Is Recursion Uniquely Human?

1. Is Recursion Uniquely Human? Hauser, Chomsky and Fitch (2002) Fitch and Hauser (2004)

2. Two Senses of the Faculty of Language • Faculty of Language in a Broad Sense (FLB) • Sensory-Motor system: speech, hearing, gestures (for sign language), etc. • Conceptual-intentional system: ability to create beliefs, meanings, and intentions that can be conveyed through language • Computational system: ability to put words or linguistic units together in a way that fits with the grammar of a language (Faculty of Language in a Narrow Sense)

3. Two Senses of the Faculty of Language • Faculty of Language in a Narrow Sense (FLN) • Only the basic computational elements of language (grammar) • Discrete Infinity, the capacity for a finite number of small units to create an infinite number of sentences • Recursion, the ability to embed units within each other and have a hierarchical structure of those units

4. Hypothesis • Hauser, Chomsky, and Fitch argue that • FLN is uniquely human and arose since humans diverged from ape ancestor • FLB is shared by non-human animals, and has evolutionary history predating emergence of language

5. Comparative Approach to Language Evolution • Necessary because fossils cannot provide evidence about language evolution • Compares empirical data from living species to draw inferences about extinct ancestors • Important to be able to test claim that FLN is unique to humans

6. Data for FLB Shared by Other Animals • Sensory-motor • Many species are capable of distinguishing between human speech sounds. • Some mammals without speech also have descended larynxes • Although birds and dolphins can imitate sounds, monkeys cannot • Imitation in birds and dolphins is analogous to human evolution • Ability to imitate in humans must have evolved after diverging from monkeys

7. Data for FLB Shared by Other Animals • Conceptual-intentional • Some studies show that chimps have Theory of Mind (although this is disputed) • Mismatch between conceptual capacities of animals and the content of their communication • Differences between human and non-human communication: • Limited repertoire of signals • Not creative • Generally appears early in development, with experience only necessary to know when signals apply

8. Data Supporting Uniqueness of FLN in Humans • Seems clear that only humans have recursion, although what specific properties other animals lack is unclear • Chimpanzee number studies • Show that chimps can assign meaning to symbols, but not relate the symbols to a recursive number sequence • Study on grammar acquisition in human adults and tamarins by Fitch and Hauser

9. Computational Constraints on Syntactic Processing in a Nonhuman Primate • Testing the abilities of cotton-top tamarins and human adults to master Finite State and Phrase Structure grammars Fitch and Hauser, 2004

10. Definitions • Finite State grammar (FSG) • Places severe constraints on hierarchy (hierarchy is fixed) • Insufficient to generate the structures of any human language • Phrase Structure grammar (PSG) • Can have unlimited embedding at different hierarchical levels • More powerful than FSGs (grammars above the FSG level are required for human languages) • Allows parts of a sentence that are related to be located far away from each other. • e.g., The KIDS who play in our street ARE loud and dirty.

11. Two Test Grammars • Researchers recorded two possible strings of consonant-vowel (CV) syllables • 8 “A” syllables in a female voice • 8 “B” syllables in a male voice • The FSG was (AB)n, with a sequence of 2 or 3 AB syllables • The PSG was AnBn, with n A syllables followed by n B syllables (in this grammar, each A syllable is connected to a B syllable through a hierarchical structure)

12. Test Grammars

13. Procedures • The tamarins were separated into 2 groups, one for each grammar • Each group had different sound strings from its grammar played to it for 20 mins. in the evening • The next morning, after a re-familiarization period, monkeys were tested individually with the same 8 stimuli in random order • 4 stimuli were consistent with the grammar in which the monkeys were trained, 4 were consistent with the other grammar • Tamarins (like infants) were expected to look at the speaker if they heard something inconsistent with the grammar to which they had been trained. • Latency and duration of looking at the speaker were recorded.

14. Cotton-top Tamarin

15. Procedures for Human Adults (Undergraduate Students) • Same grammars and syllables as tamarins, tested in essentially the same way • Habituated to grammars in less than 3 minutes, instead of 20 • Instead of the looking paradigm, subjects were just told to indicate whether stimuli were “different” from the grammar to which they had been trained

16. Results

17. Implications • While tamarins can process regular acoustic sequences, they cannot process simple phrase structure • Tamarins have a computational limitation on the ability to recognize and remember hierarchically organized acoustic structures • This hierarchical organization is one of the requirements for human language, and may represent an important evolution of human language faculty.