A behavioral genetic analysis of general writing ability was conducted using teacher assessments based on UK National Curriculum criteria for a sample of 3296 same-sex pairs of 7-year-old twins. Writing was highly heritable within the normal range (0.66) and at the low extreme (0.70). Environmental influences were almost all non-shared, with shared environmental estimates negligible (0.07 normal range, 0.06 low extreme). Bivariate genetic analyses of teacher assessments of writing and reading indicated that 69% of the genetic influences on writing were in common with those on reading, and that the two skills shared all of their shared environmental influences. In contrast, non-shared environmental factors are largely specific, with 86% independent of those on reading.