The data given on the cusp size of maxillary first permanent and second primary molars support the following conclusions. The qualitative literature on cusp size hierarchy requires revision because the quantitative data suggest that the mesiolingual cusp is largest, the distobuccal cusp second largest, the mesiobuccal cusp third largest, and the distolingual cusp smallest. Sexual dimorphism is apparent only when reflected as a collective measure of total occlusal area, a summation of cusp and ridge sizes. Antimere variations are random for individual cusps and collective measures of total occlusal areas mask observable right-side, left-side cuspal variations. Similarly, cross-twin and co-twin analyses of cusp size heritability indicate a low level of hereditary variability.