Fig. 2.10 Mating biases stemming from disproportionate levels of familiarity between affines, and their relations with known exogamy rules in humans. The individuals pictured are the same as in Fig. 2.1, except that siblingships include one more individual. (1) Initial pair-bond between female Ego and male B1. (2) Pair-bond between B1 and his wife's sister, the equivalent of sororal polygyny if Ego is still alive, or of the sororate if Ego is dead. (3) Pair-bond between Ego and her husband's brother, the equivalent of the levirate if the husband is dead. (4) Pair-bond between Ego's brother and B1's sister. In conjunction with (1), this produces the structural equivalent of sister exchange siblings-in-law "are explicable as extensions of the marital relationship" and reflect the attraction of people to persons who most closely resemble their spouse. "The persons who universally reveal the most numerous and detailed resemblances to a spouse" he wrote, "are the latter's siblings of the same sex" ... "who are likely to have similar physical characteristics" ... and "almost identical social statuses since they necessarily belong to the same kin group" (Murdock 1949, pp 268-269). Murdock's explanation has much in common with the present model. Both conceive of mate selection as being affected by informal regularities, whether the latter stem from familiarity biases, physical similarities, or social compatibilities. To Levi-Strauss, in contrast, exogamy rules were normative and part of reciprocity agreements. But viewed from an evolutionary perspective, the two types of explanations are compatible: informal regularities paved the way for normative rules; in the same manner, incest avoidances paved the way for incest prohibitions.

These considerations apply to another marriage rule: sister exchange (or daughter exchange, depending on one's viewpoint). Structurally speaking, sister exchange is simply bilateral marriage between two groups of affines. In all likelihood, the exchange dimension of the phenomenon is a further and more recent aspect of it, an aspect involving the control of sisters by their brothers (or of daughters by their fathers). As for cross-cousin marriage, it is the extension of sister exchange to the following generation, as described earlier (Fig. 2.3). Thus, from an evolutionary perspective, the relevant question for both sister exchange and cross-cousin marriage concerns the origin of bilateral marriage between affines and, again, the answer proposed here lies in familiarity differentials affecting mate selection. As illustrated in Fig. 2.10, if female Ego is already pair-bonded with male B1, bilateral marriage between affines ensues from B1's sister (B2) pair-bonding with Ego's brother (A1). From male A1's viewpoint, female B2 is a familiar affine, the sister of his brother-in-law. Reciprocally, from female B2's viewpoint, male A1 is also a familiar affine, the brother of her sister-in-law. In short, the most basic of exogamy rules - levirate, sororate, sister exchange, and cross-cousin marriage - might have originated in mate selection biases stemming from disproportionate levels of familiarity between affines.

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