Sex differences in lingual marginal ridging on the human maxillary central incisor

Although incisor dimensions are on average larger in males, at least one incisor trait, lingual marginal ridging, is significantly more common in females. This is documented for the upper central incisor in a collation of 38 samples of living populations representing six racial groups. Caucasian, Asian, Polynesian, Melanesian, and American Indian females each possess significantly higher frequencies than their male counterparts. American Blacks do not show a sex difference. Sex differences in the duration and rate of incisor crown development do not explain this dimorphism. Possible underlying genetic causes are reviewed, and the hypothesis is proposed that relevant portions of both X chromosomes in females are contributing to greater trait expression.