Source:
Natural Selection and the Analysis of Human Sociality
Richard D. Alexander (1977)
Incest avoidance may be regarded as an aspect of mate selection whereby, in general, close relatives of the opposite sex are unavailable or disfavored, while more distant relatives are favored or specified. Members of one's nuclear family are nearly always excluded as possible mates, and, in humans, certain close relatives, just outside the nuclear family, are nearly always either excluded or disfavored. Other relatives only a little more distant may be suitable, preferred, or prescribed mates. Distant relatives or nonrelatives may also be suitable, preferred, or prescribed mates. If fairly close relatives are preferred or prescribed, those more distant may as a result be more or less incidentally disfavored or excluded.
Genetic outbreeding does not necessarily imply that only the most distant possible relatives, or nonrelatives, are suitable marriage or sex partners. Restricting one's sexual or marriage partner to increasingly distant relatives involves increasing costs (Fig[. ...] 5). Thus, fewer individuals are available, greater distances may have to be traveled to locate them, and greater risks may be involved in securing them; deleterious partitioning of reproductive resources may also result from extreme outbreeding.
On the other hand, close inbreeding also has disadvantages. Thus, with outbreeding fewer deleterious recessive alleles are exposed, there is less chance of reproductive devaluing of relatives because of sexual competition (Hamilton, 1967), and reproductive resources may be advantageously partitioned, for example, to those levels at which they are likely to undergo maximal inflation of their value. The complex patterns of marriage regulations in human societies may thus be largely interpretable as aspects of nepotism in which genetic relatives are constrained against competition, for mates and other resources, which devalues collective effects on the inclusive fitnesses of parents and other relatives. This explanation of marriage patterns and rules focuses attention simultaneously on individual interests in obtaining mates and resources and collective or societal interest in minimizing competition deleterious to the inclusive fitnesses of individuals related to the competitors. Rules or practices resulting from such collective interests must often be compromises based on differences in interests, and differences in power or influence; frequently, the most satisfactory or practical compromise remains the same from one generation to the next. In this case a Darwinian model appears to draw together the phenomena of individual interests and cultural collectivity (including its continuity) in a fashion not previously accomplished with cultural or genetic models that ignored inclusive fitness (Alexander, ms.).
Since the costs of inbreeding diminish outward and the costs of outbreeding mount with its degree, the two lines necessarily cross at some point (Fig. 5). I suggest that in most human societies the lines cross somewhere near the level of first cousins, farther out when more distant cousins are readily available, and closer in when the particular heritable resources involved are deflated rather than inflated in value as a result of extensive partition. Thus, we may expect nomads with few heritable goods, such as Australian Aboriginals, to marry further out. Herdsmen and farmers, and members of titled or royal families, in particular, might tend to marry in. A very large number of factors may be involved in the exact degree of inbreeding and outbreeding, or in the selection of marriage partners at given levels of inbreeding and outbreeding. The important point is that with human marriage patterns, as with kinship systems and inheritance patterns, analyses based on cost-benefit assessments from a Darwinian model are called for, and they are feasible.
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