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The chronological distribution of enamel hypoplasias on anterior permanent teeth is presented for individuals from the Hammon-Todd osteological collection. This skeletal collection derives from lower socioeconomic class individuals from the greater Cleveland area, bom between 1855 and 1913. T he age at development of enamel hypoplasias was estimated for 185 adults by measuring the location of these defects on anterior permanent tooth crowns and referring to the calcification chronology of Massler and co-workers.

Hypoplasias most frequently develop between 2.0 and 4-0 years and are rare before 1.5 and after 5.0 years. This chronological distribution of defects is similar to that found in prior studies of non-industrial and archaeological populations. However, it is vastly different from that reported by Sarnat and Schour, who found that two-thirds of defects in a Chicago series occurred before 12 months. The present data suggest that the Chicago hypoplasia chronology is neither a universal one, as is commonly reported in review articles and dental textbooks, nor reflective of all industrialized populations. Variations in chronologies may be due to a variety of factors including differences in sampling of individuals and teeth and differences in the temporal pattern of exposure to environmental conditions potentially causative of dental defects.