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In Canon law of the Catholic Church, affinity is a relationship arising from the sexual intercourse of a man and a woman, sufficient for the generation of children, whereby the man becomes related to the woman's blood-relatives and the woman to the man's.
If this intercourse is between husband and wife, this relationship extends to the fourth degree of consanguinity, and the degree of affinity coincides with that of blood relationship.
Even if the intercourse were the result of force or committed in ignorance, e.g. in intoxication, the juridical effect would follow.
Impediment of marriage
If the intercourse is licit, it is a diriment impediment of marriage in the collateral line of the fourth degree, as also in the direct line. If the intercourse is illicit or out of marriage, the impediment today is limited to the second degree.
The Council of Trent makes no distinction with regard to the extent in either line. Though the Church has no jurisdiction over the unbaptized, yet it considers an affinity arising before baptism as a diriment impediment.
The regulations of the Mosaic law, based on considerations of relationship, are contained in Leviticus 18 and 20. It gives an exhaustive list of prohibitions; he not only gives examples of degrees of relationship, but God specifies the prohibitions which are strictly parallel to each other, e.g. son's daughter and daughter's daughter, wife's son's daughter and wife's daughter's daughter, whereas had he wished to exhibit the prohibited degree, one of these instances would have been sufficient. He prohibits marriage to a brother's widow, but not to a deceased wife's sister. Yet he requires a brother to marry his brother's widow in case the latter died without issue; and he cautions the man not to hold intercourse with his wife's sister while the wife is living.
The Roman law considered the intercourse of marriage to be a bar to marriage only with the kindred in the direct line.
Early Christian law
The Christian emperors extended it to the first degree of collateral affinity. The ecclesiastical law extended the juridical effect also to illicit intercourse. In the Council of Elvira (c. 300), the only recognized prohibition is the marriage of a widower with his deceased wife's sister. The prohibition became slowly more extensive until, in 1059, the eleventh canon of the Council of Rome recognizes the impediment of affinity as well as of consanguinity to extend to the seventh degree. This may have arisen from the need of mingling the various barbarian races through marriage, an end that was effected by the extension of prohibitions of marriage between persons related, though it is also clear that the Church used the ability to grant dispensations from the rule as a means of gaining both power and money from the nobility, as most royal marriages, given the limited circle of families involved, breached these limits. Innocent III in the Fourth Council of the Lateran (1215) limited both affinity and consanguinity to the fourth degree. The Council of Trent (Sess. XXIV, c. iv, De Ref.) limited the juridical effect of the extra-matrimonial intercourse to the second degree of affinity.
The motive for the impediment of affinity is akin to, though not as strong as, that of consanguinity; there arises from the partners' carnal intercourse a nearness and natural intimacy with the blood-relatives of the other side. The degrees of affinity are determined by the same rule as the degree of blood-relationship.
Before the Fourth Council of Lateran two other kinds of affinity were recognized as an impediment to marriage. If a man then married a widow, those who were akin to her by the previous marriage were also akin to the present husband. Moreover, if the first husband of the widow had been a widower, the blood relatives of his first wife were akin to the first husband, were also akin to the new wife, and to the last husband.
We give an example: Titius contracted and consummated marriage with Bertha. The blood-relatives of Bertha were akin to Titius. Bertha dies. Titius contracts and consummates marriage with Sarah. The blood-relatives of Bertha, akin to Titius by the first kind, became akin to Sarah by the second kind of affinity. Titius dies and Sarah contracts and consummates marriage with Robert. The blood-relatives of Bertha, akin by second kind to Sarah, become akin by the third kind of affinity to Robert.
Affinity no longer begets affinity
Affinity also, in the ancient law, arose between the children of a woman from a deceased husband and the children of her husband from a previously deceased wife. Hence a father and a son could not marry a mother and a daughter. Affinity begot affinity. But the Fourth Council of Lateran took away all but the first kind of affinity; hence the axiom that "affinity does not beget affinity".
There was some discussion in the eighteenth century as to whether a stepfather could marry the widow of his deceased stepson; but it was authoritatively decided, as Benedict XIV states (De Syn. Dioec., IX, xii) that there was no impediment to their marriage, it having been done away with by the Fourth Council of Lateran.
The impediment to marriage from affinity arises from ecclesiastical law. This is clearly recognized today by theologians with regard to collateral affinity. The Church grants dispensation in all the degrees of this affinity. In regard to affinity in the direct line, there was a serious discussion whether in the first degree it arose from a natural, Divine, or ecclesiastical law; by what law was a stepfather forbidden to marry his stepdaughter? The Church refrains from granting the dispensation, but does not disclaim the right to do so. Indeed, a decree of the Holy Office (February 20, 1888) implies that this affinity arises from ecclesiastical law: "The Holy Father permits bishops to dispense from all public impediments diriment of marriage derived from the ecclesiastical law, except from the order of the priesthood, and affinity, in the direct line, arising from lawful intercourse." Craisson states (Man. Jur. Canon, Lib. II, De affin., n. 4285) that "Collator Andegavensis" quotes (394) Sanchez and Pontius as asserting that "the Pope dispenses converted infidels married within this first degree of affinity, if they had contracted marriage in accord with the law of their country." This supposes that this affinity in the first degree of the direct line is not an impediment of the natural or Divine law. An additional argument may be drawn from the dispensation which the Church grants in this case where there has been occult unlawful intercourse. Any repugnance of nature would hold then, as where the intercourse proceeded from marriage.
If a married person should have intercourse with the marriage-partner's blood-relative of the second degree, in the direct or collateral line, a penalty is placed upon the one so sinning of forfeiting the right to ask for marital intercourse from the marriage-partner, though the innocent party does not forfeit the right to claim it. If the wrong had been done through fear, the common teaching is that the penalty is not incurred, and this is also probably so if done without knowledge of the penalty. If incurred, a dispensation from the penalty may be obtained from the bishop. The affinity would become more complicated, and add new bars to marriage, if the person had intercourse with several persons of varying degrees of affinity. By the Roman law, the affinity ceased at the death of the one from whom it originated. Thus when a remarried father died, his second wife was no longer akin to the children of his former wife. By canon law a marriage not consummated does not beget affinity. By a marriage null through a diriment impediment, the affinity probably does not extend beyond the second degree. By the French code the affinity in the direct line, and in the first degree of the collateral line, is a bar to marriage, though the privilege was given to the king to dispense in the second case.
Until the passage of the Deceased Wife's Sister's Marriage Act 1907, British marriage law forbade the marriage of a man with his deceased wife's sister, and a marriage of this kind performed in the colonies of the British Empire, where it might be allowed, was not held as valid in Great Britain. In the session of the British Parliament in 1906, a strong effort was made to enact a law to recognize as valid, in Great Britain, such a marriage, if the colonial law recognized its validity where contracted.
In Virginia this marriage is null, but it is generally recognized in the other States of the Union.
The Greek Church adheres to the law as laid down in Leviticus, xviii, 8, 14, 16, 18; xx, 11, 12, 14, 19, 21. Yet the Greek patriarchs and bishops grant dispensations from some of the affinities therein mentioned. Nestorians allow affinity to beget affinity very extensively. Armenians extend the affinity to the fourth degree. The United Orientals approach the Catholic regulations.