Marriages and sexual relationships between first cousins are stigmatized as incest in some cultures, but tolerated in much of the world. Currently, 24 US states prohibit marriages between first cousins, and another seven permit them only under special circumstances. This is notably true in the culture of Korea. In South Korea, before , anyone with the same last name and clan were prohibited from marriage. In light of this law being held unconstitutional, South Korea now only prohibits up to third cousins see Article of the Korean Civil Code.
Hmong culture prohibits the marriage of anyone with the same last name — to do so would result in being shunned by the entire community, and they are usually stripped of their last name[ citation needed ]. Some Hindu communities in India prohibit cousin marriages.
In a review of 48 studies on the children parented by cousins, the rate of birth defects was twice that of non-related couples: For example, the question of the legality and morality of a widower who wished to marry his deceased wife's sister was the subject of long and fierce debate in the United Kingdom in the 19th century, involving, among others, Matthew Boulton   and Charles La Trobe.
The marriages were entered into in Scotland and Switzerland respectively, where they were legal. In medieval Europe, standing as a godparent to a child also created a bond of affinity.
The Hebrew Bible forbids a man from marrying his brother's widow with the exception that, if his brother died childless, the man is instead required to marry his brother's widow so as to "raise up seed to him" per Deuteronomy In Islamic law, marriage among close blood relations like parents, stepparent, parents in-law, siblings, stepsiblings, the children of siblings, aunts and uncles is prohibited, while first or second cousins may marry. Marrying the widow of a brother, or the sister of deceased or divorced wife is also allowed.
Inbreeding Offspring of biologically related parents are subject to the possible impact of inbreeding. Such offspring have a higher possibility of congenital birth defects see Coefficient of relationship because it increases the proportion of zygotes that are homozygous for deleterious recessive alleles that produce such disorders  see Inbreeding depression.
Because most such alleles are rare in populations, it is unlikely that two unrelated marriage partners will both be heterozygous carriers. However, because close relatives share a large fraction of their alleles , the probability that any such rare deleterious allele present in the common ancestor will be inherited from both related parents is increased dramatically with respect to non-inbred couples. Contrary to common belief, inbreeding does not in itself alter allele frequencies, but rather increases the relative proportion of homozygotes to heterozygotes.
This has two contrary effects. In the long run, however, because of this increased exposure of deleterious recessive alleles to natural selection , their frequency decreases more rapidly in inbred population, leading to a "healthier" population with fewer deleterious recessive alleles.
The closer two persons are related, the higher the zygosity, and thus the more severe the biological costs of inbreeding. This fact likely explains why inbreeding between close relatives, such as siblings, is less common than inbreeding between cousins.
Thus, similar immune systems may be more vulnerable to infectious diseases see Major histocompatibility complex and sexual selection. Animal sexual behavior Many mammal species, including humanity's closest primate relatives, tend to avoid mating with close relatives, especially if there are alternative partners available.
Insects[ edit ] Common fruit fly females prefer to mate with their own brothers over unrelated males. North Carolina State University found that bedbugs in contrast to most other insects tolerate incest and are able to genetically withstand the effects of inbreeding quite well. Laws regarding incest Laws regarding sexual activity between 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.
Prohibition of incest laws may extend to restrictions on marriage rights, which also vary between jurisdictions. Most jurisdictions prohibit parent-child and sibling marriages, while others also prohibit first-cousin and uncle-niece and aunt-nephew marriages.
In most places, incest is illegal, regardless of the ages of the two partners. In other countries, incestuous relationships between consenting adults with the age varying by location are permitted, including in the Netherlands , France , Slovenia and Spain. Sweden is the only country that allows marriage between half-siblings and they must seek government counseling before marriage.
In some legal systems, the fact of a perpetrator being a close relative to the victim constitutes an aggravating circumstance in the case of sexual crimes such as rape and sexual conduct with a minor — this is the case in Romania. Parents and children cf. Relationships between these are particularly singled out for a curse in Deuteronomy 27 , and they are of the only two kinds incestuous relationships that are among the particularly-singled-out relationships—with the other particularly-singled-out relationships being ones of non-incestuous family betrayal cf.
Let them be married to whom they think best; only into the family of the tribe of their father shall they be married. So shall no inheritance of the children of Israel remove from tribe to tribe; for the children of Israel shall cleave every one to the inheritance of the tribe of his fathers. And every daughter, that possesseth an inheritance in any tribe of the children of Israel, shall be wife unto one of the family of the tribe of her father, that the children of Israel may possess every man the inheritance of his fathers.
So shall no inheritance remove from one tribe to another tribe; for the tribes of the children of Israel shall cleave each one to its own inheritance. For Mahlah, Tirzah, and Hoglah, and Milcah, and Noah, the daughters of Zelophehad, were married unto their father's brothers' sons.
Incestuous relationships are considered so severe among chillulim HaShem , acts which bring shame to the name of God, as to be, along with the other forbidden relationships that are mentioned in Leviticus 18, punishable by death as specified in Leviticus In the 4th century BCE, the Soferim scribes declared that there were relationships within which marriage constituted incest, in addition to those mentioned by the Torah.
These additional relationships were termed seconds Hebrew: The Fourth Lateran Council held in attempted to codify that marriage was forbidden up to and including third cousins, though permissible beyond this for fourth cousins, third cousins once removed, etc. Marrying one's godparent or deceased spouse's sibling is also prohibited, although marrying one's stepchild is not — e. Vyacheslav Ivanov exercised his right to marry his stepdaughter after her mother's his first wife's death. The Book of Common Prayer of the Anglican Communion allows marriages up to and including first cousins.
Mahram The Quran gives specific rules regarding incest, which prohibit a man from marrying or having sexual relationships with: Half relations are as sacred as are the full relations. In the post-Sasanian Zoroastrian literature, Xvaetvadatha is said to refer to marriages between cousins which have always been relatively common. Hindus believes there are both karmic and scientifically bad effects of incest and thus practice strict rules of both endogamy and exogamy , as well as same family tree gotra or bloodline Pravara.
Marriage within the gotra "swagotra" marriages are banned under the rule of exogamy in the traditional matrimonial system. Marriage with paternal cousins is strictly prohibited. Gotra is transferred down the male lineage while the Gotra of a female changes upon marriage.