The proposal by David Haig that competition between the sexes occurred during the evolution of placental mammals. This resulted in a compromise between the robustness of the offspring during their period of uterine growth and the health of the mother. In the sperm, genes that increase the cost of the offspring are switched on. Such genes are active in the placenta and cause the embryo to grow rapidly and use up the mother's resources. In the egg, genes that decrease the cost of the offspring to the mother are activated. Such genes keep the unlimited growth of the embryo in check, so the mother survives the pregnancy. The genes occur in antagonistic pairs, and parental imprinting (q.v.) controls their functioning. See Chronology, 1992, Haig.