The Qur’an is the primary source of law in Islam. Incest refers to those who are so closely related that they are forbidden to marry and have sexual intercourse as listed in the Qu’ran, Sura Four entitled The Women. The relevant verses are addressed to men and detail exactly which women are unlawful as spouses. The Qur’anic discussion opens by outlawing the pre-Islamic custom that allowed an heir to inherit his stepmother as a wife after his father’s death: “Do not marry women whom your fathers married, except what has already occurred in the past. It was obscene, abhorrent, and an evil practice” (4:22).
This prohibition is followed by a list of all the other women who are unlawful for a man to marry (Qur’an 4:23). First cousins are notably absent from this list. Although many in the West consider first-cousin marriage to be incestuous, it was an accepted pre-Islamic practice; because anyone not expressly prohibited is lawful as a marriage partner (4:24), it is still very popular among Arabs today.
The prohibited categories given in the Qur’an include close blood relatives—mothers, daughters, sisters, aunts, and nieces. These prohibitions are familiar from Western legal and religious traditions. Besides blood, mother’s milk also creates relationships that preclude marriage. Suckling establishes both a mother-child bond between a woman and the children she suckles and a sibling relationship between the children themselves that bars them from marrying each other. In-law and step-relations also may not marry.
The Qur’an forbids a man to marry his wife’s mother, or two sisters at the same time. The prohibition or permissibility of marrying a step-daughter depends on whether the marriage to her mother was consummated. Marriage to a step-daughter is prohibited if the marriage to her mother was consummated, but it is allowed if it was not. The Qur’an also distinguishes between the ex-wives of biological sons and those of adopted sons. Marriage to the former is explicitly prohibited in the Qur’an (4:23), whereas marriage to the latter is explicitly permitted (33:37).
The Qur’anic permission to marry the ex-wives of adopted sons is said to have been revealed in response to a scandal that rocked the early Muslim community when Muhammad married the former wife of his adopted son Zayd—such a marriage was considered incestuous according to pre-Islamic custom, which did not distinguish between an adopted and a biological son. The Qur’an breaks with pre-Islamic ideas of what constitutes incest. It permits marriage between a man and the ex-wife of his adopted son (33:37) because there is no biological connection, and prohibits marriage between a man and his father’s widow (4:22) because of the close biological connection of father and son. These Qur’anic prohibitions and permissions show that Islam defines incest according to an immediate biological relationship established by blood or mother’s milk, or a close in-law or step-relationship.
