" Nature is not infinite in its resources and not all organisms that are born can survive. When conditions are difficult, parents, especially mothers, must decide who is most likely to survive—including future potential children who may fare better when conditions are better—and sacrifice the rest. Daly and Wilson’s survey turned up these reasons for infanticide: disease, deformity, weakness, a twin when parents have only enough resources for one, an older sibling too close in age for resources to support both, hard economic times, no father to help raise the child, or because the infant was fathered by a different sexual partner. "

― Michael Shermer , The Moral Arc: How Science and Reason Lead Humanity toward Truth, Justice, and Freedom