I say "uncle", you say "DaJiu"

Kinship terms (mother, uncle, niece, etc.) are socially important and generally learned early in acquisition. Interestingly, different languages have different sets of terms. Mandarin, for instance, divides "uncle" into "father's older brother", "father's younger brother", and "mother's brother".
Stranger things (to an anglophone, anyway) happen, too: In Northern Paiute, the kin terms for grandparents and grandchildren are self-reciprocal: you would use the same word to refer to your grandmother (if you are female) that she uses to refer to you. (See my previous post on "mommy" across languages.)

Kinship terms in English and Northern Paiute. Ignore all the logical terms for now.

(Figure taken from Kemp & Regier, 2012)

Even so, there are a lot of similarities across languages. Disjunctions are relatively rare; that is, it's unusual to see a word that means "father or cousin". Usually there are more words to distinguish varieties of closely-related relatives (sister, brother) than distant relatives (cousin). How come? One obvious answer is that maybe the kinship systems we have are just better than the alternatives (ones with words like "facousin" = "father or cousin"), but it would be nice to show this.

Optimal Kinship Terms

In a paper earlier this year, Charles Kemp and Terry Regier did just that.

We show that major aspects of kin classification follow directly from two general principles: Categories tend to be simple, which minimizes cognitive load, and to be informative, which maximizes communicative efficiency ... The principles of simplicity and informativeness trade off against each other... A system with a single category that includes all possible relatives would be simple but uninformative because this category does not help to pick out specific relatives. A system with a different name for each relative would be complex but highly informative because it picks out individual relatives perfectly. 

That seems intuitively reasonable, but these are computational folk, so they formalized this with math. The details are in the paper, but roughly: They formalize the notion of complexity by using minimum description length in a representational language based on primitives like FEMALE and PARENT. The descriptions of the various terms in English and Northern Paiute are shown in parts C and D of the figure above. Communicativeness is formalized by measuring how ambiguous each term is (how many people it could potentially refer to).

A language is considered "better" than another if it out-scores the other on one dimension (e.g., simplicity) and no worse on the other (informativeness). A language is near-optimal if it there is hardly any possible language that is better. They looked at a number of different existing kinship systems (English, Northern Paiute, and a bunch of others) and found that all of them were near-optimal.

Evolution, Culture, or Development?

There are generally three ways of explaining any given behavior: evolution (we evolved to behave that way), culture (culture -- possibly through cultural evolution -- made us that way), or development (we learned to behave that way). For instance, it's rare to find people who chiefly eat arsenic. This could be because of evolution (we evolved to avoid arsenic because the arsenic-eaters don't have children and pass on their genes), cultural evolution (cultures that prized arsenic-eating all died out, leaving the non-arsenic cultures as the only game in town), or development (we learned as children, through trial and error, that eating arsenic is a bad idea). If I remember my Psych 101, food preferences actually involve all three.

What about kinship terms? If they are optimal, who do we credit with their optimality? Probably not development (we don't each individually create optimal kinship terms in childhood). Kemp and Regier seem to favor cultural evolution: over time, more useful kinship terms stuck in the lexicon of a given language and useless ones like "facousin" died out. It would be nice to show, however, that it is not actually genetic. This wouldn't have to be genes for kinship terms, but it could be genes that bias you to learn naming systems that are near-optimal (kinship naming systems or otherwise). One would need to show that these arose for language and not just cognition in general.

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Kemp, C., and Regier, T. (2012). Kinship Categories Across Languages Reflect General Communicative Principles Science, 336 (6084), 1049-1054 DOI: 10.1126/science.1218811