Casti Connubii (On Christian Marriage) by Pope Pius XI addresses Catholic teaching on marriage and procreation. The pope declares that marriage is a divine institution and uniquely suited to raising children.The encyclical also stresses the indissolubility and stability of marriage. Birth control is forbidden, for anything which "deliberately frustrate[s]" the procreative aspect of marital sex is unnatural and immoral; married couples who for "natural reasons either of time or of certain defects" cannot have children are exempt from this condemnation. Likewise, abortion is prohibited as "direct killing of the innocent," whether it is done for the health of the mother or justified by eugenics. The state also has no authority to forbid the union of those "naturally fit for marriage" or to forcibly sterilize them. The remainder of the encyclical discusses challenges to marriage and ways to strengthen it, including aid to poor families.