Search for secular trends in calvarial diameters, cranial base height, indices, and capacity in South African Negro crania.

Two hundred eighteen South African Negro male crania were arranged in 5-year birthdate cohorts from 1880-1884, 1890-1894, up to 1930-1934 inclusive and investigated for the presence of a secular trend in their dimensions. Maximum breadth, maximum length, basibregmatic height, cranial base height, and bi-auricular breadth (BAB) were measured, and cranial index (CI), cranial capacity (CC), and cranial base height index (CBHI) were determined. Cranial length exhibited a significant increase from 1890 to 1930. Cranial base height and CBHI exhibited statistically significant declines over the same period. The absence of secular trends in the other dimensions studied corresponds to previous analyses of the lengths of the femora and tibiae of the same sample. The secular trends in cranial length, cranial base height and its index, and the absence of trends in the other dimensions, are discussed in relation to an apparent decrease in the standard of living occurring in this population after 1900. It is unlikely that the absence of a positive secular trend denotes the attainment of a genetically determined ceiling to the physical size of the subjects represented by these skeletons. It is more likely that the environmental factors believed to produce a positive secular trend were absent in the years from which this sample was drawn and that signs of improvement in these factors, if any, would be so recent that evidence of their effect is not yet available from our cadaver-derived skeletons.