The ‘root infinitive’ phenomenon in child speech is known from major languages such as Dutch. In this case study, a child acquiring the Papuan language Nungon in a remote village setting in Papua New Guinea uses two different non-finite verb forms as predicates of main clauses (‘root’ contexts) between ages 2;3 and 3;3. The first root non-finite form is an apparent innovation of the child, unacceptable in adult-to-adult speech, which must be learned from a special auxiliary construction in child-directed speech. The second root non-finite form functions like attested adult main clause use of the same form. During the study period, the first root non-finite form increases sharply to function as a default verb form, then decreases to nil by 3;2. The second increases gradually to near-adult levels. Both forms are non-finite and have similar proportions in the input. Thus, factors other than finiteness and frequency must explain their distributions.

The clause chain is an under-investigated complex sentence type, found in hundreds of languages. In a clause chain, as many as 20 or more ‘medial’ clauses with under-specified verbal predicates combine with a single ‘final’ clause with fully-specified verbal predicate. Clause chains are of interest for three main reasons: (a) the special syntactic relationship between clauses, which is neither textbook subordination nor coordination; (b) the potential extreme length of a single chain; and (c) switch-reference marking in clause chains of some languages could require speakers to plan at least one clause ahead as they speak. Research on child production of complex sentences has largely overlooked clause chains. Longitudinal data for three children aged 1;1 to 3;3 acquiring the Papuan language Nungon show that Nungon-speaking children begin producing clause chains around the age of 2;4, with a marked increase in rate of use around age 2;11. Chain length is limited to two clauses until age 3;1. Different-subject marking in medial clauses is used by all three children early, but is first attested in one-clause, ‘root medial’ contexts, rather than in chains. Bayesian statistical models confirm the strong tendency for children to use root medials in expressions of desires and commands. Children’s production of three types of complex sentences—clause chains, subordinated final clauses, and coordinated final clauses—is shown to be subject to the same type of developmental constraint; but once development reaches an adequate level for increased complex sentence production, children acquiring Nungon produce many more clause chains than complex sentences involving subordinated or coordinated final clauses.

Papua New Guinea is home to >10% of the world’s languages and rich and varied biocultural knowledge, but the future of this diversity remains unclear. We measured language skills of 6,190 students speaking 392 languages (5.5% of the global total) and modeled their future trends using individual-level variables characterizing family language use, socioeconomic conditions, students’ skills, and language traits. This approach showed that only 58% of the students, compared to 91% of their parents, were fluent in indigenous languages, while the trends in key drivers of language skills (language use at home, proportion of mixed-language families, urbanization, students’ traditional skills) predicted accelerating decline of fluency to an estimated 26% in the next generation of students. Ethnobiological knowledge declined in close parallel with language skills. Varied medicinal plant uses known to the students speaking indigenous languages are replaced by a few, mostly nonnative species for the students speaking English or Tok Pisin, the national lingua franca. Most (88%) students want to teach indigenous language to their children. While crucial for keeping languages alive, this intention faces powerful external pressures as key factors (education, cash economy, road networks, and urbanization) associated with language attrition are valued in contemporary society.

Music is a vital part of most cultures and has a strong impact on emotions [1–5]. In Western cultures, emotive valence is strongly influenced by major and minor melodies and harmony (chords and their progressions) [6–13]. Yet, how pitch and harmony affect our emotions, and to what extent these effects are culturally mediated or universal, is hotly debated [2, 5, 14–20]. Here, we report an experiment conducted in a remote cloud forest region of Papua New Guinea, across several communities with similar traditional music but differing levels of exposure to Western-influenced tonal music. One hundred and seventy participants were presented with pairs of major and minor cadences (chord progressions) and melodies, and chose which of them made them happier. The experiment was repeated by 60 non-musicians and 19 musicians in Sydney, Australia. Bayesian analyses show that, for cadences, there is strong evidence that greater happiness was reported for major than minor in every community except one: the community with minimal exposure to Western-like music. For melodies, there is strong evidence that greater happiness was reported for those with higher mean pitch (major melodies) than those with lower mean pitch (minor melodies) in only one of the three PNG communities and in both Sydney groups. The results show that the emotive valence of major and minor is strongly associated with exposure to Western-influenced music and culture, although we cannot exclude the possibility of universality.