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| Family law |
|---|
| Family |
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Laws regarding incest (i.e. sexual activity between family members or close relatives) vary considerably between jurisdictions, and depend on the type of sexual activity and the nature of the family relationship of the parties involved, as well as the age and sex of the parties. Besides legal prohibitions, at least some forms of incest are also socially taboo or frowned upon in most cultures around the world.
Incest laws may involve restrictions on marriage, which also vary between jurisdictions. When incest involves an adult and a child (under the age of consent) it is considered to be a form of child sexual abuse.[1][2]
Degrees of relationship
Laws regarding incest are sometimes expressed in terms of degrees of relationship. The degree of relationship is calculated by adding the number of generations back to a closest common ancestor of each individual. Consanguinity (but not affinity) relationships may be summarized as follows:
| Degree of relationship |
Relationship | Average DNA shared % |
|---|---|---|
| Inbred strain | 99% | |
| 0 | identical twins; clones | 100%[3] |
| 1 | parent-offspring[4] | 50% |
| 1 | full siblings | 50% |
| 2 | 3/4-siblings or sibling-cousins | 37.5% |
| 2 | grandparent-grandchild | 25% |
| 2 | half-siblings | 25% |
| 3 | aunt/uncle-nephew/niece | 25% |
| 4 | double first cousins | 25% |
| 3 | great grandparent-great grandchild | 12.5% |
| 4 | first cousins | 12.5% |
| 6 | quadruple second cousins | 12.5% |
| 6 | triple second cousins | 9.38% |
| 4 | half-first cousins | 6.25% |
| 5 | first cousins once removed | 6.25% |
| 6 | double second cousins | 6.25% |
| 6 | second cousins | 3.13% |
| 8 | third cousins | 0.78% |
| 10 | fourth cousins | 0.20%[5] |
Most laws regarding prohibited degree of kinship concern relations of r = 25% or higher, while most permit unions of individuals with r = 12.5% or lower. In 24 states of the United States, cousin marriages are prohibited. Also, most laws make no provision for the rare case of marriage between double first cousins. Incest laws may also include prohibitions of unions between biologically unrelated individuals if there is a close legal relationship, such as adoption or step relations.
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