In a companion paper, word recognition skills assessed by telephone using the Test of Word Reading Efficiency (TOWRE) were found to correlate highly with National Curriculum (NC) teacher-assessed reading ability in 7-year-old twins. This study examined the genetic and environmental origins of this high correlation. TOWRE and NC scores were both highly heritable and the correlation between them was largely due to overlapping genetic effects. These findings were obtained both across the normal range of reading abilities and at the low extreme, defined by scores below a 13.4% cut-off on either measure. TOWRE and NC scores may provide promising phenotypes for further study of the aetiology of early reading abilities and disabilities.