In a number of pre-modern societies, relations of this
kind were the exclusive prerogative of kings and remained forbidden to
commoners. 'Royal incest' (Bixler 1982), which serves to emphasize
the supernatural qualities of the rulers (strongly associated with
mythological traditions of divine incest) and insulates ruling families
against intrusions, can be found around the globe, from the Pharaohs
and Ptolemies of ancient Egypt (e.g., Cerny 1954; Carney 1987) and
ancient Near Eastern rulers (e.g., Elam, Persia, Phoenicia, etc.)
(Kornemann 1923) to kings in Central Africa (de Heusch 1958), the Inca
of Peru and the Mixtec aristocracy of Mexico (Christensen 1998), and
the chiefs of pre-contact Hawaii (Davenport 1994). In other cases,
mostly in tribal societies, incestuous behaviour could be imagined to
confer magical powers. By contrast, habitual nuclear-family incest
outside ruling families was exceedingly rare. The census returns of
Roman Egypt, preserved on papyrus, provide quantifiable documentary
evidence of brother-sister marriage, mostly for the 2nd and early 3rd
centuries AD (Thierfelder 1960; Sidler 1971; Hopkins 1980; Shaw 1992;
Scheidel 1996a). At that time, one in five attested couples in Middle
Egypt consisted of brothers and sisters (Bagnall and Frier 1994). The
incidence of incest in the city of Arsinoe in the Fayum was higher
still, indicating that virtually every man with a living younger sister
married her instead of someone from outside the family. At that level,
this custom must have assumed the function of a cultural norm (Scheidel
1995). Mazdaean ('Zoroastrian') religious doctrine, originating
from Iran, not only legitimized but encouraged and extolled sexual
relations between parents and children and between siblings. The very
substantial corpus of pertinent evidence combines prescriptive
Zoroastrian texts (mostly from the early Middle Ages) and descriptive
accounts by outsiders, ranging from the 5th century BC to the Middle
Ages and from western Europe to Tibet and China (West 1882; Spooner
1966; Sidler 1971; Bucci 1978; Frye 1985; Herrenschmidt 1994;
Mitterauer 1994). Half-sibling unions are also attested for a number of
other societies but have so far eluded systematic investigation
(Modrzejewski 1964; Goggin & Sturtevant 1964).