Demonstrating the empirical effect of population specificity of anthropological standards in a contemporary Australian population

In a forensic context it is of paramount importance that investigators have assigned the correct skeletal sex (i.e. female or male) to a set of unknown remains. This is crucial when it comes to applying sex-specific predictive models for age, stature and ancestry, which are more accurate when sex specific. Thus, the accurate estimation of sex is inherently intertwined with the accuracy of biological analyses and other avenues of investigation performed thereafter.

In the present study, the application of cranial sex estimation methods derived using populations geographically and/or temporally removed from Australia saw a general reduction in classification accuracy (albeit to varying degrees), with nearly all cases having an unacceptably large associated sex bias value. These results indicate that the use of established sex estimation models in an Australian context would be ineffective and highly inaccurate. The reasons underpinning the ineffectiveness of these models relate to overall differences in the cranial morphology between the various populations. A comparison of the mean values of nine common cranial measurements across each of the five populations highlights such variances (Table 4). A common trend is that Australian females are on average larger than females from the USA, Japanese, South African and Greek populations. Assessment of the average measurement values identified that Australian females are more similar in size to the males from those populations. For example, the mean basion-nasion length (BNL) in the female Australian sample is 101.1 mm, which is consistent with the mean male values (101.0 to 102.5 mm) of the other populations.

Table 4 Mean values (in mm) of nine common cranial measurements taken on the five distinct populations

Similarly, the mean maximum cranial length (GOL) in the female Australian sample is 181.1 mm which is relatively similar and, for some populations, larger than the foreign males (179.0 mm—Japanese and 181.8 mm—Greek). The same pattern was also observed in minimum frontal breadth (MFB) and bifrontal breadth (BIB); see Table 4. Therefore, the application of foreign models using those measurements will result in most females being misclassified as males. This is clearly elucidated when one considers the data shown in Fig. 2: 25 of the 35 models notably misclassified Australian females as male, leading to very large sex bias values.

Morphological variation between the populations is also evident when considering the expression and magnitude of sexually dimorphic features; this can, to some degree, be discerned by examining the relative loadings of the coefficients in the discriminant equations. In examining function #2 of Iscan et al. [22], mastoid height contributed the most to the estimation of sex (i.e. had the highest loading), whereas in Swift et al. [38], which uses the same Australian population as the present study, mastoid height had a proportionately smaller loading and thus contributes less to the model (e.g. relatively lower dimorphism). A similar trend was also observed relative to basion-bregma height for function #2 of Iscan et al. [22], functions #1, #4, #6, #7, #8 and #9 of Giles and Elliot [15] and functions #3, #5 and #8 of Ogawa et al. [27]. For the aforementioned studies, basion-bregma height was ranked within the top three highest loaded (most dimorphic) cranial measurements for each population, contributing significantly toward the estimation of sex. Relative to Swift et al. [38], basion-bregma was not one of the most dimorphic measurements. What this demonstrates is that it is not only a gross size difference, but the variance in which measurements are most strongly weighted (e.g. dimorphic) in the multivariate models. To achieve accurate and reliable results, the latter needs to be optimised to suit the specific population of interest.

The underlying aetiology of cranial morphological variation is related to a combination of factors, including mechanical loading of craniofacial muscles, subsistence patterns [25] and climate [20]. Considering the loading of craniofacial muscles, Schlager and Rudell [34] investigated variation in the zygomatic region of the skull between a Chinese and German population. The investigation demonstrated that 9.7% of the overall variation of the sample was related to population, and further to this, population affinity could be reliably predicted at 97.9% accuracy. The authors hypothesised that the morphological variability of the zygomatic region was directly related to the variations in physical stress caused by mechanical loading of masticatory muscles and differences in the position of insertion points for the masseter and temporalis muscles. The lateral rim of the orbit was particularly affected, resulting in more pronounced development associated with increased muscle loading in the Chinese population.

The variation in the muscles of mastication is likely an evolutionary difference relative to subsistence, which persists in some contemporary populations. Noback and Harvati [25] investigated the effect that different subsistence methods had on cranial development in 15 discrete Homo sapiens populations. Their data suggest that individuals living on diets that comprise tougher harder foods (meat and fish) were associated with more robust, broader skulls, relative to the zygomatic and temporal regions and the alveolar processes. Those populations that survived on more agriculturally based diets, including higher amounts of grain and “processed” food, tended to have a relatively narrower craniofacial region. The theory that populations exhibiting increased loading of the masticatory muscles result in broader zygomatic and temporal regions of the skull, regardless of sex, is supported by several other studies, including Prado et al. [31], von Cramon-Taubadel [39], Noback and Harvati [26] and Paschetta et al. [29].

Climate is another factor that can explain the variations in cranial morphology exhibited between geographically disparate populations. Hubbe et al. [20] analysed craniometric data (33 measurements) in 7422 males from a total of 135 geographic populations, with the aim of exploring the impact of climate on cranial morphology. The results of this investigation indicated a statistically significant correlation between geographic location and cranial morphology, with different anatomical regions of the skull impacted disproportionately. Individuals from colder climates, such as Northern Europe, Northeast Asia and the extreme North of America, characteristically exhibited broader neurocrania. This was interpreted as being a necessary (selectively advantageous) morphological attribute that decreases the surface/volume ratio of the skull and brain, necessary for reducing heat loss through the skull. Individuals from the colder climates were also characterised by morphological changes to the viscerocranium. Those from Northern Europe experienced variations in facial projection, specifically increased nasal and frontal breadth, while those from Northeast Asia and the extreme North of America were characterised by increased nasal height, facial height and breadth. The authors postulated that the most likely cause of these adaptations relates to the need for reduced nasal indexes necessary for warming air during inhalation in cold climates [6].

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