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Duality and productivity in language
Two properties set human language apart from most other communication systems: it is built in two layers (duality) and it can say endlessly new things (productivity).
Hockett’s design features
The linguist Charles Hockett (1960; Hockett & Altmann, 1968) listed a set of “design features” for comparing human language with animal communication. They include arbitrariness (words need not resemble what they mean), displacement (talking about things that are absent or in the past or future), semanticity (signals carry meaning) and cultural transmission (languages are learned from others). Two features concern how language is built and how it grows.
Duality of patterning
Language has two layers. At the lower level, a small set of meaningless sound units (phonemes) is combined. At the higher level those combinations form meaningful units (morphemes and words), which then combine into phrases and sentences.
| Level | Units | Example |
|---|---|---|
| Sounds (no meaning on their own) | /k/, /æ/, /t/ | None of these means anything alone |
| Words (meaning) | Different orderings | cat, act, tack all use the same three sounds |
| Phrases and sentences | Words combined by grammar | The cat sat on the mat |
English uses roughly 40 to 45 sounds, depending on the accent, yet has hundreds of thousands of words. Duality is what makes such a large vocabulary possible from so few building blocks.
Productivity
Productivity, also called openness, means speakers can produce and understand sentences they have never heard before. The purple walrus polished the violin is almost certainly new to you, yet you understand it at once. Grammar allows unlimited extension, for example by embedding one clause in another: This is the house that Jack built, This is the malt that lay in the house that Jack built, and so on.
How other species compare
- Vervet monkeys give different alarm calls for different predators, and listeners react appropriately (Seyfarth et al., 1980). The calls carry meaning, but the system has few signals and no grammar.
- Honeybees’ waggle dance conveys the direction and distance of food (von Frisch, 1967), a form of displacement, but it is confined to one topic.
- Songbirds combine sounds in complex sequences, and researchers debate how closely this resembles duality of patterning.
- Claims about apes learning language are disputed; most researchers see their combinations as far more limited than a child’s.
Relevance to speech and language therapy
- Sound level: speech sound disorders disrupt the first layer, which can make words harder to understand. Phonological awareness, noticing that words are made of sounds, is an important foundation for reading.
- Productivity: in language disorders, sentences may become short, repetitive or restricted. Therapy often works on combining and expanding words, and measures such as type-token ratio track variety.
Key idea. Duality explains how a few sounds make many words. Productivity explains how a few words make unlimited sentences.
Sources and further reading
- Hockett, C. F. (1960). The origin of speech. Scientific American, 203(3), 88–96.
- Hockett, C. F., & Altmann, S. (1968). A note on design features. In T. A. Sebeok (Ed.), Animal Communication. Indiana University Press.
- Seyfarth, R. M., Cheney, D. L., & Marler, P. (1980). Vervet monkey alarm calls: Semantic communication in a free-ranging primate. Animal Behaviour, 28(4), 1070–1094.
- von Frisch, K. (1967). The Dance Language and Orientation of Bees. Harvard University Press.