The aim of this large-scale, preregistered, cross-linguistic study was to mediate between theories of the acquisition of inflectional morphology, which lie along a continuum from rule-based to analogy-based. Across three morphologically rich languages (Polish, Finnish and Estonian), 120 children (mean age 48.32 months, SD = 7.0 months) completed an experimental, elicited-production study of noun case marking. Confirmatory analyses found effects of surface-form (whole-word, token) frequency for Polish and Estonian, and phonological neighbourhood density (PND) for all three languages (using either our preregistered class-based or an exploratory form-based measure). An exploratory all-languages analysis yielded both main effects, and a predicted interaction, such that the effect of PND was greater for forms with lower surface-form frequency, which are less available for direct retrieval from memory. Cross-linguistic differences were investigated with exploratory analyses of case variance, affix syncretism and stem changes. We conclude that these findings are difficult to reconcile with accounts that posit rules or linguistic abstractions and are most naturally explained by analogy-based connectionist or exemplar accounts.The International Centre for Language and Communicative Development (LuCiD) will bring about a transformation in our understanding of how children learn to communicate, and deliver the crucial information needed to design effective interventions in child healthcare, communicative development and early years education. Learning to use language to communicate is hugely important for society. Failure to develop language and communication skills at the right age is a major predictor of educational and social inequality in later life. To tackle this problem, we need to know the answers to a number of questions: How do children learn language from what they see and hear? What do measures of children's brain activity tell us about what they know? and How do differences between children and differences in their environments affect how children learn to talk? Answering these questions is a major challenge for researchers. LuCiD will bring together researchers from a wide range of different backgrounds to address this challenge. The LuCiD Centre will be based in the North West of England and will coordinate five streams of research in the UK and abroad. It will use multiple methods to address central issues, create new technology products, and communicate evidence-based information directly to other researchers and to parents, practitioners and policy-makers. LuCiD's RESEARCH AGENDA will address four key questions in language and communicative development: 1) ENVIRONMENT: How do children combine the different kinds of information that they see and hear to learn language? 2) KNOWLEDGE: How do children learn the word meanings and grammatical categories of their language? 3) COMMUNICATION: How do children learn to use their language to communicate effectively? 4) VARIATION: How do children learn languages with different structures and in different cultural environments? The fifth stream, the LANGUAGE 0-5 PROJECT, will connect the other four streams. It will follow 80 English learning children from 6 months to 5 years, studying how and why some children's language development is different from others. A key feature of this project is that the children will take part in studies within the other four streams. This will enable us to build a complete picture of language development from the very beginning through to school readiness. Applying different methods to study children's language development will constrain the types of explanations that can be proposed, helping us create much more accurate theories of language development. We will observe and record children in natural interaction as well as studying their language in more controlled experiments, using behavioural measures and correlations with brain activity (EEG). Transcripts of children's language and interaction will be analysed and used to model how these two are related using powerful computer algorithms.

The three languages were tested on location, in nurseries in countries where the language is spoken by the majority. All tested children were reported to be typically developing, monolingual speakers of their respective languages. The ages ranged between 3;0–5;0, such that children would be sufficiently old to be able to complete the task, and sufficiently young to be expected to produce errors. We had a balanced sample size of 40 children per language group, with a span of just over two months between the mean ages of the groups. In all three languages, surface-form frequency and PND counts (both PND class and PND form) were obtained automatically from child-directed speech corpora, using a custom-written R script. The study employed an elicited-production method and within-subjects design. Due to the large number of cases in each of the languages, it was not possible for us to test children on every case. Instead, we selected the most frequently occurring singular cases, based on the child-directed speech corpora in each language. Each case was represented pictorially using a unique elicitation context involving a fox or hare character interacting with each target noun object. To prevent fatigue effects, each participant was tested on all nouns, but each in only three of the five (Polish) or six (Finnish and Estonian) case contexts. This resulted in 90 (Polish), 81 (Finnish) and 72 (Estonian) trials per participant. Each child was tested individually in a quiet setting at their nursery. The full set of trials for each participant was presented in two experimental sessions, lasting approximately 15 min each. There was a break between the experimental sessions; the second session was conducted either a few hours after the first session or on a subsequent visit during the next few days. In each experimental trial, the participant was presented with a picture of the object on the screen and told the name of the object in nominative form. S/he was then shown the stimulus picture (i.e., the object in a case elicitation context), and the experimenter produced the beginning of the context sentence, for example: 1) Experimenter: ‘This is an apple-NOM.’ [screen shows an apple] 2) Experimenter: ‘The fox looks at…’ [screen shows picture of the fox looking at an apple] 3) Participant: ‘apple-ACC’