Roger Sandall writes in his review of anthropologist Robin Fox's The Tribal Imagination in The American Interest:
Fox begins this chapter by describing New York Times columnist John Tierney’s bafflement in September 2003 upon discovering that the lavish weddings regularly taking place in his Baghdad hotel were mostly marriages of first cousins who were the children of brothers. Questioned about this practice, the young people told Tierney, “Of course we marry a cousin. What would you have us do, marry a stranger? We cannot trust strangers.”
... It is also a truth, Fox believes, that we ignore at our peril as we go stumbling about in far-away strange places where tribes rule with an authority denied the more-or-less absent state. The pride and latent violence of groups of mutually suspicious kindred must be the starting point, Fox says, for anyone venturing into this political landscape. Such men and women are not the free individual citizens of a recognized territorial jurisdiction; nor are they people with clearly defined and defensible legal rights with respect to the state, whether in Libya or Iraq or Afghanistan.
... While primates have kin, they do not have in-laws.
Unpacking this highly condensed formulation reveals a whole range of connected evolutionary phenomena: the dispersal of animal populations, the need for genetic variability and the origin of language, the last enabling social structures to form in time and space among men and women who have never seen each other and in some cases never will. Other primates don’t do this, and here Lévi-Strauss was right. The uniquely human cultural fact that arose was something new, and it formed “the enduring relationship between natal kin separated by marriage but linked by kinship, by descent from a common ancestor.”
All mammals ensure genetic variability through population dispersal. Fox argues that this observation applies as far back as “the emergence of self-replicating matter, and the crucial revolution that produced sex to replace cloning.” Sexual reproduction, plus dispersal, spontaneously produces the genetic variability natural selection needs to work on. If mammalian populations did not disperse, close inbreeding would result in a loss of such variability, and “hence mechanisms evolved to avoid it.” At the same time too great a dispersal—so great that separated bands lost contact with each other—meant that beneficial features of kinship association might be lost. So it is that “organisms breed out to avoid losing variability, but not so far out that they dissipate genetic advantages.” Not too close, but not too far; that was the evolutionary Golden Rule and, of course, the plinth of tribal society itself.
Which brings us back to what was going on in that Baghdad hotel. In human terms, the Darwinian imperatives of dispersal, variability and natural selection eventually produced a social world in which marriage with cousins was preferred. Historically, that’s how it has been in most traditional preindustrial societies until quite recently. And for Fox it is an integral part of the tribal default system of humanity everywhere.