Avunculate marriage

An avunculate marriage is a marriage with a parent's sibling or with one's sibling's child—i.e., between an uncle or aunt and their niece or nephew. Such a marriage may occur between biological (consanguine) relatives or between persons related by marriage (affinity). In some countries, avunculate marriages are prohibited by law, while in others marriages between such biological relatives are both legal and common, though now far less common.

If the partners in an avunculate marriage are biologically related, they normally have the same genetic relationship as half-siblings, or a grandparent and grandchild—that is they share approximately 25% of their genetic material. (They are therefore more closely related than partners in a marriage between first cousins, in which on average the members share 12.5% of inherited genetic material, but less than that of a marriage between, for instance, cousin-siblings, in which the partners share 37.5% of their inherited genetic material.)

Avunculate marriage is permitted in Norway, Chile, Argentina, Australia, Canada, Finland, Malaysia, The Netherlands, Germany and Russia. In the United States it is permitted in some circumstances in two states. In New York a marriage between a woman and her mother's half-brother was upheld by the New York Court of Appeals. In Rhode Island there is an exception to the general prohibition against "kindred marriages" for Jewish marriages allowed by that religion. It is not permitted in New Zealand, or the United Kingdom.