The strong and growing recognition of the importance of qualitative aspects of the input on language development was shown by the large number of submissions we received to our call for a special issue on this topic, such that the issue spilled over into two parts. In Part 1, in our January 2020 issue, contributors discussed many aspects of the characteristics and importance of infant-directed or child-directed speech, from infancy to preschool age, and examined the input to special populations.

Here in Part 2, we examine a number of themes across the studies in the context of the impact of input on language outcomes for the child. We begin by continuing the theme of unique populations by examining the development of minority or heritage languages. Antonijevic, Muckley and Müller describe the influence of consistency and accuracy in the development of Irish, a minority language undergoing rapid change. Bosma and Blom compare the relative influence of different activities on lexical and morphological development and find a greater influence of book reading for Frisian (minority language) than Dutch (majority language). The study of Sun, Yussof, Vijayakumar, Lai, O'Brien and Ong, situated in Singapore, continues the theme of minority/heritage languages but introduces our next theme of language input in a school context by examining the impact of teacher code-switching behaviours on a variety of measures of child language and cognitive development. In a sample of largely African American children in Head Start pre-K classrooms, Barnes, Griefenhagen and Dickinson used cluster analysis and multi-level modeling to describe the relationship between different teacher-child discourse behaviours in a lunchtime setting and child vocabulary development in English.

Next, a cluster of papers explore various aspects of the home environment and parental behaviours and relate these to child language characteristics. Alper, Hurtig and McGregor employ a randomized control trial approach to provide evidence in favour of the efficacy of a parent-training program focused on parental perceived self-efficacy and locus of control. Treat, Sheffield Morris, Hays-Grudo and Williamson study the relationship of maternal depression longitudinally to maternal behaviours and child vocalisations and conversational turns. Chang and Luo, in a Taiwanese sample, find changes over time in mother-child interaction behaviours during book reading and correlations between specific behaviours and child language measures. Grimminger, Rohlfing, Luke, Liszkowski and Ritterfeld find evidence for decontextualized talk in caregiver speech to infants as young as 12 months, and assess the relationship between this behaviour and child linguistic development at 12 and 24 months. Mimeau, Cantin, Tremblay, Boivin and Dionne, follow a large sample of infants and their caregivers from 5 months to 5 years. They show associations both from maternal to child characteristics and vice versa, highlighting the importance of considering these bidirectional influences.

Our final cluster of papers takes a more experimental approach to considering input factors. De Pablo, Murillo and Romero return to the theme of minority/heritage languages by studying infants’ responses to infant-directed speech in Spanish and Basque – infants attend more and produced more multimodal communicative behaviours in response to infant-directed speech regardless of the language used. Fais and Vatikiotis-Bateson find evidence for stronger word-object associations when the stimuli were based on speech to a (verbal) 24-month-old than to a (preverbal) 5-month-old, although this effect was found only for female infants. Last but not least, Lany and Shoaib also observe gender differences in the relationship between measures of infants’ exposure to infant-directed speech at home (and their own language receptive language skills) on sensitivity to non-adjacent statistical dependencies.

These twelve papers showcase the diversity of research being conducted on the complex topic of input quality and interaction in children's language learning experiences. Important longstanding topics are still going strong, such as the role of infant-directed speech, or associations between input and outcome variables. At the same time, new directions are clearly emerging, such as combining measures of the home environment with experimental approaches, or examining aspects of multilingual input.