Empiric evidence indicates that the general distribution of the cephalic index is explicable in terms of climatic adaptation. Based on a sample of 339 populations, the magnitude of the index is statistically different between zones of predominantly dry heat, wet heat, wet cold and dry cold. There is an inverse relationship between the mean cephalic index and temperature. It is argued that the occupation of cold climates is one of the circumstances increasing the frequency of brachycephaly through time.
Significant associations of cranial shape for 82 ethnic groups and seven climate variables are described. Variation among current populations is partially attributed to cold adaptation throughout the Pleistocene. Application of data files tabulated by the authors is described for a number of problems. Temporal distribution of 115 specimens indicates a geometric trend (CI = 76.7-1.96 log time X 10(3) ). Cranial indices are summarized within alternative taxonomic models and between climatic ecotypes. Evidence supports the hypothesis of cold adaptation among "Classic" Neandertals. Limitations of the thermodynamic model are discussed. It is probable that a decrease of the cranial index occurs from the Middle to Upper Paleolithic. During the Holocene, the index increases under all climatic conditions.
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