Consanguinity
Christianity

Under Roman civil law, which the early canon law of the Catholic Church followed, couples were forbidden to marry if they were within four degrees of consanguinity. Around the ninth century the church raised the number of prohibited degrees to seven and changed the method by which they were calculated; instead of the former Roman practice of counting each generational link up to the common ancestor and then down again to the proposed spouse, the new method computed consanguinity only by counting back the number of generations to the common ancestor. Intermarriage was now prohibited to anyone more closely related than seventh cousins, which meant that in particular the nobility struggled to find partners to marry, the pool of non-related prospective spouses having become substantially smaller. They had to either defy the church's position or look elsewhere for eligible marriage candidates. In the Roman Catholic Church, unknowingly marrying a closely consanguineous blood relative was grounds for a declaration of nullity, but during the eleventh and twelfth centuries dispensations were granted with increasing frequency due to the thousands of persons encompassed in the prohibition at seven degrees and the hardships this posed for finding potential spouses.

In 1215, the Fourth Lateran Council made what they believed was a necessary change to canon law reducing the number of prohibited degrees of consanguinity from seven back to four, but retaining the later method of calculating degrees. After 1215, the general rule was that fourth cousins could marry without dispensation, greatly reducing the need for dispensations. In fourteenth century England, for example, papal dispensations for annulments due to consanguinity (and affinity) were relatively few.

The ban on marriage to minor degrees of relationship imposed by the Roman Catholic Church was met with heavy criticism in the Croatian society in the 11th century, which led to a schism in the Croatian church.

Among the Christian Habesha highlanders of Ethiopia and Eritrea (the predominantly orthodox Christian Amhara and Tigray - Tigrinya), it is a tradition to be able to recount one's paternal ancestors at least seven generations away starting from early childhood, because "those with a common patrilineal ancestor less than seven generations away are considered 'brother and sister' and may not marry." The rule is less strict on the mother's side, where the limit is about four generations back, but still determined patrilinearly. This rule does not apply to Muslims or other ethnic groups.
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