Skull Shape Study Cranial Shape Helps Identify Gender

The authors of the paper sought to demonstrate that one could differentiate between discrete populations of humans in a small geographic region by analyzing the shape and size of their skulls. They analyzed differences between two Spanish populations separated by time, and between these two populations and a Portuguese population, using both traditional and three-dimensional (3D) cranium measurements. They were able to demonstrate a quantifiable but moderate difference between the three populations, with the cranial measurements being able to distinguish the men of each population from the women fairly accurately.

Anthropologists have been trying to tease out the geographical origins of different human populations based on the differences in their cranial morphology— or skull size and shape. The purpose for this kind of analysis is to be able to identify individuals, especially those who either may be victims of violent crimes or have gone missing. The researchers hoped to achieve their goals by examining the craniums of two Spanish populations that lived during different centuries, and of a Portuguese population, to account for regional variation within the Iberian Peninsula. Additionally, they were hoping to illustrate that the physical differences between men and women, and hence the degree of cranio-facial differences between the sexes within each population, would help them to distinguish between the three populations.

* The researchers compared 129 skull samples from the Spanish regional 19th century Oloriz collection of cadavers, 93 crania from a local 16th century collection from Wamba, Spain, and 54 samples from a 19th century Bocage Museum, Lisbon collection.
* Traditional cranial measurements using calipers to map distances between distinct features in the skull were carried out.
* A modern 3D geometric morphology assessment tool was also employed to classify the samples.

* Using the Mahalanobis D2 statistical method to analyze results from the traditional measurements, they showed differences in skull morphology between the three populations, with the two 19th-century collections being more related. This methodology was only 52 to 76 percent accurate at identifying the regional source of the skull, but nearly 83 to 86 percent accurate at determining the sex of the specimen.
* Using an index to compute sexual dimorphism or distinctness, researchers demonstrated that the highest differences in cranial morphology existed between the men and women from Wamba. Additionally, they demonstrated a similar number of changes in morphology for both sexes going from the 16th to the 19th century.
* When the 3D geometric assessment tool was used, the ability to classify samples by sex and region correctly was inadequate.

Experiments to classify Iberian cranial samples based on region, using both traditional and modern approaches to studying cranial morphology did not yield good results. The Iberian Peninsula has witnessed a number of invasions and migrations over the centuries and this could account for a lot of variability in the population samples. There is therefore a need to structure such studies by taking into account other factors such as the biology or genetics of these local populations.

When both the traditional and modern 3D methods of assessing differences in human cranial structure were used to identify ancestry or region, and sex, the process was found to be problematic when the populations in question were closely related geographically. As men and women are different structurally, with men in general being larger than women, sexual distinctness in cranial structure could be teased out more easily. Additionally, the two Spanish populations that were separated temporally could be distinguished, perhaps due to size related changes in the more recent population from better diet and living conditions. However, the high degree of variation in the Iberian Peninsula populations, brought about by invasions and migrations, perhaps accounted for the difficulties faced by the research team in classifying the skull samples by region. The methodology to distinguish between local populations therefore needs to be further refined, perhaps with the incorporation of other non-morphological data in the analysis.

For More Information:
A Role for Sexual Dimorphism, Population Variation and Secular Change in Determining Differences between Populations in the Iberian Peninsula
Publication Journal: Forensic Science International, February 2011
By Ann H. Ross; Douglas H. Ubelaker; NC State University, Raleigh, North Carolina and Smithsonian Institution, Washington, DC

*FYI Living Lab Reports Are Summaries of the Original Research.

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