Temperament and vagal tone in boys with fragile X syndrome.

Physiological hyperarousal, an elevated state of physiological arousal and poor modulation, has been postulated to be a significant source of behavior problems in children with fragile X syndrome (FXS). Temperament has been associated with behavior problems and may also reflect biological reactivity. Young boys with FXS display poorly modulated and low levels of vagal tone (Roberts, Dev Psychobiol 2001;39:107-123) and high activity, poor attention, low adaptability, poor persistence, and low intensity when compared with a reference sample of typically developing (Hatton, Dev Med Child Neurol 1991;41:625-632). In this study, we integrated physiological indices of vagal tone with temperament ratings and compared a sample of 29 young boys with FXS to 31 typically developing boys matched on chronological age and ethnicity. Boys with FXS were more active, less adaptable, and less persistent than the comparison group. Boys with FXS also showed lower baseline levels and less suppression of vagal tone in response to changing task demands. A relationship between baseline vagal tone and persistence was shown across both groups. However, group differences in temperament dimensions did not appear to be mediated or moderated by vagal tone.