Gonial inversion: facial architecture, not sex

Abstract

Traditionally, gonial eversion has been used as an adult male sex indicator in the mandible. However, like a number of morphologic features, the sexing accuracy of this trait has never been quantified on a large, diverse sample. Moreover, it has been suggested that gonial eversion is associated with race rather than sex (ANGEL & KELLEY 1990). Thus, the purpose of this study is to systematically assess the development, population specificity, and sexing accuracy of gonial eversion on a large multiracial sample. The sample consisted of 609 known sex Africans, American whites and blacks, and Paleoamerindians from birth to over 90 years. Each mandible was assessed as having clearly expressed bilateral eversion, no eversion, or «indeterminate» in cases with unilateral eversion, or only «fringes» at the gonial base. Results indicated that gonial form averages only 45% sexing accuracy in adults (males, 35%; females, 58%). Further analyses revealed that several factors contributed to this very low level of discrimination. First, bilateral eversion is rare, occurring in only 23% of mandibles. Since gaonial eversion is present at birth and the same proportion of eversion was found in subadults (0-19 years) as in adults, this trait does not appear to be developmental. To make matters worse, a full third of individuals (of both sexes) are indeterminate. Females were consistently diagnosed better than males because nearly twice as many (44%) mandibles are bilaterally not everted than are everted. However, little more than half of females were correctly sexed by this trait. Differences were observed among races and populations. Chi squared tests revealed significant population differences between African and U.S. blacks, yet it is likely that there would have been others if the proportions held in larger samples. Several findings explain the ineffectiveness of gonial eversion as a sex predictor. Population differences in the rate of eversion and its presence at birth suggest a stronger association with hereditary factors, such as overall facial architecture, than sex. The strongest evidence against the misconception that eversion reflects male robusticity are the results for Paleoamerinds and African blacks. These were the most robust bones in the sample, but had the lowest rates of eversion. This is also supported by the fact that the least robust group (whites) had the highest level of eversion over all. These observations along with those of pathologic specimens indicated that eversion depends primarily on the relative strength of the muscles of mastication that attach there. Thus, eversion may be traced to the selective robusticity of the masseter whereas a more robust medial pterygoid inverts the gonial region. The fact that most mandibles are not everted reflects that the norm is either a tendency for the medial pterygoid to be somewhat stronger than the masseter, or that there is a relative balance between these muscles (no matter how well developed they are). In conclusion, at only 45% accuracy, gonial form is not a reliable sexing trait. Apart from the hereditary nature of this feature, bilateral eversion is uncommon and not limited to males, at least a third of individuals are indeterminate, and little more that half of females have no eversion.