The constraints of racialization: How classification and valuation hinder scientific research on human variation

The Trotter collection: A review of Mildred Trotter's hair research and an update for studies of human variation

OBJECTIVES

Mildred Trotter was an anatomist and physical anthropologist whose studies on hair morphology, growth, somatic distribution, and trait relationships to age and ethnogeographic population were foundational to the field of microscopical hair analysis. The collection of human hair samples she assembled for her research has been an underutilized resource for studies on human hair variation. We applied updated methods and reviewed Trotter's original data to reassess the relationship hair traits have to diverse population labels.

METHODS

Hair form and pigmentation patterns were measured from a subset of the hair samples accumulated by Trotter and we compared our data to Trotter's original results. Variability in hair traits were tested within individuals, within populations, and among ethnogeographic groups.

RESULTS

Measured hair cross-section dimensions and melanosome density and distribution revealed substantial variability within individuals and ethnogeographic populations. Hair traits were found to not be distinctly separable by ancestry but instead showed continuous variation across human populations. Trotter's measurements were precise and the dataset she compiled remains valid, though the conclusions should be reviewed in light of our current understanding of human variation.

DISCUSSION

Our findings support moving away from categorical ancestry classifications and eliminating the use of outdated racial typologies in favor of more descriptive trait analysis. Detailed analysis of trait pattern distributions are presented that may be useful for future research on human variation. We point to the need for additional research on human variation and hair trait relationships with reference to known population affinity.

Investigating identification disparities in forensic anthropology casework

Forensic anthropology is shifting to reflect on the impact of its practices within the criminal justice context in important ways. Here, we contribute to this essential work by examining how decedent demographics as well as estimations of biological profile components are related to identification trends in forensic anthropology cases. The study uses data from more than 1,200 identified and unidentified forensic anthropology cases from three agencies (together representing a nation-wide sample). We found the following: i) multivariate analyses indicated that decedent sex, age, and race and/or ethnicity are not related to case identification rates in the pooled United States sample, ii) when identification rate differences do occur, they appear to be smaller effects, more agency-specific, and/or related to the context of a particular agency, iii) for the agency-specific sample with available data, there was no consistent evidence for a discrepancy in the duration of an identification investigation based on a decedent's sex, age, or race and/or ethnicity, iv) forensic anthropological estimations of sex, age, and ancestry can improve the odds of identification for decedents, although these are small effects, and v) reporting an ancestry estimation does not appear to impact decedent race representation among resolved unidentified person cases. Although previous studies have identified demographic discrepancies in other areas of the criminal justice system, the results presented here suggest that decedent demographic estimation practices by forensic anthropologists in general do not appear to be related to discrepancies in identification trends, but more research is needed to examine whether these findings hold. Contextual factors and practices specific to each investigative agency likely contribute to identification trends.

Deconstructing Eurocentrism in skin pigmentation research via the incorporation of diverse populations and theoretical perspectives

The evolution of skin pigmentation has been shaped by numerous biological and cultural shifts throughout human history. Vitamin D is considered a driver of depigmentation evolution in humans, given the deleterious health effects associated with vitamin D deficiency, which is often shaped by cultural factors. New advancements in genomics and epigenomics have opened the door to a deeper exploration of skin pigmentation evolution in both contemporary and ancient populations. Data from ancient Europeans has offered great context to the spread of depigmentation alleles via the evaluation of migration events and cultural shifts that occurred during the Neolithic. However, novel insights can further be gained via the inclusion of diverse ancient and contemporary populations. Here we present on how potential biases and limitations in skin pigmentation research can be overcome with the integration of interdisciplinary data that includes both cultural and biological elements, which have shaped the evolutionary history of skin pigmentation in humans.

The Eumelanin Human Skin Colour Scale: A Proof-of-Concept Study

BACKGROUND

At present there is no standard nomenclature for describing the diversity of human constitutive skin colour.

OBJECTIVES

To develop a standard nomenclature for describing human constitutive skin colour.

METHODS

Monthly focus group discussions by a multi-disciplinary group of specialists over a 7-month period. Topics covered were: 1) Limitations and unmet needs of current nomenclature(s) pertaining to human cutaneous diversity; 2) Practical considerations about the function and role of any proposed nomenclature pertaining to human cutaneous diversity; 3) Review of the cellular basis and current molecular genetic understanding of variation in human skin pigmentation; 4) In vivo methods to evaluate human skin pigmentation. In addition, a preliminary review of the published literature was undertaken to collate data on published skin reflectance measurements, notably melanin index (MI) values for well-referenced human populations.

RESULTS

Development of a 5-point scale to describe the full spectrum of human constitutive skin colour, termed The Eumelanin Human Skin Colour Scale. The nomenclature of the scale uses Eumelanin, the dominant chromophore of human skin, as a central descriptive word. The categories of the scale (nomenclature and MI values) are: Eumelanin Low (EML)- <25; Eumelanin Intermediate Low (EMIL)- 25-<50; Eumelanin Intermediate (EMI)- 50-<75; Eumelanin Intermediate High (EMIH)- 75-<100; Eumelanin High (EH)- ≥100.

CONCLUSIONS

The Eumelanin Human Skin Colour Scale enables the complete range of human constitutive skin colour to be described in an objective, equitable and understandable manner. In future, this scale can be used as the basis for developing other scales that address the specific functional aspects of human skin, such as response to ultraviolet radiation.

High-throughput phenotyping methods for quantifying hair fiber morphology

Quantifying the continuous variation in human scalp hair morphology is of interest to anthropologists, geneticists, dermatologists and forensic scientists, but existing methods for studying hair form are time-consuming and not widely used. Here, we present a high-throughput sample preparation protocol for the imaging of both longitudinal (curvature) and cross-sectional scalp hair morphology. Additionally, we describe and validate a new Python package designed to process longitudinal and cross-sectional hair images, segment them, and provide measurements of interest. Lastly, we apply our methods to an admixed African-European sample (n = 140), demonstrating the benefit of quantifying hair morphology over classification, and providing evidence that the relationship between cross-sectional morphology and curvature may be an artefact of population stratification rather than a causal link.