Cognitive Development 24 (2009) 430–449 Contents lists available at ScienceDirect Cognitive Development The trajectory of mathematics skills and working memory thresholds in girls with fragile X syndrome Melissa M. Murphy a , Michèle M.M. Mazzocco b,c,∗ a School of Education, College of Notre Dame of Maryland, Baltimore, MD, USA b Math Skills Development Project, Kennedy Krieger Institute, 3825 Greenspring Avenue, Painter Building, Baltimore, MD, USA c Department of Psychiatry and Behavioral Sciences, Johns Hopkins University School of Medicine, 3825 Greenspring Avenue, Painter Building, Baltimore, MD, USA article info Keywords: Mathematics skills Working memory Fragile X syndrome Mathematical learning disability abstract Fragile X syndrome is a common genetic disorder associated with executive function deficits and poor mathematics achievement. In the present study, we examined changes in math perfor- mance during the elementary and middle school years in girls with fragile X syndrome, changes in the working memory loads under which children could complete a cognitive switching task, and the association between these two areas of function, in girls with fragile X syndrome relative to their peers. Our find- ings indicate that the trajectory of math and executive function skills of girls with fragile X differs from that of their peers and that these skills contribute to predicting math achievement and growth in math performance over time. Also, changes in math performance were associated with incremental increases in work- ing memory demands, suggesting that girls with fragile X have a lower threshold for being able to perform under increasing task demands. Still, we found improvement in executive func- tion performance between 10 and 12 years in girls with fragile X rather than a performance plateau as has been reported in other studies. The findings implicate the importance of early ∗ Corresponding author at: Math Skills Development Project, Kennedy Krieger Institute, 3825 Greenspring Avenue, Painter Building, Baltimore, MD, USA. E-mail address: Mazzocco@jhu.edu (M.M.M. Mazzocco). 0885-2014/$ – see front matter © 2009 Elsevier Inc. All rights reserved. doi:10.1016/j.cogdev.2009.09.004