Harry Tily

Word lengths are optimized for efficient communication

We demonstrate a substantial improvement on one of the most celebrated empirical laws in the study of language, Zipfs 75-y-old theory that word length is primarily determined by frequency of use. In accord with rational theories of communication, we show across 10 languages that average information content is a much better predictor of word length than frequency. This indicates that human lexicons are efficiently structured for communication by taking into account interword statistical dependencies. Lexical systems result from an optimization of communicative pressures, coding meanings efficiently given the complex statistics of natural language use.

The communicative function of ambiguity in language.

We present a general information-theoretic argument that all efficient communication systems will be ambiguous, assuming that context is informative about meaning. We also argue that ambiguity allows for greater ease of processing by permitting efficient linguistic units to be re-used. We test predictions of this theory in English, German, and Dutch. Our results and theoretical analysis suggest that ambiguity is a functional property of language that allows for greater communicative efficiency. This provides theoretical and empirical arguments against recent suggestions that core features of linguistic systems are not designed for communication.

The time-course of lexical and structural processes in sentence comprehension.

Online sentence comprehension involves multiple types of cognitive processes: lexical processes such as lexical access, which call on the users knowledge of the meaning of words in the language, and structural processes such as the integration of incoming words into an emerging representation. In this article, we investigate the temporal dynamics of lexical access and syntactic integration. Unlike much previous work that has relied on temporary ambiguity to investigate this question, we manipulate the frequency of the verb in unambiguous structures involving a well-studied syntactic complexity manipulation (subject- vs. object-extracted clefts). The results demonstrate that for high-frequency verbs, the difficulty of reading a more structurally complex object-extracted cleft structure relative to a less structurally complex subject-extracted cleft structure is largely experienced in the cleft region, whereas for low-frequency verbs this difficulty is largely experienced in the postcleft region. We interpret these results as evidence that some stages of structural processing follow lexical processing. Furthermore, we find evidence that structural processing may be delayed if lexical processing is costly, and that the delay is proportional to the difficulty of the lexical process. Implications of these results for contemporary accounts of sentence comprehension are discussed.

Complementing quantitative typology with behavioral approaches: Evidence for typological universals

Two main classes of theory have been advanced to explain correlations between linguistic features like those observed by Greenberg (1963). arbitrary constraint theories argue that certain sets of features patterm together because they have a single underlying cause in the innate language faculty (e.g., the Principles and Parameters program; see Chomsky & Lasnik 1993). functional theories argue that languages are less likely to have certain combinations of properties because, although possible in principle, they are harder to learn or to process, or less suitable for efficient communication (Hockett 1960, Bates & MacWhinney 1989, Hawkins 2004, Dryer 2007, Christiansen & Chater 2008; for further discussion see Hawkins 2007 and Jaeger & Tily 2011). The failure of Dunn, Greenhill, Levinson & Gray (2011) to find systematic feature correlations using their novel computational phylogenetic methods calls into question both of these classes of theory. Dunn et al.’s methodology is a principled and powerful new way of evaluating change-based theories of language typology, but it faces fundamental challenges common throughout quantitative typology. Typological data is usually sparse, and additional data hard or impossible to obtain. The statistical power to detect an effect (e.g., to detect universal tendencies) is reduced by uncertainty about the genealogical relations between languages, the period of time during which a language was spoken, and the amount of contact with other languages. It is hence possible that the failure to find support for word order universals reported in Dunn et al. 2011 is a spurious null effect (see Croft et al. 2011, in this issue, who also discuss other potential methodological issues with that study). To some extent, this is a critique that can be leveled at much work on quantitative typology: sparse and uncertain data weakens the conclusions that can be drawn from it. If we desire a higher level of certainty when evaluatAUTHOR’S COPY | AUTORENEXEMPLAR

Reply to Reilly and Kean: Clarifications on word length and information content

First, we disagree with Reilly and Kean (1) that our results on word length (2) contradicted Zipfs principle of least effort. Our findings were in the same spirit, except that we measured effort in a more principled way than Zipf could have (2). Assigning word length by information content is least effort under an assumption of a superlinear relationship between effort and information content (3), and it is optimal under a desire to stay just under the channel capacity of linguistic systems (4).