A diagnostic issue relating to repetitive behaviour in autism is the difficulty in differentiating it from normal early development. Sallustro and Atwell’s observations of normally developing children indicated that rhyth-mic behaviours are common in infants [25]. Research conducted by Thelen reported that a variety ofrhythmic and stereotyped behaviours are frequently performed by normal infants within their first year oflife [33].It was suggested that these behaviours are manifestations of “incomplete cortical control in maturing neuromus-cular pathways” correlated with motor skill acquisition [33,p.699].
Research indicates that frequencies ofcompulsive-like behaviour fluctuate in normally developing chil-dren aged between 2 and 4 years [9].This behaviour may include an insistence on sameness,such as always want-ing to read the same book.As early as 1928, Gesell pro-posed that these ritualistic,repetitive behaviours might have an adaptive purpose, helping individuals to deal with environmental change [11].
Another challenge to early diagnosis is differentiat-ing the features of autism and intellectual disability or developmental delay. Because 70–80% of people with autism also have an intellectual disability [3,37],Lord has suggested that, in diagnosis, the behavioural fea-tures of autism must be differentiated from those due to developmental delay [15].The symptoms of autism can be difficult to distinguish from behavioural features of developmental delay and may account for a delay in the diagnosis of autism [18,23].It is,therefore,essential that research examining the early diagnosis ofautism com-pares children with autism to those with developmental delay who do not have autism in order to identify symp-toms that are specific to autism.