Reproduction is an economically important complex composite trait in sheep. Genetic improvement of composite traits can occur by selection for individual components traits, some combination of individual component traits, or by direct selection for the composite trait. This review discusses the responses of selecting directly for litter weight weaned compared to selection responses for one of its component traits. Litter weight weaned is concluded to be a biological selection index determined by environmental factors under which it is selected for. Selection for litter weight weaned can result in a balanced biological composite trait with favorable responses in component traits such as fertility, number of lambs born, lamb survival, lactation, and lamb growth. It is concluded that selection to improve reproductive efficiency under most production and environmental systems would benefit from selection for a composite trait such as litter weight weaned rather than for a single component trait.