Interpreting sex differences in enamel hypoplasia in human and non-human primates: Developmental, environmental, and cultural considerations

The purpose of this review is to provide a synoptic, critical evaluation of the evidence of, and potential etiological factors contributing to, sex di fferences in the expression of enamel hypoplasia (EH). Specifically, this r eview considers theoretical expectations and empirical evidence bearing on two central issues. The first of these is the impact of a theorized inheren t male vulnerability to physiological stress on sex differences in EH. The second issue is the potential contribution to sex differences in EH of intr insic differences in male and female enamel composition and development. To address this first issue, EH frequencies by sex are examined in samples su bject to a high degree of physiological stress. Based on the concept of inh erent male vulnerability (or female buffering), males in stressful environm ents would be expected to exhibit higher EH frequencies than females. This expectation is evaluated in light of cultural practices of sex-biased inves tment that mediate the relationship between environmental stress and EH exp ression. Defects forming prenatally afford an opportunity to study this rel ationship without the confounding effects of sex-biased postnatal investmen t. Data bearing on this issue derive from previously conducted studies of E H in permanent and deciduous teeth in both modern and archaeological sample s as well as from new data on Indian schoolchildren. To address the second issue, fundamental male-female enamel differences are evaluated for their p otential impact on EH expression. A large sex difference in the duration of canine crown formation in non-human primates suggests that male canines ma y have greater opportunity to record stress events than those of females. T his expectation is examined in great apes, whose canines often record multi ple episodes of stress and are sexually dimorphic in crown formation times. With respect to the first issue, in most studies, sex differences in EH pr evalence are statistically nonsignificant. However, when sex differences ar e significant, there is a slight trend for them to be greater in males than in females, suggesting a weak influence of greater male vulnerability. Cul tural practices of sex-biased investment in children appear to have greater impact on EH expression than does male vulnerability/female buffering. Wit h respect to the second issue, sex differences in the composition and devel opment of enamel were reviewed and determined to have limited or unknown im pact on EH expression. Of these factors, only the duration of crown formati on was expected to affect EH expression by sex within the great apes. The d ata support an association between higher defect counts in the canines of g reat ape males relative to those of females that may be the result of longe r crown formation times in the canines of great ape males. This review conc ludes with an assessment of the nature of the evidence currently available to examine these issues and suggests future avenues for research focused on elucidating them. (C) 1999 Wiley-Liss, Inc.