to imitate gesture or vocalization may be deficient (Sigman and Ungerer, 1984b; Sigman et al., 1997). The difficulties in imitation begin early (Prior et al., 1975) and are persistent (Sigman and Ungerer, 1984b; Otah, 1987). The specificity of these difficulties has been the topic of some debate (Smith and Bryson, 1994), although it is clear that children with autism usually have major difficulties in combining and integrating different kinds of information and their responses (Rogers, 1998).
Although sensorimotor skills may not appear to be highly deviant in some younger children with autism, aspects of symbolic play and imagination, which typically develop during the preoperational period, are clearly impaired (Wing et al., 1977; Riquet et al., 1981). Children with autism are less likely to explore objects in unstructured situations (Kasari et al., 1993; Sigman et al., 1986). Younger children with autism do exhibit a range of various play activities, but the play is less symbolic, less developmentally sophisticated, and less varied than that of other children (Sigman and Ungerer, 1984a). These problems may be the earliest manifestations of what later will be seen to be difficulties in organization and planning (“executive functioning”) (Rogers and Pennington, 1991). Thus, younger children with autism exhibit specific areas of deficiency that primarily involve representational knowledge. These problems are often most dramatically apparent in the areas of play and social imitation. As Leslie has noted (e.g., Leslie, 1987), the capacity to engage in more representational play, especially shared symbolic play, involves some ability for metarepresentation. Shared symbolic play also involves capacities for social attention, orientation, and knowledge, which are areas of difficulty for children with autism.
IQ scores have been important in the study of autism and autistic spectrum disorders. To date, scores on intelligence tests, particularly verbal IQ, have been the most consistent predictors of adult independence and functioning (Howlin, 1997). IQ scores have generally been as stable for children with autism as for children with other disabilities or with typical development (Venter et al., 1992). Though fluctuations of 10– 20 points within tests (and even more between tests) are common, within a broad range, nonverbal IQ scores are relatively stable, especially after children with autism enter school. Thus, nonverbal intelligence serves, along with the presence of communicative language, as an important prognostic factor. Epidemiological studies typically estimate that about 70 percent of children with autism score within the range of mental retardation, although there is some suggestion in several recent studies that this proportion has decreased (Fombonne, 1997). This change may be a function of more complete identification of children with autism who are