Consanguinity

Consanguinity is the quality of being descended from the same ancestor as another person.

The degree of relative consanguinity can be illustrated with a consanguinity table, in which each level of lineal consanguinity (i.e., generation) appears as a row, and individuals with a collaterally-consanguinious relationship share the same row.

The connotations of degree of consanguinity varies by context (e.g., Canon law, Roman law, et al.). Most cultures define a degree of consanguinity below which sexual interrelationships are regarded as incestuous (the "prohibited degree of kinship"). In the Roman Catholic Church, unwittingly marrying a closely-consanguinious blood relative is grounds for an annulment, but dispensations were granted, actually almost routinely (the Catholic Church's ban on marriage within the fourth degree of relationship (third cousins) lasted from 1550 to 1917; before that, the prohibition was to marriages between as much as seventh degree of kinship). Adoption may or may not be considered at law to create such a bond; in most Western societies, adoptive relationships are considered blood relationships for these purposes, but in others, including both Japan and ancient Rome, it was common for a couple with only daughters to adopt a son-in-law, making the marriage one between adoptive siblings.

Historically, some European nobles cited a close degree of consanguinity when they required convenient grounds for divorce, especially in contexts where religious doctrine forbade the voluntary dissolution of an unhappy or childless marriage. Conversely, the consanguinity law of succession requires the next monarch to be of the same blood of the previous one; allowing, for example, illegitimate children to inherit.

Rates of consanguinity in Europe and the Americas in recent times are low; significantly higher rates have been reported in Asia and the Middle East, reportedly as high as 78% of marriages are between first and second cousins in some tribes, and nationally as much as 50% in Iraq. (See the background to and global incidence external link below.)