Thus, a parent and child pair has a value of "r"=0.5 (sharing 50% of genes), siblings have a value of "r"=0.5, a parent's sibling has "r"=0.25 (25% of genes), and first cousins have "r"=0.125 (12.5% of genes). These are often expressed in terms of a percentage of shared DNA. As a working definition, unions contracted between persons biologically related as second cousins or closer ("r" ≥ 0.03125) are categorized as consanguineous. This arbitrary limit has been chosen because the genetic influence in marriages between couples related to a lesser degree would usually be expected to differ only slightly from that observed in the general population. Globally it is estimated that at least 8.5% of children have consanguineous parents. In clinical genetics, consanguinity is defined as a union between two individuals who are related as second cousins or closer, with the inbreeding coefficient (F) equal or higher than 0.0156.where (F) represents the proportion of genetic loci at which the child of a consanguineous couple might inherit identical gene copies from both parents. It is common to distinguish first-degree cousins, second-degree cousins, and often also third-degree cousins. Since comparatively few people can trace their full family tree for more than four generations, the identity of fourth-degree cousins often cannot be established. Also, at a genetic level, half-fourth cousins typically do not exhibit greater genetic similarity with one another than with any other individual from the same population. Reasons favoring consanguinous marriage have been listed as higher compatibility between husband and wife sharing same social relationships, couples stability, enforcing family solidarity, easier financial negotiations and others. Consanguinity is a deeply rooted phenomenon in 20% of the world population mostly the Middle East, West Asia and North Africa. Globally, the most common form of consanguineous union is between first cousins, in which the spouses share of their genes inherited from a common ancestor, and so their progeny are homozygous (or more correctly autozygous) at of all loci ("r" = 0.0625). Due to variation in geographical and ethnic background and the loci chosen to genotype there is some 2.4% variation expected. Historically, some European nobles cited a close degree of consanguinity when they required convenient grounds for divorce, especially in contexts where religious doctrine forbade the voluntary dissolution of an unhappy or childless marriage.