Quantifying the Behavioural Relationship Between ASD and Nonverbal Learning Disabilities: More Than Social Imperception

Background:  The first description of nonverbal learning disabilities (NLD) highlighted disordered social perception (Johnson & Myklebust, 1967). Social imperception, or the inability to fully understand how oneself and others relate to social environments, is also a defining feature of Autism Spectrum Disorder (ASD). Whether social difficulties in ASD and NLD are the same, either in kind or by degree, is unknown. Clinical reports maintain that social impairments in children and adults with NLD are less severe than in those with ASD, and some researchers separate NLD from ASD according to an absence of stereotyped and repetitive behaviours.

Objectives:  The purpose of the present study was to quantify the degree of impairment in social relatedness and other autism symptoms in adults with NLD, and to compare these results to adults with ASD and to those without developmental disorders.

Methods:  Two surveys designed to screen for ASD were given to participants, 19 - 44 years, who had a community diagnosis of ASD or NLD, and to controls. Both the Ritvo Autism Asperger Diagnostic Scale – Revised (RAADS-R) and the Social Responsiveness Scale – Adult version (SRS-A) focus on social relatedness, but also have items that address circumscribed interests, repetitive behaviours, and other symptoms defined as specific to autism (DSM-5, 2013). Individual subscales on the RAADS-R that separate social relatedness from other types of symptoms were analyzed for potential group differences.

Results:  More adults with ASD scored above the threshold score indicating the presence of ASD on the RAADS-R, a self-report tool, than did adults with ASD on the SRS-A, a survey completed by others who know the participant well. Adults with NLD also had higher than threshold RAADS-R and SRS-A scores, as did a small number of participants who did not have any diagnosis by self-report. The overall RAADS-R and SRS-A score patterns, in which NLD participants as a group scored below the ASD group and above the Control group, also emerged for each of the individual subscales on the RAADS-R. All mean group scores were significantly different. Adults with NLD endorsed sensory and motor impairments, detail-oriented processing, restricted interests, and stereotyped behaviours such as hand flapping, albeit not to the same degree as did those with ASD.

Conclusions:  Results quantified a clinical impression that social difficulties in NLD exist but are less severe than in ASD. The second clinical impression tested here, that NLD can be differentiated from ASD according to an absence of stereotyped and repetitive behaviours, was not supported. Rates of specificity and sensitivity for these surveys were lower than expected in the present sample. Potential reasons for these findings were explored, including: endorsement of a broader range of symptoms by NLD participants than anticipated; the possibility of missed diagnoses; use of psychiatric scales rather than autism surveys to compare NLD and ASD; validation of autism surveys with comparison participants with psychiatric but not learning disorders; and the exclusion in other research of participants without clinical diagnoses who nonetheless scored above threshold on the RAADS-R and SRS-A.