AbstractFragile X syndrome (FXS) is a well‐recognized cause of mental retardation and developmental delay in males. Alongside the well‐documented clinical characteristics of the condition, recent advances in technology and methodology have begun to define FXS at a number of different levels: genetic, brain structure and function, cognition, and behavior. This article suggests that the FXS phenotype is not merely a juxtaposition of spared and impaired functions but rather may be characterized by an inhibitory control deficit that interferes with the individual's ability to modulate output causing perseverative responding across various skill areas. It is further suggested that an inability to modulate arousal may be at least one cause for the inhibitory control deficit that typifies the FXS phenotype. The approach to understanding atypical development outlined here holds exciting promise for future research in FXS and other developmental disorders. MRDD Research Reviews 2004;10:11–16. © 2004 Wiley‐L