Unusually low sexual dimorphism of endocranial capacity in a Zulu cranial series.

The mean cranial capacities of 50 male and 50 female Zulu crania were found to be 1373.3 +/- 107.4 ml for males and 1251.2 +/- 101.1 ml for females (means +/- SD). The male value resembles that of other Negro groups, while the female value is somewhat higher than the value for Negro crania as a whole. The index of sexual dimorphism is 8.9%, which is low when compared with those of other Negroid series and other populations. The possible causes for this form of a low sexual dimorphism are as follows: A negative secular trend, with the assumption that the Zulu crania were larger than those of the reference populations of African Negroids before the start of the secular trend change. This would seem to be the most likely possibility, with some supporting evidence for both parts of the explanation. An absence of secular trend, with a demographic sampling aberration, in which large females and small males of the population are sampled. This possibility cannot be totally excluded. An absence of secular trend, with a genetic difference in sexual dimorphism for cranial capacity between the Zulu and the reference populations. While this possibility cannot be excluded, it would be the least preferable explanation.