Talk of linguistic universals has given cognitive scientists the impression that languages are all built to a common pattern. In fact, there are vanishingly few universals of language in the direct sense that all languages exhibit them. Instead, diversity can be found at almost every level of linguistic organization. This fundamentally changes the object of enquiry from a cognitive science perspective. This target article summarizes decades of cross-linguistic work by typologists and descriptive linguists, showing just how few and unprofound the universal characteristics of language are, once we honestly confront the diversity offered to us by the world's 6,000 to 8,000 languages. After surveying the various uses of “universal,” we illustrate the ways languages vary radically in sound, meaning, and syntactic organization, and then we examine in more detail the core grammatical machinery of recursion, constituency, and grammatical relations. Although there are significant recurrent patterns in organization, these are better explained as stable engineering solutions satisfying multiple design constraints, reflecting both cultural-historical factors and the constraints of human cognition.

Linguistic diversity then becomes the crucial datum for cognitive science: we are the only species with a communication system that is fundamentally variable at all levels. Recognizing the true extent of structural diversity in human language opens up exciting new research directions for cognitive scientists, offering thousands of different natural experiments given by different languages, with new opportunities for dialogue with biological paradigms concerned with change and diversity, and confronting us with the extraordinary plasticity of the highest human skills.

I present the so-called Verb-Object Constraint as a serious proposal for a true linguistic universal. It provides an example of the kind of abstraction in linguistic analysis that seems warranted, of how different languages can confirm such a universal in different ways, and why approaches that avoid all abstractness miss important linguistic generalizations.

Evans & Levinson (E&L) argue against Universal Grammar on the basis of language diversity. A related and fundamental issue is whether the language input provides sufficient information for a child to acquire it. I briefly discuss the more integrated approaches to language acquisition which focus on the mechanisms, and research showing that input cues provide valuable information for the language learner.

Evans & Levinson (E&L) are right to hold theories of language accountable for language diversity, but typological data alone cannot determine the structure of mental phonological grammars. Grammatical universals are nonetheless testable by formal and experimental methods, and the growing research in experimental phonology demonstrates the viability of a comparative experimental evaluation of the Universal Grammar (UG) hypothesis.

This commentary argues that Evans & Levinson (E&L) should expand their two-track model to a three-track model in which biological and cultural evolution interact with the evolution of an individual's language repertories in ontogeny. It also comments on the relevance of the argument from the poverty of the stimulus and offers a caveat, based on analogous issues in biology, on the metaphor of language as a container, whether of meanings or of other content.

Evans & Levinson (E&L) argue that language universals are a myth. Christiansen and Chater (2008) have recently suggested that innate universal grammar is also a myth. This commentary explores the connection between these two theses, and draws wider implications for the cognitive science of language.

Evans & Levinson (E&L) perform a major service for cognitive science. The assumption of Chomskyan generative linguistics – that there are absolute unrestricted universals of grammatical structure – is empirically untenable. However, E&L are too reluctant to abandon word classes and grammatical relations in syntax. Also, a cognitive scientist can already draw on a substantial linguistics literature on variationist, evolutionary models of language.

Evans & Levinson's (E&L's) critique of Universal Grammar fails because their methodology is flawed, as illustrated in their discussion of the Subjacency Condition. The lack of explicit analysis leads the authors to a false conclusion that is refuted by work published in this journal twenty years ago. They miss the point that unanalyzed data cannot disprove grammatical hypotheses.

The recognition that contentful universals are rare and often “banal” does not undermine the fact that most non-universal but recurring patterns of language are amenable to explanation. These patterns are sensical or motivated solutions to interacting and often conflicting factors. As implied by the Evans & Levinson's (E&L's) article, linguistics would be well served to move beyond the essentialist bias that seeks universal, innate, unchanging categories with rigid boundaries.

Evans & Levinson (E&L) claim Kiowa number as a prime example of the semantically unexpected, threatening both Universal Grammar and Linguistic Universals. This commentary, besides correcting factual errors, shows that the primitives required for Kiowa also explain two unrelated semantically unexpected patterns and derive two robust Linguistic Universals. Consequently, such apparent exceptionality argues strongly for Universal Grammar and against E&L.

Conditional universals have always interested linguists more than unrestricted universals, which are often impossible to demonstrate empirically because categories cannot be defined in a cross-linguistically meaningful way. But deep dependencies have not been confirmed by more recent empirical research, and those universals with solid empirical support mostly relate to scalar patterns that can plausibly be related to processing cost.

Universal Grammar promotes the study of an idealization of language behavior and language learning. In examining the diversity of actual behavioral strategies used to achieve linguistic goals, Evans & Levinson (E&L) move towards studying language as a behavior. This approach can benefit from studying communicative and cognitive capacities more broadly – across species. We exhort like-minded linguists to cast off the remaining intellectual shackles of linguistic speciesism.

In documenting the dizzying diversity of human languages, Evans & Levinson (E&L) highlight the lack of universals. This suggests the need for complex learning. Yet, just as there is no universal structure, there may be no universal learning mechanism responsible for language. Language is a behavior assembled by many processes, an assembly guided by the language being learned.

Conflation of our unique human endowment for language with innate, so-called universal, grammar has banished language from its biological home. The facts reviewed by Evans & Levinson (E&L) fit the biology of cultural transmission. My commentary highlights our dedicated learning capacity for vocal production learning as the form of our language endowment compatible with those facts.

Understanding the universal aspects of human language structure requires comparison at multiple levels of analysis. While Evans & Levinson (E&L) focus mostly on substantive variation in language, equally revealing insights can come from studying formal universals. I first discuss how Artificial Grammar Experiments can test universal preferences for certain types of abstract phonological generalizations over others. I then discuss moraic onsets in the language Arrernte, and how its apparent substantive variation ultimately rests on a formal universal regarding syllable-weight sensitivity.

In our opinion, the discontinuity between extant human and nonhuman minds is much broader and deeper than most researchers admit. We are happy to report that Evans & Levinson's (E&L's) target article strongly corroborates our unpopular hypothesis, and that the comparative evidence, in turn, bolsters E&L's provocative argument. Both a Universal Grammar and the “mental continuity” between human and nonhuman minds turn out to be modern myths.

Evans & Levinson (E&L) advocate taking linguistic diversity at “face value.” Their argument consists of a list of diverse phenomena and the assertion that no non-vacuous theory could possibly uncover a meaningful unity underlying them. I argue, with evidence from Tlingit and Warlpiri, that E&L's list itself should not be taken at face value – and that the actual research record already demonstrates unity amidst diversity.

While endorsing Evans & Levinson's (E&L's) call for rigorous documentation of variation, we defend the idea of Universal Grammar as a toolkit of language acquisition mechanisms. The authors exaggerate diversity by ignoring the space of conceivable but nonexistent languages, trivializing major design universals, conflating quantitative with qualitative variation, and assuming that the utility of a linguistic feature suffices to explain how children acquire it.

Evans & Levinson's (E&L's) major point is that human languages are intriguingly diverse rather than (like animal communication systems) uniform within the species. This does not establish a “myth” about language universals, or advance the ill-framed pseudo-debate over universal grammar. The target article does, however, repeat a troublesome myth about Fitch and Hauser's (2004) work on pattern learning in cotton-top tamarins.

Modern linguistics has highlighted the fundamental invariance of human language: A rich invariant structure has emerged from comparative studies nourished by sophisticated formal models; languages also differ along important dimensions, but variation is constrained in severe and systematic ways. I illustrate this research direction in the domains of island constraints, word order restrictions, and the expression of referential dependencies. Both language invariance and language variability within systematic limits are highly relevant for the cognitive sciences.