Factors affecting the incidence of non-metrical skeletal variants.

Non-metrical variants of the human cranium have been studied in 186 London crania of known age, sex and date of birth. The incidence of several variants was different in the two sexes, and these results were compared with those of other workers from different parts of the world. Few variants persistently favoured one sex: the majority behaved inconsistently. Age dependency was only demonstrated for one variant, while year of birth, presence of rickets, and spina bifida occulta, showed negligible influence on variant incidence. 20% of vertebral columns examined included an anomalous vertebra, usually sacral spina bifida occulta. Although family studies were largely inconclusive, this investigation provides no reason to doubt the basic genetical control of these variants.