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Optimality Theory

Optimality Theory (OT) is a theory of grammar, originally developed in the field of phonology (speech sounds), which I’ve found useful in explaining how people categorize their kin.

One way to understand OT is to compare it with theories of optimization in economics and behavioral ecology. In these theories, people are typically faced with the problem of finding the best outcome in the face of tradeoffs and constraints, where the best outcome maximizes some quantity (utility, fitness). In OT too people are trying to find the best grammatical output in the face of constraints. But in OT constraints are tradeoffs between constraints are handled by ranking. Isaac Asimov’s Laws of Robotics work this way:

  1. A robot may not injure a human being or, through inaction, allow a human being to come to harm.
  2. A robot must obey the orders given it by human beings, except where such orders would conflict with the First Law.
  3. A robot must protect its own existence as long as such protection does not conflict with the First or Second Laws

You follow rule the first rule if at all possible, the second rule as long as this doesn’t violate the first rule, and so on. (This is like alphabetical order, where you sort by the first letter, then by the second letter if the first letter is tied, and so on. Economists call this a lexicographic preference.) In OT, there are two types of constraints, corresponding to two imperatives of communication: give as much relevant information as possible, and keep difficult, non-prototypical forms to a minimum.

Here’s an example of how this works for English kin terms




Wife’s Sister

Describe Distance

Describe Affinity

Minimize Spouse’s Sibling









Spouse’s Sister 




This very simple example can be extended with more constraints and different inputs to account for why sister-in-law and grandfather are complex, multi-part words, while sister and father are simple, why English has one word for Mother’s Sister and Father’s Sister (aunt) and another one for Mother, and why siblings but not cousins are distinguished by sex.

And there is a further payoff. OT is meant to account not just for grammar in one language, but for variation of grammar across cultures. In the example above, consider what happens if we change the constraint ranking: we can get a language that classifies a Wife’s Sister as a Wife, or as a Sister. In an analysis involving more kin terms and more constraints, we may find, however, there are some Input-Output mappings that can’t be produced by any constraint ranking. These correspond to logically possible but empirically non-existent terminologies.

Finally, there may be one more payoff to the OT analysis of kinship. In my published work I consider kin terminology. But in my latest work (under review), I show that the same machinery that produces kin terminology can also generate marriage rules: dividing kin into those one should, can’t, and may marry. Some scholars have proposed the existence of a human moral grammar faculty, comparable to the faculty for linguistic grammar. My recent research suggests that linguistic grammar and moral grammar use some of the same psychological machinery. This implies that Claude Lévi-Strauss was onto something when he wrote, “linguists and sociologists do not merely apply the same methods but are studying the same thing”

PDF: human kinship grammar


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