human hair form and color within individuals and among groups. Using TEM, we examined traits related to hair shape, thickness of the outer cuticle, and density and distribution of pigment granules (melanosomes) that impart color. These traits were then assessed relative to genetic ancestry to study the strength of any relationships. Differences in hair form and cuticle thickness are apparent among broadly defined geographic ancestry groups, but these categories do not necessarily reflect our modern, genetically admixed world. Measurable differences were found in cuticle thickness and in the number of cuticle layers among the populations studied. Hairs from people with African ancestry tend to have thinner cuticles with fewer layers making up the cuticle, while those from people with European and East Asian ancestry tend to have thicker cuticles made up of more layers. Samples of hairs from people with primarily European and East Asian ancestry did not reveal statistically significant differences in cuticle thickness or number of cuticle layers when compared to each other. Testing European and East Asian hairs against African hairs did show statistically significant differences in the cuticle thickness and number of layers. Analyses of hair dimensions and the number and distribution of melanosomes within a hair cross section revealed variability among diverse populations, and demonstrated that hair traits are not easily separable into distinct categories by ancestry.

An inter-laboratory test was designed to assess the accuracy and reliability of microscopical comparisons of human hair and ancestry determinations for hairs that share a mitochondrial haplogroup. Practicing forensic scientists in accredited US forensic laboratories were recruited to take these tests. Included in the tests were samples from identical twins, hairs that shared similar macroscopic appearance, ancestry, and length. Conclusions from microscopical comparisons of these hair samples were found to be accurate in 85% of the comparisons while ancestry classifications were more variable. Hair length was shown to impact accuracy, with shorter lengths having higher levels of false inclusion/exclusions. Further research is needed on which traits are reliable indicators of ancestry and the extent that ancestry classifications are useful or misleading.

Overall, we found that variation in hair traits exists within and among human populations in hair color, cross-sectional shape and diameter, and in melanosome size and distribution. These findings and the variability in ancestry classifications based on these traits by practicing forensic scientists offer support for eliminating the use of categorical racial typologies. General trait patterns were observed among broadly defined population groups that may be useful for future research that focuses on the effects of genetic admixture to assess traits associated with ancestry in human hair. This research helps to improve the current understanding of human variation in hair form and serves as a foundation for future studies on the genetic and developmental bases of variation in hair form and microstructure related to genetic ancestry.

Last Modified: 04/30/2019
Modified by: Nina G Jablonski