Onset is in the early developmental period. Stereotypic movements typically begin within the first 3 years of life. Simple stereotypic movements are common in infancy and may be involved in acquisition of motor mastery. In children who develop complex motor stereotypes, approximately 80% exhibit symptoms before age 24 months, 12% between 24 and 35 months, and 8% at 36 months or older. In most typically developing children, the severity and frequency of stereotyped movements diminish over time. Onset of complex motor stereotypies may be in infancy or later in the developmental period. Among individuals with intellectual developmental disorder, the stereotyped, self-injurious behaviors may persist for years, even though the typography or pattern of self-injury may change.