Stabilizing selection in man: the effect of mother's age on birthweight variance

In order to study the effect of mother's age on birthweight variance, data were collected from Official Vital Statistics of the USA, Japan and Italy. Birthweight means and variances were calculated for all single births and for live births only. The results demonstrated that birthweight variance regularly increases in babies born to mothers of increasingly older age (with the exception of the youngest mothers). Moreover, the higher the variance in all births, the more the variance diminishes after late fetal selection. Therefore, a feedback phenomenon takes place in the stabilizing component of selection, i.e. the difference between the variance of all births and that of live births. As a consequence, at least part of the higher number of fetal losses experienced by older mothers can be attributed to the increased birthweight variance of their children.