To examine whether children’s acquisition of perspective-marking language supports development in their ability to reason about mental states, we conducted a longitudinal study testing whether proficiency with complement clauses around age three explained variance in false-belief reasoning six months later. 45 English-speaking 2-and-3-year-olds (23 female, Time 1 age range: 33-41 months) from middle-class families in the North-West of England took part in the study, which addresses a series of uncertainties in previous studies: We avoided the confound of using complement clauses in the false-belief tests, assessed complement-clause proficiency with a new comprehensive test designed to capture gradual development, and controlled for individual differences in executive functioning that could affect both linguistic and sociocognitive performance. Further, we aimed to disentangle the influence of two aspects of complement-clause acquisition: proficiency with the perspective-marking syntactic structure itself and understanding of the specific mental verbs used in this syntactic structure. To investigate direction of causality, we also tested whether early false-belief reasoning predicted later complement-clause proficiency. The results provide strong support for the hypothesis that complement-clause acquisition promotes development in false-belief reasoning. Proficiency with the general structure of complement-clause constructions and understanding of the specific mental verbs “think” and “know” in 3rd-person complements at Time 1 both contributed uniquely to predicting false-belief performance at Time 2. However, false-belief performance at Time 1 also contributed uniquely to predicting complement-clause proficiency at Time 2. Together, these results indicate a bidirectional relationship between linguistic and sociocognitive development.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. LuciD's TECHNOLOGY AGENDA will develop new multi-method approaches and create new technology products for researchers, healthcare and education professionals. We will build a 'big data' management and sharing system to make all our data freely available; create a toolkit of software (LANGUAGE RESEARCHER'S TOOLKIT) so that researchers can analyse speech more easily and more accurately; and develop a smartphone app (the BABYTALK APP) that will allow parents, researchers and practitioners to monitor, assess and promote children's language development. With the help of six IMPACT CHAMPIONS, LuCiD's COMMUNICATIONS AGENDA will ensure that parents know how they can best help their children learn to talk, and give healthcare and education professionals and policy-makers the information they need to create intervention programmes that are firmly rooted in the latest research findings.

Participants were recruited from Lancaster University Babylab’s database. The vast majority of participants in the database come from a white middle-class background and more than 70% of the caregivers attended higher education, with about one third holding a postgraduate degree. Fifty one 2-and-3-year-olds took part. Three were excluded due to not engaging with multiple tasks, leaving 48 children (24 female, age range: 33-41 months, M = 36.85, SD = 2.34) included at T12. We recruited eight boys and eight girls in each of three 3-month-windows (2;9-2;11 years; 3;0-3;2 years; 3;3-3;5 years). All participants were monolingual English speakers with no known history of language or hearing impairment. Forty four percent had no siblings, 37.5% were the youngest sibling, 14.5% were middle children and 4% were the oldest child in their family. The retention rate for T2 was 93.75% (n = 45) with three participants dropping out between T1 and T2. The ages of the children at T2 ranged from 39 to 47 months (M = 42.93, SD = 2.46). The study, “Language and social cognition in pre-schoolers”, received ethical approval from the Faculty of Arts and Social Sciences and Lancaster Management School’s Research Ethics Committee at Lancaster University (UREC: FL16283).