The authors investigated the etiology of low mathematics performance in 7-year-olds in the context of normal variation. The lowest 15% were selected from a representative U.K. sample of 2,178 same-sex twin pairs rated by their teachers according to National Curriculum criteria in 3 domains of mathematics. Model-fitting analyses of mathematics performance and low performance yielded remarkably similar results. The heritability estimate for the teacher-reported mathematics performance composite is .66, whereas the group heritability estimate for low performance is .65. These results are consistent with the hypothesis that the same genetic and environmental factors are largely responsible for low mathematics performance as for normal variation in mathematics. The limitations of the present study and further directions for research are discussed.