The results of our study indicate that the Autism-Spectrum Quotient is a valid and reliable instrument to assess individual differences in autistic traits. The Dutch AQ was found to have satisfactory internal consistency and test–retest reliability. Moreover, high AQ scores were specific to ASC patients. Previously, an exploratory factor analysis performed in a British student population suggested three underlying factors within the AQ, encompassing Social skill, Details/patterns, and Communication/mind reading (Austin 2005
). A recent study using the child version of the AQ found evidence for 4 of the 5 domains (Auyeung et al. 2008
). Our confirmatory factor analyses in both a general population and a student sample indicated that 4 of the 5 domains of the AQ (Social skill, Communication, Attention switching, and Imagination) were highly correlated. Rather than four separate domains, we propose a hierarchical model allowing these domains to cluster together. This way, one broad band “Social interaction” factor (incorporating the 4 highly correlated domains) could be identified, together with a small second factor, consisting of items focusing on a preference for details and patterns (the domain “Attention to detail”).
It may seem surprising that the broad band scale also includes the domain Attention switching. As put forward by Courchesne and colleagues (1994
), difficulties in Attention switching make it harder to keep track of social information. Social interaction usually involves frequent and rapid changes in the source of information (visually or auditory information, change in objects or actions, etc.), and requires the ability to follow the flow of social cues (words, gestures, postures, background context, etc). The marked deficit in attention switching in ASC subjects may directly harm their social and communication abilities, which explains why this domain is included within the Social interaction factor.
The internal consistency and test-retest reliability of the total AQ score (α = .81/.71 and r = .78) and the broad band social interaction factor (α = .84/.77 and r = .79) were satisfactory. The internal consistency of the Attention to detail factor was moderate (α = .63/68) and its test–retest reliability was acceptable (r = .71). The internal consistencies of the four domains encompassing the broad band social interaction factor varied but were generally moderate (α between .39 and .76), the test–retest reliability varied between .60 and .81. Our results indicate that future studies using the AQ to assess autistic traits may benefit from a focus on the total AQ score and the 2 factor scores, rather than examining differences and similarities in the five original domains. The power to detect differences will be higher using the 2 factors; this could be of importance especially in linkage or association studies into autistic traits.
A small but significant correlation (r
= .19) was found between the two Social interaction factor and the Attention to detail factor. This result is in agreement with findings from a British twin project (Ronald et al. 2005
) which assessed both social and non-social behaviors characteristic for autism using parent and teacher ratings. Social and non-social behaviors were weakly correlated with each other, both in the teacher (r
= .15) and the parent data (r
Group differences in AQ scores were in line with previous studies using the AQ (Austin 2005
; Baron-Cohen et al. 2001
; Hoekstra et al. 2007
; Wakabayashi et al. 2006
). Males scored higher than females on the total AQ score and the Social interaction factor, although no sex difference on the Attention to detail factor was found. Similar to the findings reported by Baron-Cohen et al. (2001
) and Austin (2005
), science students obtained significantly higher scores than humanities and social science students. In our study, students enrolled in a social science degree in turn scored significantly lower than humanities students. Rather than a remarkable discrepancy with the British results (no differences between humanities and social sciences students), these findings are probably due to differences in the student sample. The majority of the social science students included in our study was enrolled in a psychology or education degree. Both these studies have a strong focus on human interaction on an individual level and are likely to attract students who enjoy social interaction. If social sciences focusing on a broader level (such as economics and political science) had been included, the difference in AQ score might not have been significant.
Subjects diagnosed with an ASC scored significantly higher on the total AQ than the general population and the other patient groups. This is a satisfying result, considering the potential symptom overlap between these patient groups and ASC. The overlap between ASC subjects and SAD patients entails problems with social interaction in both groups; the overlap between ASC and OCD subjects encompasses repetitive behaviors that occur in both groups. Relatives of autistic individuals are reported to have up to 10-fold higher rates of social phobia compared to control families (Smalley et al. 1995
; Piven and Palmer 1999
). Similarly, an increased incidence of OCD is found in autism relatives (Bolton et al. 1998
), and the occurrence of obsessive–compulsive traits in parents of an autistic child is significantly more likely if the child displays strong repetitive behavior (Hollander et al. 2003
). Recent research suggests that a common genetic pathway, the serotonin transporter gene, could explain a small part of the association between OCD and rigid compulsive behaviors in autism (Ozaki et al. 2003
; Sutcliffe et al. 2005
). Our study however shows that high AQ scores are specific to the ASC patients. ASC patients scored significantly higher than the other patient groups on both the Social interaction factor and the Attention to detail factor.
Moreover, all subjects with an AS/HFA diagnosis could be distinguished from the other samples, as no subjects without an ASC diagnosis obtained a score >145. The difference is less clear-cut for subjects with PDD-NOS, who obtained intermediate AQ scores. These findings could explain the discrepancy in previous studies using the AQ. Woodbury-Smith et al. (2005
) reported satisfying ability of the AQ to distinguish patients with an AS/HFA diagnosis from non-AS/HFA patients in a clinic for adults suspected of having AS or HFA. On the other hand, Ketelaars et al. (2007
) found little differences in AQ scores between mild ASC patients and patients referred to the general outpatient clinic. The latter study mainly included PDD-NOS patients and few patients with AS or HFA. PDD-NOS constitutes a broad diagnostic category with criteria less stringent than for autistic disorder (American Psychiatric Association 2000
) and is often seen as a milder form of autism. A lower AQ score in PDD-NOS patients compared to AS/HFA patients, yet higher than in most subjects diagnosed with another psychiatric disorder or subjects from the general population, is compatible with the notion of a continuum of autistic traits.
This study had some limitations. Firstly, the data collection in the student and general population sample was based on anonymous reports. We could therefore not verify whether any subjects in these groups had an ASC diagnosis or should warrant a diagnosis. Furthermore, the sample sizes of the patient groups were small and findings from these groups should be interpreted with care. Future studies in clinical samples should explore the criterion validity of the AQ more extensively, and should especially focus on the differences between severely impaired ASC patients and mildly impaired PDD-NOS subjects.
In conclusion, this study shows that the AQ is a reliable instrument for examining variation in autistic traits. The AQ can be divided into two reliable sub factors, focusing on difficulties in social interaction and on marked interests and attention to details and patterns. Total AQ scores follow a continuous distribution in the general population, and patients with an ASC diagnosis fall in the upper end of this distribution. OCD and SAD patient obtain scores between the general population mean and scores typical for ASC patients. These findings indicate that the AQ is a valuable instrument to assess where an individual lies on the autism spectrum.