Parallel and cross cousins

In many "classificatory" systems of kinship terminology, relatives far beyond genealogical first cousins are referred to using the terms for parallel and cross-cousins.

The remaining types of kinship terminology (the "Hawaiian", "Eskimo" and "Sudanese") do not group parallel cousins together in opposition to cross-cousins.

John Maynard Smith, in The Evolution of Sex (1978)[1] notes that Richard D. Alexander suggested that uncertainty regarding paternity may help account for the intermarriage taboo on parallel, but not on cross-cousins.

Likewise, mothers who are also sisters may overtly or covertly share sexual access to the husband of one or the other, raising the possibility that apparent parallel cousins are in fact half-siblings, sired by the same father.

Andrey Korotayev claimed that Islamization was a strong and significant predictor of parallel cousin (father's brother's daughter – FBD) marriage.