Cousin

More generally, in the kinship system used in the English-speaking world, a cousin is a type of relationship in which relatives are two or more generations away from their most recent common ancestor.

[1][2] Various governmental entities have established systems for legal use that can precisely specify kinship with common ancestors any number of generations in the past; for example, in medicine and in law, a first cousin is a type of third-degree relative.

[2] In the English system the cousin relationship is further detailed by the concepts of degree and removal.

The degree is the number of generations subsequent to the common ancestor before a parent of one of the cousins is found.

[1] Two people can be removed but be around the same age due to differences in birth dates of parents, children, and other relevant ancestors.

[12][13] Step-cousins are either stepchildren of an individual's aunt or uncle, nieces and nephews of one's step-parent, or the children of one's parent's step-sibling.

Consanguinity decreases by half for every generation of separation from the most recent common ancestor, as there are two parents for each child.

For each additional removal of the cousin relationship, consanguinity is reduced by half, as the generations of separation increase by one.

For each additional degree of the cousin relationship, consanguinity is reduced by a quarter as the generations of separation increase by one on both sides.

If there are half-siblings on one side and full siblings on the other, they would have three-halves the consanguinity of ordinary first cousins.

Some groups encourage cousin marriage while others attach a strong social stigma to it.

[24][25] Supporters of cousin marriage often view the prohibition as discrimination,[26][27] while opponents claim potential immorality.

[28] Couples that are closely related have an increased chance of sharing genes, including mutations that occurred in their family tree.

[30] 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.

[31] This risks of inbreeding - particularly through common practice of cousin marriage - extend beyond the immediate descendants of those involved.

[32] Increased genetic literacy within a population can lead to a reduction in cousin marriage.