The researchers in this study were interested in looking at differences in physiological arousal between infants and toddlers with fragile X syndrome and those with typical development.

They looked at physiological predictors early in development to autism severity later in development in fragile X syndrome. Participants were 31 male children with fragile X syndrome aged between 8 and 40 months and 25 age-matched control participants.

They found that the group with fragile X syndrome showed shorter interbeat intervals  (the time interval between individual beats of the heart usually measured in milliseconds). lower vagal tone and less modulation of the interbeat intervals. They found a nonlinear effect with interbeat intervals and autistic behaviour; but they did find a linear effect with vagal tome and autistic behaviour.

They suggest that atypical physiological arousal might emerge within the first year and could predict severity of autistic behaviour in fragile X syndrome. The authors conclude that their findings have

important implications for the early identification and treatment of autistic behaviours in young children with fragile X syndrome.

Heart Activity and Autistic Behavior in Infants and Toddlers With Fragile X Syndrome, Roberts J et al., in  American Journal on Intellectual and Developmental Disabilities, 117,. 2, 90-102.