Source: Ottenheimer 1996
The Biblical Model
This model is based on the incest prohibitions listed in the 18th chapter of Leviticus, which are noteworthy for their extensive restrictions on affinal relatives and the absence of bans on cousin marriage. Specific state legislation derives its origin from Church of England canon law that was applied in most of the original American colonies. It represent an abridged version of the biblical prohibitions and focus on banning marriages within the nuclear family and between a marriage partner and his/her spouse's parents or children, specifically between:
The Anglican regulations, and those of many Protestant churches, excluded any restrictions on cousin marriage, which were critically perceived as a Catholic misinterpretation of scripture. They also once included a ban on marriage between a brother-in-law and sister-in-law, even after the death a connecting spouse. However, no states currently include this prohibition, nor does the Church of England. The states whose legislation conforms to this pattern are clustered in the eastern part of the country, especially in New England and the South. South Dakota, Oklahoma and Georgia prohibit marriages between stepparents and stepchildren only.
The Western Model
This form contrasts with the biblical model in that shifts its focus from affinal restrictions to consanguineal ones. In-laws of any kind are allowed to marry, but first cousins are not. The states in this group are located primarily in the Mid-West and West. Most of them entered the Union and formulated their marriage legislation after the Civil War (Ottenheimer 1996).
Ottenheimer advances an interesting theory of the reasons for this complex and puzzling difference among the states. He observes that the different models of incest prohibition reflect a marked change in 19th century social values. The biblical model represents a carryover of older, pre-industrial attitude towards family life. It focuses on adherence to scriptural authority and on the maintenance of social order within the family, through the elimination of possible occasions for role conflicts and personal antagonisms among closely related people. The western model represents a change to a conceptualization of the family as an instrumental reproductive unit geared to producing optimally healthy offspring. This view owes its origin to Victorian minded physicians and anthropologists, including Lewis Henry Morgan, who believed that cousin marriages led to the production of mentally and physically deformed children. It was also supported by the evolutionist position that attributed the biological and moral advance of the human species to the imposition of ever widening restrictions on marriages among kin.