Using a combination of observational and parent report measures of sensory symptoms, significant associations were found between the hyperresponsive sensory construct and the presence of repetitive behaviors in children with autism and those with developmental delays. Thus, higher hyperresponsive scores were related to a variety of repetitive behaviors in both clinical groups; however, primarily non-significant associations were found between sensory seeking (craving/fascination with sensory stimuli) or hyporesponsiveness (behavioral under-reactivity to sensory stimuli) and repetitive behaviors. The one exception was the significant association found between sensory seeking and ritualistic/sameness behaviors. Preliminary findings suggest that sensory hyporesponsiveness and sensory seeking may be more associated with deficits in social-communication skills, at least in children with autism [Gay, Watson, Baranek, Poe, & Boyd, 2008
]. In addition, the associations between sensory features and repetitive behaviors did not vary between the clinical groups. However, children with autism in our sample did have significantly higher levels of the three types of sensory patterns and most types of repetitive behavior in comparison to the group of children with DD.
Further, the association between sensory features and repetitive behaviors remained the same (i.e., higher hyper scores were associated with higher repetitive behavior scores) after controlling for participants' MAs. It is important to highlight that previous research has suggested that both sensory features and repetitive behaviors are independently related to the cognitive abilities or developmental maturation of individuals with autism. For example, Baranek et al. 
reported that MA was a predictor of hyperresponsiveness in children with autism and those with DD. With repetitive behaviors, it has been found that individuals with lower IQ levels, in particular nonverbal IQ, displayed more types of repetitive behaviors [Bishop, Reichler, & Lord, 2006
; Bodfish, Symons, Parker, & Lewis, 2000
]. Thus, developmental or cognitive maturation was predictive of the actual expression of these behaviors in children even though this variable may not have played a significant role in how these behaviors were related to one another. Other shared mechanisms may be responsible for their co-existence. A study by Gabriels et al. 
provided support for this finding. Those researchers also found that the actual relationship between abnormal sensory responses and repetitive behaviors was not affected by the IQ levels of the participants with autism, although their participants were older in age than our study sample (mean age = 10.8 years), and the researchers did not explore the different patterns of sensory features (i.e., hyper, hypo, and seeking). Understanding the relationship between these behaviors has both theoretical and clinical implications for the role of shared neurobiological mechanisms and treatment development.
While the present study did not directly test for physiological or biological correlates of these behaviors, previous research allows us to make theoretical postulations about the mechanisms underlying their relationship. First, it is important to highlight that research links both abnormal sensory features and repetitive behaviors to neuropathological mechanisms occurring early in development, which result in atypical sensory processing (e.g., long range underconnectivity in the cerebral cortex; and/or lack of “functional” connectivity) [Gomot, Belmonte, Bullmore, Bernard, & Baron-Cohen, 2008
; Tommerdahl, Tannan, Holden, & Baranek, 2008
] or aberrant restricted/repetitive behaviors (e.g., structural or functional differences in the caudate nucleus or frontal lobe) [Langen, Durston, Staal, Palmen, & van England, 2007
; Shafritz, Dichter, Baranek, & Belger, 2008
; Thakkar et al., 2008
]. We found a specific association between hyperresponsiveness and compulsive or ritualistic/sameness behaviors in both the individual and simultaneous data analysis models. Based on behavioral and physiological research on obsessive–compulsive disorder (OCD), a neurodevelopmental disorder that shares pathogenic and phenomenological similarities with autism [Rapoport & Inoff-Germain, 2000
], researchers have found that anxiety plays a major role in the expression of compulsive/ritualistic behaviors [Abramowitz, Whiteside, & Deacon, 2005
; Aouizerate et al., 2004
; Piacentini & Langley, 2004
]. Individuals with OCD often engage in these behaviors to alleviate anxiety that reoccurs as a result of obsessive thoughts. Hyperawareness of, or hypersensitivity to, stimuli in the environment (e.g., tags on the back of a child's shirt) may lead to similar feelings of anxiety, and subsequent engagement in compulsive or ritualistic behaviors to alleviate those feelings.
In typical development, researchers have speculated that ritualistic or OCD-like behaviors serve as compensatory mechanisms in fearful situations [Evans, Gray, & Leckman, 1999
; Zohar & Felz, 2001
], and allow individuals to establish predictability and control in a perceived chaotic environment [Fiske & Haslam, 1997
; Zohar & Felz, 2001
]. Compulsive and ritualistic types of repetitive behaviors in typically developing children begin to abate around six years of age [Evans et al., 1999
]; yet, for children with developmental disorders these behaviors may continue to serve a compensatory strategy due to other cognitive capacities taking longer to come on-line, such as inhibitory control [Mosconi et al., 2009
]. Further, at a biological level, some studies have linked the amygdala, which plays a role in regulating our reaction to environmental stimuli that are perceived as fearful or anxiety-provoking, to repetitive behavior in autism [Dziobek, Fleck, Rogers, Wolf, & Convit, 2006
]. It is possible that the amygdala also plays a role in the expression of hyperresponsive sensory symptoms. We also found that hyperresponsiveness and stereotypy were correlated, and one of the long standing psychological theories has been that engagement in stereotypies served a homeostatic function and increased in situations associated with high levels of arousal [Turner, 1999
]. In our study, sensory seeking was only significantly associated with ritualistic/sameness behaviors. This may indicate that there is a different pathogenesis associated with children who display hyperresponsiveness and ritualistic behaviors versus those who seek out certain stimuli in their environment but also display ritualistic/sameness behaviors. Further research is needed to fully explicate the neurobiological underpinnings of our behavioral findings.
A multi-method, multi-trait approach to characterizing key sensory constructs, as used in this study, provides a more reliable and valid phenotypic characterization of sensory features that can be used as a basis for future experimental studies of pathogenesis in autism. We need additional research to address how co-presentation of aberrant sensory and repetitive behavior features relates to the child's adaptive functioning. It is plausible that a subgroup of children who present with high levels of both types of behaviors could have more trouble with everyday functioning in the home and community. Alternately, some theories [Baron-Cohen, Ashwin, Ashwin, Tavassoli, & Chakrabarti, 2009
] postulate that hypersensitivities (i.e., enhanced sensory perception; lower sensory thresholds) may lead to greater attention to detail and thus facilitate hypersystemizing “talents” in high-functioning persons with autism. Thus, it is unclear to what extent sensory processing differences as found in this study may have adaptive vs. maladaptive effects on different individuals.
In conclusion, this study suggests that there is primarily a co-occurrence of hyperresponsive sensory features and repetitive behaviors in children with autism and DD, and these associations may be related to shared neurobiological mechanisms and have implications for diagnostic classification. Behavioral or educational treatments may need to address the co-presentation of these symptoms to maximize intervention effectiveness. Further research is needed to understand why the three sensory constructs are differentially related to core features of autism, and to determine neurobiological mechanisms that underlie these associations.