Study title

England

Individual

We tested 92 children and 17 adult controls. The children were recruited through nurseries and primary schools in the Manchester area (North-West of England), and at the Manchester Museum. Prior informed consent was obtained from caregivers/parents. All children were monolingual, native speakers of English without any known history of speech or language problems or developmental delays. Of the 92 child participants, 40 were between 3;6 to 4;5 years old (M = 48 months, SD = 3 months, 17 girls), 40 were between 4;6 and 5;5 years old (M = 60 months, SD = 2.8 months, 22 girls), and 12 were between 7;1 and 9;6 years old (M = 8;3, SD = 0;1, 7 girls). We will refer to the first group as the four-year-olds, the second group as the five-year-olds, and the third group as the eight-year-olds. Nine additional children were tested, but their data had to be excluded because they didn’t understand the task (five children), turned later out to be older than the targeted age range (three children), or because they did not want to continue after the warm-up (one child). In addition, one child chose not to do the second session, and one child did not do the digit-span (memory) task. For one child, the data for the dimensional change card sort (inhibition) were lost due to experimenter error. As mixed-effects models deal well with missing data, the data of these three participants were retained in the final data set. The adult participants (N = 17, M = 35 years, 13 women) were visitors to the Manchester Museum and students or staff members from the University of Manchester, and native speakers of English. One additional adult participant’s data were excluded because he was a non-native speaker of English. Due to a technical error, for two adult participants, the final trials were not recorded, resulting in the loss of two trials. Note that the initial study plan (see pre-registration) contained only four- and five-year-olds and adults. Because we later found that five-year-olds were far from adult-like, we tested an additional, smaller sample of eight-year-olds to get a more comprehensive picture of the developmental trend. For reasons of interpretation we present the eight-year-olds’ data together with the other data.The four- and five-year-old children were tested in a quiet area in their nurseries and primary schools. In addition to the sentence comprehension test, the children completed six tasks on general language ability and vocabulary, working and short-term memory, and inhibitory control, which are detailed below. The tasks were spread over two sessions on two separate (and typically consecutive) days. Each session lasted between 25 and 40 min. Children completed half of all items of the sentence comprehension task in session one, and the other half in session two. In each session, the children also completed one inhibition task, one general language task, and one memory task. With the exception of the first inhibition task (Flanker task), children always first completed the sentence comprehension task before doing the other tasks (see Appendix 1 for details). The eight-year-olds and the adults were tested in the Study area at the Manchester Museum and at the Child Study Centre at the University of Manchester. They only did the sentence comprehension task and the digit span (memory) task. They completed all items in one session, with a short break between two blocks. The allocation of trials across sessions and the experimental lists are described in Experimental lists (Section 2.3.5) below.