Cousin marriage is a form of consanguinity (marriages among couples who are related as second cousins or closer). As of 2003, an average of 45% of married couples were related in the Arab world. While consanguinity is not unique to the Arab or Islamic world, Arab countries have had "some of the highest rates of consanguineous marriages in the world". The bint 'amm marriage, or marriage with one's father's brother's daughter (bint al-'amm) is especially common, especially in tribal and traditional Muslim communities, where men and women seldom meet potential spouses outside the extended family.
Anthropologists have debated the significance of the practice; some view it as the defining feature of the Middle Eastern kinship system while others note that overall rates of cousin marriage have varied sharply between different Middle Eastern communities. In pre-modern times rates of cousin marriage were seldom recorded. In recent times, geneticists have warned that the tradition of cousin marriage over centuries has led to recessive genetic disorders, due to inbreeding.
History
Pre-Islamic
Prior to the origins of Islam cousin marriage was an acceptable practice in the Middle East according to writings in the Christian Bible. Abraham sent his servant back to his brother to get a wife for his son Isaac. Isaac eventually married his second cousin Rebekah (daughter of Bethuel the son of Nahor, Abraham's brother.)
The Persian king Ardashir I of the Sasanian Empire advised his lawyers, secretaries, officers, and husbandmen to "marry near relatives for the sympathy of kinship is kept alive thereby." The same motivation is given in ancient Arabic sources referring to the practice of marriage between paternal cousins prevalent in pre-Islamic Arabia. The Kitab al-Aghani similarly features the story of Qays ibn Dharih, who was not allowed by his father to marry a beautiful maiden from another tribe because, in the words of the father felt that as a rich and wealthy man he did not want his son to take the side of a stranger. There is the related consideration that a man who grows up with a cousin in the intimate setting of one extended family knows her and so may develop his own liking or love for her. There is also the benefit of knowing the qualities of the spouse: a Syrian proverb reads, "Ill luck which you know is better than good luck with which you get acquainted." Keeping property in the family is a final reason for cousin marriage. One of the earliest examples of this is the five daughters of Zelophehad from ancient Israel () who upon inheriting from their father all married their father's brother's sons.
Relation with spread of Islam
A 2000 study (by Andrey Korotayev) found that parallel-cousin (Father's Brother's Daughter – FBD) marriage is likely to be common in areas that were part of the eighth-century Umayyad Caliphate and remained in the Islamic world, i.e. North Africa and Middle East. Korotayev argues that while there is some functional connection between Islam and FBD marriage, the permission to marry a FBD does not appear to be sufficient to persuade people to actually marry FBD, even if the marriage brings with it economic advantages. According to Korotayev, a systematic acceptance and practice of parallel-cousin marriage took place when Islamized non-Arab groups adopted Arab norms and practices even if they had no direct connection with Islam to raise their social standing.
Religious aspects
In the holy book of Islam, the Quran, Sura An-Nisa (Q.4:22-5) gives a fairly detailed list of what sort of marriages are prohibited in Islam, (including "... your fathers' sisters, and your mothers' sisters, and brother's daughters, and sister's daughters, and your foster-mothers ...") but does not include first cousins, and ends by saying: "Lawful to you are all beyond these".
Pious Muslims look to the life of the Islamic Prophet Muhammad and early Muslims as examples to be followed, and "several ... members of the Prophet's family and inner circle were married to their cousins." One of Muhammad's wives – Zaynab bint Jahsh was the daughter of Muhammad's aunt. However, Muhammad didn't have children with Zaynab, his children being all children of him and Khadija bint Khuwaylid, except one with Maria al-Qibtiyya. Ali, cousin of Prophet Muhammad and the fourth Rashidun caliph, was married to the Prophet's daughter Fatimah.