Abstract

Due to their frequent uneven cognitive profiles, genetic developmental disorders allow researchers to investigate which numerical sub-system of those present in typically developing infants best predicts subsequent numerical abilities. More importantly, they can provide evidence of which other cognitive abilities outside number are necessary for the successful development of these numerical sub-systems. We discuss evidence from cross-syndrome comparisons of adults, adolescents, children, and infants with Williams syndrome and those with Down syndrome to show that the approximate magnitude sub-system is crucial for later number development. In addition, we show that specific problems outside the number domain, and within basic-level visual and attention systems contribute to an explanation of the difficulties and proficiencies observed within each genetic disorder. Finally, we argue that a truly developmental approach is critical when using the cross-syndrome design in order to reveal subtle differences that impact over time on the development of cognitive abilities.