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: 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.
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.
[65] Laws regarding incest in the United States vary widely among jurisdictions regarding both the definition of the offense and penalties for its commission.
[71] In Hong Kong, a special administrative region of China, it is illegal to have sexual intercourse with certain close relatives, even if they are consenting adults.
[72] The law does not cover sexual intercourse with more distant relatives, such as an aunt, uncle, niece, nephew and cousin.
It only addresses male-on-female and female-on-male sexual intercourse, and it appears that consensual same-sex incest is not illegal.
[75]) While it is unclear to which family members the incest law applies, a verdict from the High Court in Sabah and Sarawak in 2011 provided some indication about the sentencing guidelines.
It described incest as a "heinous crime" but that the degree of kinship between the parties dictates the "level of repulsion" which the court translates into a sentence imposed.
It said an uncle and his maternal niece committing incest is not on that same level and, if there was no violence involved, the length of the sentence should reflect it.
[87] Countries that allow consensual incest if both are over 18 include Belarus, Belgium, Estonia, Latvia, Lithuania, France, Russia, Spain, Ukraine, and Portugal.
Section 188 of the Czech Criminal code[93] prohibits incest between lineal ancestors and descendants and siblings.
[citation needed] In Denmark, incest is sex between lineal ancestors and descendants and between full siblings.
On 16 September 2011, the Constitutional Council repealed article 222-31-1 of the penal code, saying that if it was possible for the legislator to institute a particular penal qualification to designate incestuous sexual acts, they could not, without disregarding the principle of legality of offences and penalties, refrain from precisely designating the persons who must be considered, within the meaning of this qualification, as members of the family.
"[102] In Germany, illegal incest is defined as vaginal intercourse between lineal ancestors and descendants (parents, grandparents, great-grandparents and their children, grandchildren, great-grandchildren) and between full and half-siblings (due to this definition other sexual practices, including homosexual intercourse, are not punishable).
[105] In September 2014 the majority of the German Ethics Council recommended that the government abolish laws criminalizing consensual incest between adult siblings, while not broaching the question of to what extent criminal liability for incest between parents and children of legal age might be abrogated.
Paragraph 2 of Article 345 Penal Code also states that if the descending relative and the half or full siblings were under 18 years old, they might be cleared of any charge.
Also, Article 1357[111] of the Greek Civil Code prohibits the marriage of relatives of direct blood line in totality, and up the four degrees of consanguinity of the secondary blood line (for example you can marry the first cousin of your mother / father, but not your first cousin), also the marriage of relatives five or six degrees of consanguinity is under permission only.
It is illegal for a male to have sexual intercourse with his granddaughter, mother, daughter, sister, or half-sister; and for a female (over 16 years of age) with her grandfather, father, son, brother, or half-brother.
[118] Incest is not criminally prohibited in Latvia except as part of the general protection of adolescents from sexual abuse.
Thereby, the law explicitly foresees a criminal punishment for incest between a parent and a child or for persons of similar standing.
Marriage between blood relations of the third and fourth degree are possible, but require both partners to sign a declaration of consent (Dutch civil law book 1, articles 41 and 41a[122]).
[123] In Poland, incest is defined in Article 201 of the Penal Code[124] as sexual intercourse with an ancestor, descendant, guardian, ward, brother, or sister, and is punishable by imprisonment for no less than three months and no more than five years.
][129] However, under the Family Code of Russia, persons who are related lineally, siblings, half-siblings, and a stepparent and a stepchild may not marry.
Intercourse among siblings or other persons related by blood in direct line is punishable by up to three years' imprisonment.
However in each UK nation the relevant legislatures have enacted legislation prohibiting incest, regardless of age, or perceived consent.
[148] However, under federal law, sexual conduct between consenting adults (18 years of age or older) is legal,[149][150] which also applies to close family members.
[152] In Queensland, unlawful incest also includes sexual intercourse between an uncle or aunt with their niece or nephew, although here its application is curtailed by the effect of the federal Marriage Act 1961, as the Queensland Criminal Code states that the crime of incest does not apply to "persons who are lawfully married or entitled to be lawfully married".
This is particularly relevant where a certain form of sexual conduct between related persons falls outside of the legal definition of incest in a particular jurisdiction.
[154] Incest was the only sexual crime punishable by seven years or more imprisonment that was not subject to the country's "three-strikes" law before they were repealed in 2022.