Stereotypic Movement Disorder 79
of stereotypic movements is in the early deve lopmental period (Criterion C). Criterion D
states that the repetitive, stereotyped behavior  in stereotypic movement disorder is not at-
tributable to the physiological effects of a su bstance or neurological  condition and is not
better explained by another ne urodevelopmental or mental disorder. The presence of
stereotypic movements may indicate an unde tected neurodevelopmental problem, espe-
cially in children ages 1–3 years.
Prevalence 
Simple stereotypic movements (e.g., rocking) ar e common in young typically developing chil-
dren. Complex stereoty pic movements are much less common (occurring in approximately
3%–4%). Between 4% and 16% of individuals with intellectual disability (intellectual develop-
mental disorder) engage in stereotypy and self-injury. The risk is greater in individuals with
severe intellectual disability. Among individual s with intellectual disability living in res-
idential facilities, 10%–15% may have stereo typic movement disord er with self-injury.
Development and Course 
Stereotypic movements typically begin within the first 3 years of life. Simple stereotypic move-
ments are common in infancy and may be involve d in acquisition of motor mastery. In chil-
dren who develop complex motor stereotypies, approximately 80% exhibit symptoms before
24 months of age, 12% between 24  and 35 months, and 8% at 36 months or older. In most typ-
ically developing children, these movements resolve over time or can be suppressed. Onset of
complex motor stereotypies may be in infancy or later in the develo pmental period. Among