As described earlier, Levi-Strauss's theory of reciprocal exogamy features kinship-constrained marriage rules, the most basic of these being sister (or daughter) exchange, the levirate, the sororate, and cross-cousin marriage. Where do these rules come from? The answer proposed here is that they were ultimately derived from the atom of between-group alliances and the ensuing familiarity biases that affected mate selection in the nascent tribe. As illustrated in Fig. 2.10, upon pair-bonding with male B1 and moving into group B permanently, female Ego underwent long-lasting familiarity biases with her husband's close kin, including her brothers-in-law and sisters-in-law. Such biases were likely to translate into amicable relationships between them for reasons already given. In this context, if Ego's husband were to die, Ego might well form a pair-bond with her husband's brother (B3), in which case one obtains the structural equivalent of the levirate - a widow marrying the brother of her deceased husband. Similarly, upon pair-bonding with Ego, male B1 experienced long-lasting familiarity biases with his wife's close kin, including his brothers-in-law and sisters-in-law. If Ego were to die, male B1 could form a pair-bond with his wife's sister (A2), this producing the structural equivalent of the sororate - a widower marrying his deceased wife's sister. If, however, male B1 were to form a pair-bond with Ego's sister A2 while Ego is still alive, this produces a form of sororal polygyny, another widespread practice in human societies.
Interestingly, simple processes akin to those described here have been invoked by cultural anthropologists to explain the levirate and the sororate. Citing figures based on 250 societies, Murdock described the closely related phenomenon of privileged relationships between siblings-in-law of opposite sex, "within which sexual intercourse is permitted before marriage and frequently afterwards as well." Murdock argued that both permissive sex and preferred marriage between
Kin group A
Kin group B
Al A2 Ego B1
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