Affinity (canon law)

In Catholic canon law, affinity is an impediment to marriage of a couple due to the relationship which either party has as a result of a kinship relationship created by another marriage or as a result of extramarital intercourse. The relationships that give rise to the impediment have varied over time. Marriages and sexual relations between people in an affinity relationship are regarded as incestuous.

Today, the relevant principle within the Catholic Church is that "affinity does not beget affinity"—i.e., there is no affinity between one spouse's relatives and the other spouse's relatives. Canon 109 of the Code of Canon Law of the Catholic Church provides that affinity is an impediment to the marriage of a couple, and is a relationship which "arises from a valid marriage, even if not consummated, and exists between a man and the blood relatives of the woman and between the woman and the blood relatives of the man." Also, affinity "is reckoned in such a way that the blood relations of the man are related by affinity to the woman in the same line and the same degree, and vice versa."

In the Hebrew Bible, Leviticus 18:8–18 and 20:11–21 contain prohibitions of sexual relations between a couple in a consanguineous relationship, as well as a number of prohibitions of certain affinity relationships, e.g., Leviticus 18:8 (father’s wife), 18:14 (father’s brother's wife), 18:16 (brother’s wife), 18:18 (wife’s sister), 20:11–12 (father’s wife, daughter-in-law), 20:14 (woman and her mother), 20:19 (sister of either one's mother or father) and 20:21 (brother’s wife). Marriage to a brother's widow is prohibited, but not to a deceased wife's sister. However, as an exception, Deuteronomy 25:5–10 requires a brother to marry his brother's widow if the brother died without issue, in a so-called levirate marriage.

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