Affinity (law)

Laws, traditions and customs relating to affinity vary considerably, sometimes ceasing with the death of one of the marriage partners through whom affinity is traced, and sometimes with the divorce of the marriage partners. In addition to kinship by marriage, "affinity" can sometimes also include kinship by adoption or a step relationship.

Unlike blood relationships (consanguinity), which may have genetic consequences, affinity is essentially a social or moral construct, at times backed by legal consequences.

In law, affinity may be relevant in relation to prohibitions on incestuous sexual relations and in relation to whether particular couples are prohibited from marrying. Which relationships are prohibited vary from jurisdiction to jurisdiction, and have varied over time. In some countries, especially in the past, the prohibited relationships were based on religious laws. In some countries, the prohibition on sexual relations between persons in an affinity relationship may be expressed in terms of degrees of relationship. The degree of affinity is considered the same as the consanguineal level of the couple that was joined, so that, for example, the degree of affinity of a husband to his sister-in-law is two, the same as the wife would be to her sister on the basis of consanguinity. The degree to the wife’s parent or child is one, and to an aunt or niece it is three, and first cousin it is four. Though adoption and step relationships are cases of affinity, they are normally treated as consanguinity.

Terminology

In law, affinity relatives by marriage are known as affines.

More commonly, they are known as in-laws or family-in-law, with affinity being usually signified by adding "-in-law" to a degree of kinship. This is standard for the closest degrees of kinship, such as parent-in-law, child-in-law, sibling-in-law, etc., but is frequently omitted in the case of more extended relations. As uncle and aunt are frequently used to refer indifferently to unrelated friends of the family, the terms may be used without specifying whether the person is a cognate or affine. Similarly, the spouse of a cousin may not be called a relation at all or may be referenced as a "cousin by marriage". "By-marriage" can also be used with "uncle" or "aunt", e.g. Princess Léa of Belgium is an aunt by marriage of King Philippe of Belgium.