Written in the 18th century during the Qing Dynasty, my favorite classic Chinese novel is Hong Lou Meng (Dream of the Red Room, also translated The Stone Tale). The story revolves around Jia Baoyu, the male heir to a wealthy noble family, and his two cousins: Lin Daiyu, daughter of his paternal aunt, whom he loved the most but died at a young age; and Xue Baochai, daughter of his maternal aunt, whom he eventually married. Apparently, at that time, it was not a problem at all and sometimes even better to marry the child of your aunt or maternal uncle, as marriage would further strengthen the bond between the two families. Marriages are considered “related by blood” when couples are either second-degree cousins or more closely related. In the English upper and upper middle classes, the prevalence of marriage to first-degree cousins was high for much of the 19th century. It remained constant between 4% and 5% in the nineteenth century.  After World War I, however, there was a sudden change and marriage between cousins became very unusual. In the 1930s, only one in 6,000 marriages was associated with a first-degree cousin. A study of a London middle class conducted in the 1960s found that only one in 25,000 marriages was between first cousins.  A recent study of 70 countries found a statistically significant negative correlation between inbred kinship networks and democracy. The authors note that other factors, such as restricted genetic conditions, may also explain this relationship.  This follows a 2003 essay by Steve Sailer published for The American Conservative, in which he argued that high rates of marriage between cousins play an important role in discouraging political democracy. Sailer believes that because families who practice cousin marriages are more related than usual, their feelings of family loyalty are exceptionally intense and encourage nepotism. States have various laws regarding marriage between cousins and other close relatives, which include factors, including whether or not the parties to the marriage are half-cousins, double cousins, infertile cousins, over the age of 65, or whether it is a widespread tradition in an Indigenous culture or of ancestry, adoption status, in-laws, whether genetic counseling is necessary or not, and whether it is permissible to marry a first cousin once it has been removed. In the Far East, South Korea is particularly restrictive with marriage bans for third-degree cousins, with not all couples with the same surname and region of origin allowed to marry until 1997.  Marriage between first cousins was generally permitted during most of China`s dynastic era. However, there were exceptions. The main exception was marriages between patrilateral parallel cousins – the children of two male siblings. Such marriages are strictly prohibited. They were considered as such between siblings because they had the same surname. But marrying the child of his paternal aunt, maternal uncle or aunt was generally accepted in Chinese history. (Zhaoxiong Qin, Rethinking Cousin Marrige in Rural China, 40(4) ETHNOLOGY 347 (Fall 2001). In many Middle Eastern countries, marriage to the daughter of the father`s brother (FBD) is considered ideal, although this type does not always outperform other types.  One anthropologist, Ladislav Holý, argues that it is important to distinguish between the ideal of FBD marriage and marriage as it is actually practiced, which always includes other types of cousins and unrelated spouses.
Holý cites the Berti in Sudan, who consider FBD to be the closest relative of a man outside the forbidden zone. If there is more than one relationship between spouses, as is often the result of successive generations of cousins` marriages, only the patrilineal relationship is counted. Marriage within the lineage is preferred to marriage outside the line, even if no exact genealogical relationship is known. Out of 277 first marriages, only 84 took place between couples who could not prove a genealogical relationship between them. Among these, in 64, the spouses were of the same descendants. However, out of 85 marriages with a second or third wife, the spouses were of different descent in 60.   The Marri have a very limited set of incest prohibitions that apply only to direct relatives, sisters and aunts other than the wife of the mother`s brother.