Affinity

The relationship that a person has to the blood relatives of a spouse by virtue of the marriage.

The doctrine of affinity developed from a Maxim of Canon Law that a Husband and Wife were made one by their marriage. There are three types of affinity. Direct affinity exists between the husband and his wife's relations by blood, or between the wife and the husband's relations by blood. Secondary affinity is between a spouse and the other spouse's relatives by marriage. Collateral affinity exists between a spouse and the relatives of the other spouse's relatives. The determination of affinity is important in various legal matters, such as deciding whether to prosecute a person for Incest or whether to disqualify a juror for bias.

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AFFINITY. A connexion formed by marriage, which places the husband in the same degree of nominal propinquity to the relations of the wife, as that in which she herself stands towards them, and gives to the wife the same reciprocal connexion with the relations of the husband. It is used in contradistinction to consanguinity. (q.v.) It is no real kindred.
     2. Affinity or alliance is very different from kindred. Kindred are relations. by blood; affinity is the tie which exists between one of the spouses with the kindred of the other; thus, the relations, of my wife, her brothers, her sisters, her uncles, are allied to me by affinity, and my brothers, sisters, &c., are allied in the same way to my wife. But my brother and the sister of my wife are not allied by the ties of affinity: This will appear by the following paradigms

          My wife's father             ---|

          |                               |

          |                               |

     -----------------|                   |

     |                |                   |-- are all allied to me.
Ego ----- My Wife     My wife's sister ---|

                |                         |

                My wife's niece        ---|

                My wife's father       ---|

      My Father     |                     |              |My brother

          |         |                     |              |and my wife's

          |         |                     |              |sister are

    |---------------|          |----------|              |not allied

    |               |          |          |              |to each other
My brother          Ego ---- My wife,  My wife's sister  |

     3. A person cannot, by legal succession, receive an inheritance from a relation by affinity; neither does it extend to the nearest relations of husband and wife, so as to create a mutual relation between them. The degrees of affinity are computed in the same way as those of consanguinity. See Pothier, Traite du Mariage, part 3, ch. 3, art. 2, and see 5 M. R. 296; Inst. 1, 10, 6; Dig. 38, 10, 4, 3; 1 Phillim. R. 210; S. C. 1 Eng. Eccl. R. 72; article Marriage.