Kinship rules and linguistic rules, notes Jones, “are intriguingly similar in form.” In each case, we have not graded and correspondingly disputable meanings but sharp digital contrasts and correspondingly abstract logical computations. To explain this, the author initially posits an “innate conceptual structure of kinship” interacting with the language faculty's “optimal grammatical communication” principles. Toward the end of the article, however, this idea is abandoned. “The alternative,” the author now explains, “is that constraints are neither innate not culturally acquired, but discovered.” Humans are equipped neither with an innate Kin Term Acquisition Device nor with a hard-wired Language Acquisition Device. Instead, the relevant hard-wiring has a broader function: It equips us to play “coordination games.”

The idea has support among many evolutionary psychologists and anthropologists. What's needed here, however, is some Darwinism. If we're to come up with a testable model, we need to specify at least some details of the “coordination games” we envisage our evolving ancestors to have been playing. Sex isn't just in the head, any more than are costly burdens such as pregnancy, birth, lactation, or childcare. Behavioural ecology is the relevant discipline here, dealing as it does with the costs and benefits of real biological strategies operating in the real world.

At some point during the evolution of our species, sex and reproduction became subject to the rule of law (Knight Reference Knight2008; Reference Knight, Botha and Knight2009). Like any legal system, a formal kinship system is designed to minimize scope for disputation: It does this by eliminating shades of grey. You are either a sister or a wife; to a relative of the opposite sex, you can't be somewhere in between. Jones understands that this principle – critical to both language and kinship – is a “discovery” and not a consequence of cognitive hard-wiring. If humans are to coordinate their efforts in defining and enforcing the law, they must necessarily draw distinctions in this way. To suggest this, however, is only to make a start in asking the really interesting questions. Who initially benefited from this extraordinary development? Did males drive the whole process, as Lévi-Strauss (Reference Lévi-Strauss1969) suggests? Or were females actively involved? As soon as sex differences are taken into account, we can start constructing a model detailed enough to be testable in the light of archaeological, ethnographic and other evidence.