In order to accurately identify skulls as male or female, forensic anthropologists need to have a good understanding of how the characteristics of male and female skulls differ between populations. A new study from North Carolina State University shows that these differences can be significant, even between populations that are geographically close to one another.

The researchers looked at the skulls of 27 women and 28 men who died in Lisbon, Portugal, between 1880 and 1975. They also evaluated the skulls of 40 women and 39 men who died between 1895 and 1903 in the rural area of Coimbra, just over 120 miles north of Lisbon.

The researchers found significant variation between female skulls from Lisbon and those from Coimbra. “The differences were in the shape of the skull, not the size,” says Dr. Ann Ross, professor of anthropology at NC State and co-author of a paper describing the study. “This indicates that the variation is due to genetic differences, rather than differences of diet or nutrition.” The researchers found little difference between the male skulls.

Specifically, the researchers found that the female skulls from Lisbon exhibited greater intraorbital distance than the skulls of Coimbra females. In other words, the women from Lisbon had broader noses and eyes that were spaced further apart.

This difference in craniofacial characteristics may stem from an influx of immigrants into Lisbon, which is a port city, Ross says. However, it may also be a result of preferential mate selection – meaning Lisbon men were finding mates abroad, or were more attracted to women with those facial features.

“Finding this level of dimorphism between groups in such close proximity to each other highlights the importance of examining human variation if we hope to make informed assessments of skeletal remains,” Ross says. “That’s true whether you’re working in a biohistorical context or engaged in forensic analysis with law enforcement.”

The paper, “Craniofacial Sexual Dimorphism in Two Portuguese Skeletal Samples,” is forthcoming in the journal Anthropologie. The lead author is Ashley Humphries, who received her master’s degree at NC State and is now in the Ph.D. program at the University of South Florida. The research was supported by the National Institute of Justice.

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Note to Editors: The study abstract follows.

“Craniofacial Sexual Dimorphism in Two Portuguese Skeletal Samples”

Authors: Ashley L. Humphries, University of South Florida; Ann H. Ross, North Carolina State University

Published: forthcoming, Anthropologie

Abstract: One of the goals of anthropological research is to investigate biological human variation of past and present populations. Of particular interest is the study of sexual dimorphism, which can shed light on the human condition and aid in the identification of unidentified remains. When dealing with human skeletal remains, one of the four pillars of the anthropological protocol is the estimation of sex. Problems arise when applying sexing methods to different populations. Consequently, a skeletally robust female may appear to be “male”, particularly in light of cross-population comparisons. The purpose of this study is to evaluate sexual dimorphism of the craniofacial complex in two local Portuguese samples (Coimbra, F= 40, M=39; Lisbon, F=27, M=28). An index of sexual dimorphism or ISD was used to assess the level of sexual dimorphism within each sample (Lisbon ISD=3.71; Coimbra ISD=3.07). The Student’s t-test indicates that the degree of sexual dimorphism is not significantly different between Coimbra and Lisbon (P=0.31). However, Mahalanobis D2, which was computed to examine differences among the groups, indicates that Lisbon females differ significantly from the other samples and the pattern of sexual dimorphism coincides with the ISD results. The disparity of the Lisbon females may indicate the possible influence of immigration or genetic diversity left behind by the numerous population influxes on the Iberian Peninsula and warrants further study.