Cognitive abilities of children in Costa Rica and Austria were compared using three age groups (N=385/366). Cognitive ability tests (mental speed, culture reduced/fluid intelligence, literacy/crystallized intelligence) were applied that differed in the extent to which they refer to school-related knowledge. Preschool children (kindergarten, 5–6 years old, NCR=80, NAu=51) were assessed with the Coloured Progressive Matrices (CPM), primary school children (4th grade, 9–11 years old, NCR=71, NAu=71) with ZVT (a trail-making test), Standard Progressive Matrices (SPM) and items from PIRLS-Reading and TIMSS-Mathematics, and secondary school students (15–16 years old, NCR=48, NAu=48) with ZVT, Advanced Progressive Matrices (APM) and items from PISA-Reading and PISA-Mathematics. Additionally, parents and pupils were given questionnaires covering family characteristics and instruction. Average cognitive abilities were higher in Austria (Greenwich-IQ MCR=87 and MAu=99, dIQ=12 points) and differences were smaller in preschool than in secondary school (dIQ=7 vs 20 points). Differences in crystallized intelligence were larger than in fluid intelligence (mental speed: dIQ=12, Raven: dIQ=10, student achievement tests: dIQ=17 IQ points). Differences were larger in comparisons at the level of g-factors. Austrian children were also taller (6.80 cm, d=1.07 SD), but had lower body mass index (BMICR=19.35 vs BMIAu=17.59, d=−0.89 SD). Different causal hypotheses explaining these differences are compared.