Children differ in their ability to learn what is taught at school. Evidence from twin studies suggests that genetic effects contribute to such differences. The aim of the present study was to systematically review the existing literature, including 61 studies from 11 cohorts, on twin studies of educational achievement in primary school children. The meta-analysis estimated heritability, based on up to 5330 MZ and 7084 DZ twin pairs, at 73% for reading, 49% for reading comprehension, 57% for mathematics, 44% for spelling, 64% for language and 66% for educational achievement. The importance of genetic effects on educational achievement differed between countries. Heritability was consistently high in the Netherlands across educational domains, while this was not always true for the USA and the UK. It can be concluded that genetic variation is an important contributor to the individual differences in educational achievement, with some indication for interaction with country.