Our research on statistical language learning shows that infants, young children, and adults can compute, online and with remarkable speed, how consistently sounds co-occur, how frequently words occur in similar contexts, and the like, and can utilize these statistics to find candidate words in a speech stream, discover grammatical categories, and acquire simple syntactic structure in miniature languages. However, statistical learning is not merely learning the patterns presented in the input. When their input is inconsistent, children sharpen these statistics and produce a more systematic language than the one to which they are exposed. When input languages inconsistently violate tendencies that are widespread in human languages, learners shift these languages to be more aligned with language universals, and children do so much more than adults. These processes explain why children acquire language (and other patterns) more effectively than adults, and also may explain how systematic language structures emerge in communities where usages are varied and inconsistent. Most especially, they suggest that usage-based learning approaches must account for differences between adults and children in how usage properties are acquired, and must also account for substantial changes made by adult and child learners in how input usage properties are represented during learning.

Numerous factors are thought to be advantageous for non-native language learning although they are typically investigated in isolation, and the interaction between them is not understood. Firstly, bilinguals are claimed to acquire a third language easier than monolinguals acquire a second. Secondly, closely related languages may be easier to learn. Thirdly, certain phonetic features could be universally more difficult to acquire. We tested these hypotheses used as explanations by having adults learn vocabularies that differentiated words using foreign phonetic contrasts. In Experiment 1, Mandarin–English bilinguals outlearned English monolinguals, and the Mandarin-like (retroflex) artificial language was better learned than the English-like (fricative voicing). In Experiment 2, bilinguals again outlearned English monolinguals for the Mandarin-like artificial language. However, only Korean–English bilinguals showed an advantage for the more difficult Korean-like (lenition) language. Bilinguals, relative to monolinguals, show a general advantage when learning ‘easy’ contrasts, but phonetic similarity to the native language is useful for learning universally ‘difficult’ contrasts.

As more of the world's languages are described and compared, more absolute universals have joined the class of statistical tendencies. However, few have questioned the universality of the duality of patterning. Following Hockett, most linguists assume that in all human languages, discrete meaningless parts combine to form meaningful units that, themselves, recombine. However, an alternative interpretation, explored in this article, is that duality, like other proposed linguistic universals, is a statistical tendency reflecting a complex set of factors, and most centrally, the need for some minimal number of basic units that can recombine to yield a potentially infinite set of form-meaning correspondences. If this is the essence of duality, then we expect: languages where duality is not a central component of grammar; languages where most, but not all, utterances are decomposable into meaningless phonological units; and different types of phonological building blocks in different languages. These expectations appear to be confirmed by natural language data.

Many people have argued that the evolution of the human language faculty cannot be explained by Darwinian natural selection. Chomsky and Gould have suggested that language may have evolved as the by-product of selection for other abilities or as a consequence of as-yet unknown laws of growth and form. Others have argued that a biological specialization for grammar is incompatible with every tenet of Darwinian theory – that it shows no genetic variation, could not exist in any intermediate forms, confers no selective advantage, and would require more evolutionary time and genomic space than is available. We examine these arguments and show that they depend on inaccurate assumptions about biology or language or both. Evolutionary theory offers clear criteria for when a trait should be attributed to natural selection: complex design for some function, and the absence of alternative processes capable of explaining such complexity. Human language meets these criteria: Grammar is a complex mechanism tailored to the transmission of propositional structures through a serial interface. Autonomous and arbitrary grammatical phenomena have been offered as counterexamples to the position that language is an adaptation, but this reasoning is unsound: Communication protocols depend on arbitrary conventions that are adaptive as long as they are shared. Consequently, language acquisition in the child should systematically differ from language evolution in the species, and attempts to analogize them are misleading. Reviewing other arguments and data, we conclude that there is every reason to believe that a specialization for grammar evolved by a conventional neo-Darwinian process.