albeit at lower frequency, in typically developing children. However, in autistic spectrum disorders, the intensity, frequency, duration, or persistence of the behaviors distinguish them from similar behaviors of normally developing young children. For example, several studies have shown that self-injurious and stereotyped behaviors occur in normal infants and then decrease, although they do not necessarily disappear, as locomotion develops in these children during the first and second years of life (Thelen, 1979; Werry et al., 1983). Body-rocking occurred in 19 percent and head-banging in 5 percent of one sample of typical children ages 3 to 6 years (Sallustro and Atwell, 1978). Similar levels of body-rocking have also been reported in normal college students (Berkson et al., 1999; Rafaeli-Mor et al., 1999).
These repetitive movements and potentially self-injurious behaviors are presumed to serve some function in normal development (Berkson and Tupa, 2000). Berkson and Tupa (2000) found that about 5 percent of toddlers with developmental disabilities (including autistic spectrum disorders) engaged in head-banging, about the same percentage as reported for typically developing children. The incidence of head-banging with actual injuries in the group with developmental disabilities is presumably greater: between 1.3 and 3.3 percent, depending on the type of measurement. This rate is similar to the prevalence rates reported for older, noninstitutionalized populations of children and adults with developmental disabilities (Rojahn, 1986; Griffin et al., 1987).
Understanding what causes these problem behaviors to emerge during the early childhood and preschool years, what maintains them, and what evokes their moment-to-moment expression holds promise of treatments to prevent them from becoming permanent and abnormal (Berkson and Tupa, 2000). Once moderate to severe problem behaviors become an established part of a child’s repertoire, unlike children with typical development, children with autistic spectrum disorders or other disabilities do not usually outgrow them. Without appropriate intervention, these behaviors persist and worsen (Schroeder et al., 1986).
With increasing research into the neurobiology and genetics of autism, the organicity of some aspects of behavior in autism is becoming clearer. For example, Lewis (1996) has attempted to explicate some of the underlying neurobiology of repetitive or stereotyped behaviors. Other researchers (Symons et al., 1999) have demonstrated that the locus of some types of self-injurious behavior might show different genetic patterns. Thompson and his colleagues (Thompson et al., 1995) argue that self-injurious behaviors have consequences other than social changes. For example, some self injurious behaviors involve the release of neurochemical transmitters and modulators that subsequently bind to specific brain receptors. By using sophisticated methods that study form, location, and intensity of self-injury, these researchers conclude that some people may