Cousin

A maternal cousin is a cousin that is related to the mother's side of the family, while a paternal cousin is a cousin that is related to the father's side of the family. This relationship is not necessarily reciprocal, as the maternal cousin of one person could be the paternal cousin of the other. In the example Basic family tree Julie is Joseph's maternal cousin and Joseph is Julie's paternal cousin.

Parallel and cross cousins on the other hand are reciprocal relationships. Parallel cousins are descended from same-sex siblings. Cousins that are related to same-sex siblings of their most recent common ancestor are parallel cousins.[9] A parallel first cousin relationship exist when both the subject and relative are maternal cousins, or both are paternal cousins.

Cross cousins are descendants from opposite-sex siblings. A cross first cousin relationship exist when the subject and the relative are maternal cousins and paternal cousin to each other. In the example Basic family tree Joseph and Julie are both cross cousins.

Double cousins are relatives that are cousins on their maternal side and cousins on their paternal side. This occurs when siblings, respectively, reproduce with different siblings from another family.[10] This may also be referred to as 'cousins on both sides.' The resulting children are related to each other through both of their parents and are thus doubly related. Double first cousins share both sets of grandparents.

Half cousins are descended from half siblings and would share 1 grandparent.[11] The children of two half siblings are first half cousins. If half siblings have children with another pair of half siblings, the resulting children would be double half first cousins.

While there is no agreed upon term, it is possible for cousins to share three grandparents if a pair of half siblings had children with a pair of full siblings.[12][13]

Step-cousins are either stepchildren of an individual's aunt or uncle, nieces and nephews of one's stepparent, or the children of one's parent's stepsibling.[14] A cousin-in-law is the cousin of a person's spouse or the spouse of a person's cousin.[15] In the Basic family tree example Joseph and Roger are both Cousin-in-laws. None of these relationships have consanguinity.

Couples that possess higher than normal consanguinity, shared identical DNA and genetic material, have an increased chance of sharing genes for recessive traits.[17] Therefore, children of high consanguinity parents may have an increased risk of genetic disorders, particularly if their parents both carry a harmful recessive mutation. See inbreeding for more information.

Scientists through multiple studies have established a substantial and consistent positive correlation between the kinship of couples and the number of children and grandchildren they have. The 2008 deCODE study results show that couples related at the level of third cousins have the greatest number of offspring, with the greatest reproductive success observed for couples related at the level of third and fourth cousins.[18] This study provides the most comprehensive answer yet to the longstanding question of how kinship affects human fertility. The study result was somewhat counterintuitive from an evolutionary perspective because closely related parents have a higher probability of having offspring homozygous for deleterious recessive mutations, although closer parental kinship can also decrease the likelihood of immunological incompatibility between mother and offspring, for example in rhesus factor blood type.[19] The study confirmed that the offspring of first and second cousins died younger and reproduced less.[20]

Cousin marriage is important in several anthropological theories which often differentiate between matriarchal and patriarchal parallel and cross cousins.

Currently about 10% and historically as high as 80% of all marriages are between first or second cousins.[21][22] Cousin marriages are often arranged.[21][22][23][24][25] Anthropologists believe it is used as a tool to strengthen the family, conserve its wealth, protect its cultural heritage, and retain the power structure of the family and its place in the community. Some groups encourage cousin marriage while others attach a strong social stigma to it. In some regions in the Middle East over half of all marriages are between first and second cousins. In some of the countries in this region this may exceed 70%.[26] Just outside this region it is often legal but infrequent. Many cultures have encouraged specifically cross-cousin marriages.[27] In other places it is legally prohibited and culturally equivalent to incest.[28][29] Supporters of cousin marriage often view the prohibition as discrimination,[30][31] while opponents cite the potential immorality.[32]