What do I call the gentleman who has married my wife’s sister? In several Indian languages, I know him as my shaddaka or sadoo. And in Indian-English, people say that he is my “co-brother” or “co-brother-in law” (one of the most awkward (if I may) terms created). In England, you do not have an equivalent kin-term to specify him; you simply call him by his name. Then again, shaddaka or sadoo does not tell you whether he is my wife’s elder sister’s husband or younger sister’s.

Talk of brothers and their wives, the situation changes. My wife calls my brother’s wife as orpadi in Tamil (or Totikodali in Telugu), again regardless of whether my brother is younger or elder to me. But in Hindi, my elder brother in called Jeth by my wife (and his wife is called Jethani). She calls my younger brother Devar and his wife Devrani. (Here, thankfully, we do not use the term “co-sister-in law” in Indian-English!)

Such elaborate kin-specific terms are used only in certain languages and communities, not in all. In England or U.S., there are no such kin terms, leave alone kin-specific terms. They only talk of uncles and aunts, siblings (brother and sisters) and niblings (yes, niblings is a term used to denote nephews and nieces), and grandparents and grandchildren.

Why do some communities, cultures and subcultures use elaborate kin terms and kin-specific terms while others are more prosaic and spartan?

And how far down the relatedness or relationship web do those terms extend? Cognitive scientists are trying to understand the logic behind kin terms and whether such logic also applies to other semantic domains.

Kinship is crucial for the transmission of not only genes but also property and assets, culture and social mores, and language. Of course, we are most concerned with our immediate family or genetic connections. Since each of them has other links, the kinship spectrum widens rapidly. And our concern is how far we need to fan out in creating kin terms for each in the network and addressing them using kin-specific terms.

Such nomenclature is community or culture-dependent. Professor George Murdock of the University of Pittsburgh has made a painstaking collection of kin term patterns from 566 communities of 200 cultural provinces across the world, and some of his findings are occasionally surprising.

For example, among the American Indians who use the Northern Paiute language, a girl and her maternal grandmother use the same term to refer to each other! In contrast, in India we specify the relationship and the gender as well. Cognitive scientists point out that in specifying kinship and using kin terms, we practice a trade-off between simplicity and informativeness. In England where respect for elders is not obligatory, or family hierarchy is not imposed or widely practised, the terms brother and sister are simple and suffice, but among the Tamils, kin-specific terms such as Anna (for elder brother) and Akka (for elder sister), and Tambi (younger brother) and Thangai (younger sister) become important.

So are the terms for first cousins, “co-sisters” and others. They are informative and specify the role that each has in the family traditions and practices, alliances, partition of property and assets, and so forth. Thus, depending on a community’s gene-sharing, family practices, the vocabulary in the language for kin-specific terms is economized or expanded. There is thus a logic to the vocabulary relating to the ‘perceptual space’ of relationships — biological or sociological.

Does this trade-off between simplicity and informativeness work in other areas or semantic domains? The answer appears to be yes, and an example is that of colour perception and description.

VIBGYOR describes the basic seven colours (just as our immediate family is our genetic VIBGYOR), but when we get into specifics such as peacock-blue versus sky-blue, or canary-yellow vs lemon-yellow, we go beyond simplicity into informativeness.

However, as Drs Kemp and Regier point out in their analysis of kinship categories described above (the 25 May 2012 issue of Science, pp. 1049-54; see also commentary by SC Levinson in the same issue, pp.988-89), kin terms describe individuals while colour terms pick out a region in a continuous perception space. But the interesting thing is that a common analytical principle can be applied in categorizing such diverse semantic domains.

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