Abstract The relationship between cranial capacity and age of eruption of the mandibular first permanent molar is examined in modern anthropoid primates and used to infer age of eruption in fossil hominids. In developing and evaluating the predictive model, emphasis is placed on (i) distinguishing among cranial capacity, brain weight and brain volume as measures of brain size; (ii) selection of an appropriate population of modern Homo sapiens, and determination of both brain size and eruption age in this population; (iii) re-examination of the published literature on anthropoid dental eruption; (iv) application of a new modification in statistical procedures for prediction; and (v) attention to the implications of confidence intervals around the predicted point estimates. The limited data available on extant primate eruption ages restrict the inferences that can be made from the statistical relationship. It is not appropriate with the equation presently available to use cranial capacity to distinguish between a human-like and pongid-like M 1 eruption age for any individual hominid species. However, the data do allow for the conclusion that there is a relationship between cranial capacity and eruption age among anthropoid primates as a whole. While suggesting that australopithecines probably erupted permanent first molars at a pongid-like 3-4 years of age, rather than at the later ages characteristic of modern humans, the results differ in detail from age estimates by dental histology (perikymata), pattern of dental development, or previous work on the brain size-molar eruption relationship.