Predictive structure emerges during generalisation of kin terms to new referents

Despite crosslinguistic diversity in how kin relations map to terminology, there are constraints on which kin may be categorised together. But what are the constraints on kin term variation, and where do they come from? One proposed constraint is internal co-selection -- an evolutionary process where terminological changes in one generation of kin co-occur with parallel changes in other generations. This results in kin terms which are predictable on the basis of other kin terms, a property we call predictive structure. To determine the strength of this constraint, we measured the predictive structure of systems of kinship terminology from 1135 languages. We found that kinship systems exhibit a significant degree of predictive structure, and we argue that its prevalence reflects a pressure for simplicity. We expected that this pressure would be imposed during the generalisation of known kin terms to new referents, and we tested this claim using an artificial kin term generalisation task. Our results suggest that generalisation of kin terms does indeed favour predictive structure, but that people's preference for predictive structure faces interference from other pressures to distinguish kin by features like gender.