Although classically considered a unitary domain of behavior, the current study adds to a growing body of work suggesting meaningful subtypes of restricted repetitive behavior exist in autism. Most of these studies have suggested two potential subtypes of RRB (‘lower order’ RSMB and ‘higher order’ IS). The current study closely replicates these two factors (our IS factor is identical to those found by Cuccaro et al., (2003)
, Shao et al., (2003)
, and Szatmari et al. 
and our RMB factor is quite similar to these previous studies with the exception of the removal of the ‘Unusual Sensory Interests’ item). However, we also found evidence for an additional factor, Circumscribed Interests. To examine whether or not our novel finding was due to our exclusion of the sensory item, we ran the PCA and EFA again with ‘Unusual Sensory Interests’ included, which still revealed a three-factor solution with a distinct CI factor. We also repeated our factor analyses on the ‘ever’ ratings, which also replicated the three-factor solution reported here. Therefore, our data strongly suggest that circumscribed interests are a distinct subgroup of repetitive behavior in autism, which may have potentially important implications in both research and clinical settings.
Circumscribed interests, as identified in this study, include behaviors such as intense, focused hobbies, strong preoccupations with odd topics (such as sewer systems or garage doors), and unusually strong attachment to certain objects. This is similar to previous descriptions of circumscribed interests (e.g., Boyd, Conroy, Mancil, Nakao, & Alter, 2007
; South, Ozonoff, & McMahon, 2005
) with the exception of our inclusion of object attachment. Our findings are in line with previous research examining the structure of RRB using the RBS-R (Repetitive Behavior Scale Revised; Bodfish et al., 2000
) where an empirically-derived restricted/circumscribed interest factor was revealed, which also contained both verbally and non-verbally mediated focused interests, including object attachment (Lam & Aman, 2007
). Perhaps the essential element of CI is the limited range of focus, interest, or activity
, and is not confounded by the verbal capabilities of the individual. This may allow us to identify CI in a broad range of cases; this is consistent with previous research that has shown that circumscribed interests can be found in higher- and lower-functioning individuals with autism (Bartak & Rutter, 1976
; Freeman et al., 1981
Unlike other restricted repetitive behaviors which are found among a variety of genetic, developmental and psychiatric disorders, we are unaware of any disorders outside the autism spectrum that include circumscribed interests as a manifestation of repetitive behaviors. However, relatively little research has examined these behaviors in detail. In a recent study of 40 children and adolescents with high-functioning autism and Asperger’s syndrome, South and colleagues (2005)
examined 4 types of repetitive behaviors as defined in the DSM-IV: (1) Repetitive Object Use, (2) Motor Movements, (3) Rigid Routines, and (4) Circumscribed Interests. They found an increase in circumscribed interests over time while the other types of RRB peaked in severity earlier in life. Although this increase in CI was not observed in the current sample, this may be due to a couple of factors. First, our sample had an average age of 9.02 years at the time of the ADI-R administration; considerably younger than the sample reported by South et al., which had a mean age of 14 years (South et al., 2005
). It is possible that an increase in CI may occur in adolescence and early adulthood. In addition, the lack of an age effect in the current sample may also be due to a limitation of the ADI-R to measure the full breadth and severity of circumscribed interests, so further research is needed. If CI do show a unique developmental course relative to other types or RRB, this would provide further evidence that CI may be a distinct subtype of behavior.
It has been postulated that CI may interfere with the development of peer relationships due to the fact that the individual only cares to converse with others about his/her interest (Atwood, 1998
). In severe cases, individuals with CI may not be able to inhibit the preoccupations in order to participate in school or home life, which could contribute to academic and adaptive delays. Parents of children with ASD have also reported that CI symptoms, especially incessant talking about a certain topic, were among the most difficult aspects of autism they had to deal with on a daily basis (South et al., 2005
). More research is needed on the specificity of CI to autism, and given that they are often an area of clinical concern, the ability to distinctly measure them may be of use to researchers and clinicians (Boyd et al., 2007
When examining the relations between the three RRB factors and other subject characteristics (such as IQ, sex, age, and loss of skills/regression), several things became clear. First, each of the three RRB factors was differentially related to these subject variables, which serves as an additional validity check that they are indeed measuring distinct aspects of the behavioral phenotype. The RMB factor was most closely associated with other subject characteristics; significant relations were found with younger age, lower verbal IQ, greater social deficits, greater communication impairments (in verbal subjects), and loss of language. These findings are similar to those of Hus and colleagues (2007)
, who found that these behaviors were correlated with verbal and nonverbal IQ and all symptom areas on the ADI-R.Cuccaro et al. (2003)
and Bishop et al. (2006)
also found significant relations between RSMA scores and adaptive behavior and IQ.
In the present study, our IS factor showed significant correlations with both the social and communication domains on the ADI-R, indicating that it may be tied to overall severity of autism in our sample. This is in line with the findings of Hus et al. (2007)
, who found similar statistically significant (albeit weak) correlations between the IS factor and ADI-R social and communication domains. Szatmari and colleagues (2006)
found the IS factor to be associated with the ADI-R communication domain only. Lastly, the CI factor in the present study was shown to be quite independent; no significant relations were found between this factor score and sex, age, IQ, presence of language loss/regression, and autism symptom domains on the ADI-R.
Examination of the sib-pair data revealed significant familial associations for both the CI and IS factors, but not for the RMB factor. Overall, our results are consistent with previous research showing clear familial aggregation of the IS factor but not of the RSMB factor (Cuccaro et al., 2003
; Szatmari et al., 2006
). Our findings suggest that both the CI and IS factors may be of use in genetic investigation. However, the independence of the CI factor suggests that it may be of particular use in stratifying samples.
To begin to address the clinical heterogeneity of autism, we examined the presence of repetitive behavior at the case level to see if the presence of multiple types of RRB was associated with other features of autism. In order to maximize power for the analyses, we employed a median split to identify individuals with elevated levels of RRB. Future studies may want to examine more stringent criteria to classify more clinically-meaningful subgroups. However, these preliminary findings suggest that individuals that present with multiple types of RRBs tend to have more severe impairments in the social and communication domains. Current diagnostic guidelines (DSM-IV; APA, 1994
) require the presence of only one restricted repetitive behavior for a diagnosis of autism. However, it is interesting to revisit Kanner’s original report on autism where he described a series of children who, in addition to social and communication impairments, showed a variety of restricted repetitive behaviors (Kanner, 1943
). Perhaps it is this co-occurrence of multiple forms of RRB (e.g., stereotyped body movements, the need for sameness, and
intense interests) that distinguishes more severe, ‘Kannerian’ autism from milder variants that still meet current diagnostic criteria.
A major limitation of this study is the use of the ADI-R as the sole measure of restricted repetitive behavior. The ADI-R is a diagnostic parent interview, and although it has been shown to be a reliable and valid way of identifying children with autism (e.g., Lord et al., 1997
), it was not intended as a measure of RRB in any detail. In fact, some work has suggested that the domain of restricted repetitive behavior is undersampled by the ADI-R (Lecavalier et al., 2006
). Therefore, our findings must be viewed as preliminary, and future studies should examine this domain in more detail, using questionnaires designed to capture the full breadth of RRB in autism (e.g., the RBS-R). In addition, direct observational studies of repetitive behaviors are needed.