Consanguinity (Lat. con, 'together,' and san-guis, 'blood'), the relationship which subsists between persons who are of the same blood. It is either direct, which is the relationship between ascendants and descendants, or collateral, between persons sprung from a common ancestor. In the direct line, a son is said to stand in the first degree to his father; a grandson, in the second degree to his grandfather; and so on.—Affinity (q.v.) is the relationship brought about by marriage between a husband and the blood-relations of his wife, or between a wife and the blood-relations of her husband.
Consanguinity and affinity have been in all parts of the world more or less looked on as impediments to marriage between the parties related. Among the ancient Persians and Egyptians, marriages were sometimes sanctioned between brother and sister, and even father and daughter; and in the book of Genesis we read of Abraham marrying his half-sister.
The Roman law prohibited marriage between ascendants and descendants, a prohibition extended to relations by adoption. In the collateral line, the prohibited degrees included brother and sister, and all cases where one party stood in loco parentis to the other, as uncle and niece. Marriage between cousins-german, at one time prohibited, was declared lawful by Arcadius and Honorius. The degrees prohibited in consanguinity were by Constantine also prohibited in affinity.
By the old canon law and early decretals, marriages were prohibited between persons as far removed as the seventh degree of consanguinity or affinity. The fourth council of Lateran, 1215 A.D., narrowed the prohibition from the seventh to the fourth degree; i.e. the grandchildren of cousins-german. A marriage between persons related in any of these ways was accounted incestuous, and the children bastards. The pope assumed the right of granting dispensations from impediments to marriage arising from consanguinity and affinity, a power which seems to have been first exercised in the 12th century.
In the countries which embraced the Reformation, a general relaxation took place in the prohibitions to marriage from consanguinity and affinity. In England, an act of 1547 allowed all persons to marry who were not prohibited by the Levitical law; and according to the interpretation put on this statute, the prohibitions included all relations in the direct line, brother and sister, and collaterals, when one party is brother or sister to the direct ascendant or descendant of the other; the degrees prohibited in consanguinity being equally prohibited in affinity. In Scotland, acts of 1567, professing to take the Levitical law as the standard, assimilated the prohibitions from consanguinity and affinity to those of England. In France, the Code Napoléon prohibits marriage between ascendants and descendants lawful or natural, and persons similarly connected by affinity; and in the collateral line between brothers and sisters lawful or natural, and persons similarly connected by affinity. Marriage between uncle and niece, and aunt and nephew, is also prohibited. In various countries of Europe, as Denmark, no prohibitions from affinity, except in the direct line, are recognised. In most of the United States of America, marriage is allowed between uncle and niece. See AFFINITY, DECEASED WIFE'S SISTER; and for exogamy and curious savage methods of counting relationship, MARRIAGE.
On the much-vexed question whether the marriage of relations tends to injure the constitution of their offspring, either by the intensification of hereditary taint or more directly, see The Marriage of Near Kin (2d ed. 1888), by A. H. Huth (who takes the negative view), and the bibliography there given of works on both sides of the question. See also BREED, CATTLE.