The paper examines the relationship between cognitive ability at thirteen years of age and children's academic performance assessments at aged nine. Alongside cognitive ability, other variables considered predictive of academic success were assessed including personality measures, birthweight, handedness, socio-economic background, parental education, home language, and child-rearing practices such as breast-feeding and access to video-games. The final sample comprised 7525 children who participated in both wave 1 and wave 2 of the Growing Up in Ireland (GUI) longitudinal study. Participants in the study were selected through the state school system using a 2-stage sampling method producing a large sample representative of the national population of nine-year-old children. Linear multiple regression identified five variables which significantly explained both reading and mathematics test scores: two cognitive ability measures, birthweight, wealthier households, and high attendance at parent-teacher meetings. Gender, parental education, and home language also made a contribution to reading test scores, while a general factor of personality was significant for mathematics. Overall the cognitive ability measures accounted for almost all of the explained variance, and other factors, while sometimes statistically significant, were of relatively minor importance.