A twin-case study of developmental number sense impairment.

The current study reports on 9-year-old monozygotic twin girls who fail to make any progress in learning basic mathematics in primary education. We tested the hypothesis that the twins' core maths problems were deficits in number sense that manifested as impairments in approximate and small number systems, resulting in impairment in nonsymbolic as well as in symbolic processing. While age-matched controls (eight typically developing girls) scored highly, the twins scored at chance on all number sense tasks. More specifically, on a nonsymbolic comparison task, even in the simplest ratio condition of 1:2, and on a subitizing task including only numbers under 4, the twins performed at chance and significantly below the same age control group. Responsiveness to an intervention promoting number sense is discussed. As differences between verbal and performance IQ suggest, there seems to be a high degree of specificity in the twins' developmental number sense delays. The concomitant impairments for visual-spatial processing and working memory in the twins might explain the failure to develop number sense.