one side and full siblings on the other, they would have three-halves the consanguinity of ordinary first cousins. In a scenario where two monozygotic (identical) twins mate with another pair of monozygotic twins, the resulting double cousins would test as genetically similar as siblings. Reproduction Couples that are closely related have an increased chance of sharing genes, including mutations that occurred in their family tree. If the mutation is a recessive trait, it will not reveal itself unless both father and mother share it. Due to the risk that the trait is harmful, children of high-consanguinity parents have an increased risk of recessive genetic disorders. See inbreeding for more information. Closely related couples have more children. Couples related with consanguinity equivalent to that of third cousins have the greatest reproductive success. This seems counterintuitive as closely related parents have a higher probability of having offspring that are unfit, yet closer kinship can also decrease the likelihood of immunological incompatibility during pregnancy. Cousin marriage 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. Cousin marriages are often arranged. 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, more than half of all marriages are between first or second cousins (some of the countries in this region this may exceed 70%). Just outside this region, it is often legal but infrequent. Many cultures have encouraged specifically cross-cousin marriages. In other places, it is legally prohibited and culturally equivalent to incest. Supporters of cousin marriage often view the prohibition as discrimination, while opponents claim potential immorality and cite the increased rate of birth defects in children of cousin marriages. See also Collateral descendant Consanguinity Cousin marriage Family Sibling Second-degree relative References External links European kinship system Genealogy.com definition of various cousins Genealogy.com: What makes a cousin? Genetic Genealogy Family Kinship and descent de:Verwandtschaftsbeziehung#Cousin und Cousine