the proximity of primary schools to students' homes constitutes a major factor in primary school enrollment, the socioeconomic status (SES) of the catchment area in which the schools were situated could be considered proxy to the SES of their students. In this regard, the schools in Hong Kong and UK were located in predominantly middle-class areas. Testing in Hong Kong was done in August while testing in the UK was done in October of the same year. To ensure similarity of age and years in school, Hong Kong students were at the end of their first and third grade while UK students were at the start of their second and fourth grade. Written informed consent was obtained from parents of all participants. The study was approved by theCentral University Research Ethics Committee of University of Oxford.

At the Chinese-medium school in Hong Kong (henceforth HK-C), Cantonese was the first language for all children, who came from Chinese-speaking homes. They received a Chinese-medium education, and were taught Mathematics in Cantonese. There were 25 first-graders and 25 third-graders from HK-C. At the English-medium school in Hong Kong (henceforth HK-E), Cantonese was the first language of the children, and they spoke Chinese at home. However, they received education in English for most school subjects including Mathematics. There were 37 first-graders and 16 third-graders from HK-E. At the British school in Oxford (henceforth UK), English was the first language of the children. They spoke English at home and at school, with no exposure to the Chinese language. There were 26