Selection intensities in mothers of twins and in mothers of singletons

Abstract

Opportunity for selection was assessed in a group of mothers with twins and was compared with estimates obtained in mothers who gave birth to singletons only from the same population. Both groups were matched on a number of confounding variables in the framework of a case/control methodology. Two time periods involving demographic changes were studied separately. After appropriate adjustment for the increased family size of mothers of twins, results show that selection intensities are greater in twin‐bearing mothers than in controls. The fertility component of the selection index is lower than that of mortality if childless women are taken into account. However, the index of opportunity for selection is rather low by current standards but, given the methodology used, should reflect genetic rather than environmental selection. Notwithstanding, it would be difficult to explain the recent change in the twinning rates in the population by the actual level of selection indices.