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Intro linguistics courses usually begin by asking what makes human language different from other communication systems. In 1960 the linguist Charles Hockett proposed a list of "design features," including arbitrariness (no natural link between a word's form and its meaning), displacement (talking about things not present in time or space), productivity (creating and understanding new sentences without limit), duality of patterning (meaningless sounds combine into meaningful units), and cultural transmission (each generation learns the language from others). Many animal systems share some of these features, but no known animal system has been shown to combine all of them, especially open-ended productivity.
Take one feature at a time and test it against a case: honeybee dances, vervet monkey alarm calls, a dog responding to commands, and a human sign language. Have students build a table marking which features each system shows. Ask students to produce a sentence that has probably never been said before, which makes productivity concrete. Discuss onomatopoeia as a partial exception to arbitrariness.
Many animals communicate. Birds sing, bees dance, and monkeys call out warnings. What, then, is special about human language? In 1960 the American linguist Charles Hockett tried to answer this by listing "design features," properties that together set human language apart. His list grew over the following years. A handful of the features do most of the work in intro courses.
Arbitrariness. The sound of a word usually has no natural connection to its meaning. Nothing about "tree" looks or sounds like a tree, and other languages use completely different words for the same thing. Onomatopoeia, such as "buzz," is a partial exception, and even onomatopoeia differs across languages.
Displacement. Humans can talk about things that are not here and now: yesterday's game, a friend in another city, a dragon that never existed.
Productivity. Speakers can produce and understand sentences they have never heard before. You have probably never read the sentence "The purple giraffe borrowed my bicycle," yet you understand it at once.
Duality of patterning. Language works on two levels. Sounds such as /k/, /æ/, and /t/ mean nothing alone, but they combine into meaningful words: cat, act, tack. A few dozen sounds produce a huge vocabulary.
Cultural transmission. Children learn the particular language of the people around them. A child adopted at birth learns the language of the adoptive family, not of the birth parents.
Comparing animal systems shows how the features cluster. Karl von Frisch showed that honeybees perform a "waggle dance" that tells other bees the direction and distance of food, which is a form of displacement. But the dance cannot express new kinds of messages. Vervet monkeys give different alarm calls for leopards, eagles, and snakes, and other vervets respond appropriately; the calls carry meaning but do not combine into new sentences. Researchers have taught apes such as Washoe and Kanzi to use signs or symbols, and the results remain debated, especially on whether the apes produced anything like syntax.
No animal system studied so far has been shown to combine all these features, above all open-ended productivity. That combination is what linguists mean when they call human language unique. The features also make predictions. Any human language, spoken or signed, should show them, and decades of research on languages around the world, including sign languages, support that prediction.