In discussing consanguineal kinship in anthropology, a parallel cousin or ortho-cousin is a cousin from a parent's same-sex sibling, while a cross-cousin is from a parent's opposite-sex sibling. Thus, a parallel cousin is the child of the father's brother (paternal uncle's child) or of the mother's sister (maternal aunt's child), while a cross-cousin is the child of the mother's brother (maternal uncle's child) or of the father's sister (paternal aunt's child). Where there are unilineal descent groups in a society (i.e. matrilineal and/or patrilineal), one's parallel cousins on one or both sides will belong to one's own descent group, while cross-cousins will not (assuming descent group exogamy). Role The role of cross-cousins is especially important in some cultures. For example, marriage is promoted between them in the Iroquois system. Parallel cousins are occasionally the subject of promoted marriage, such as the preferential marriage of a male to his father's brother's daughter, common among some pastoral peoples. Such a marriage helps keep property within a lineage. On the other hand, parallel cousin unions in some cultures would fall under an incest taboo, since parallel cousins are part of the subject's unilineage whereas cross-cousins are not. Kinship terminologies In many "classificatory" systems of kinship terminology, relatives far beyond genealogical first cousins are referred to using the terms for parallel and cross-cousins. And in many societies, parallel cousins (but not cross-cousins) are also referred to by the same terms that are used for siblings. For instance, it is characteristic of the "Iroquois" system of kinship terminology, its variants the "Crow" and "Omaha", and most Australian Aboriginal systems, that a male parallel cousin is referred to as "brother", and a female parallel cousin is "sister". In an Iroquois type of terminology, if the terms used to refer to cross-cousins are assimilated to those for other relatives, it is generally in-laws (since marriage with cross-cousins is often preferentially favored), so that the terms for "male cross-cousin" and "brother-in-law" are the same, as are the terms for "female cross-cousin" and "sister-in-law". The remaining types of kinship terminology (the "Hawaiian", "Eskimo" and "Sudanese") do not group parallel cousins together in opposition to cross-cousins. Taboos John Maynard Smith, in The Evolution of Sex (1978) notes that Richard D. Alexander suggested that uncertainty regarding paternity may help account for the intermarriage taboo on parallel, but not on cross-cousins. Fathers who are also brothers may overtly or covertly share sexual