Recent research, from a variety of perspectives— genomics, neurodevelopment, psychology, psychiatry, and evolutionary biology— is casting new light on these conditions. Two new excellent summaries are particularly informative. In one recent review, Bernard J. Crespi argues that the bulk of the evidence suggests both partially-overlapping etiologies and diametric causes of autism and schizophrenia. In another recent review of a similar literature, Bryan King and Catherine Lord “consider the possibility that autism and schizophrenia share more in common than might be immediately clinically apparent and that the line of demarcation between them is arguably blurry.”

My aim here is not to sort out the precise relationship between autism and schizophrenia. That would take quite a bit more than a single blog post! Instead, I want to focus on the relationship of these spectrums in the general population. It has become clear that both autism and schizophrenia exist on spectrums (i.e., all of us have some level of autistic and schizophrenic traits). Since there are obviously a lot more people in a non-clinical setting than a clinical one, we have the potential to increase our understanding of both autism and schizophrenia using extremely large samples and looking at how autistic-like and schizophrenia-like traits are related in everyday life.

Unfortunately, our current methods of assessing the relationship between milder forms of autism and schizophrenia are hindering us from making progress in this direction. First I’ll take a look at how autism spectrum disorders and schizotypal personality disorders are distinguished in the latest version of the  Diagnostic and Statistical Manual of Mental Disorders. Then I’ll show how our current methods of distinguishing between these two classifications in the general population are woefully incomplete.

Classification Differences Between Asperger’s and Schizotypy

In the  DSM-IV-TR, Asperger’s Disorder (AD) is listed as an Axis I pervasive developmental disorder and is included within the autism spectrum. Schizotypal Personality Disorder (SPD) is listed as one of the Axis II Cluster A Personality Disorders and is considered to be a schizophrenia-spectrum disorder. AD tends to be diagnosed early in childhood, whereas SPD is typically diagnosed after the age of 18. There are both overlaps and differences among the diagnostic criteria for AD and SPD.

At the broadest level, both AD and SPD include criteria for social and communicative deficits, and odd or restricted patterns of behavior. When you look at the specific criteria within each category, however, important differences in the flavor of the “deficit” emerges. Excessive social anxiety and paranoia is included in the social deficits column for SPD but not AD. In the communication domain, the SPD criteria focuses on odd speech that is vague, circumstantial, metaphorical, and over-elaborate, whereas the AD criteria focuses on abnormal non-vocal communication behaviors, such as impairment in the use of non-verbal cues and body language (e.g., eye contact). In terms of repetitive-restrictive behaviors, again the focus in SPD is on verbal behavior (stereotyped thinking and speech), whereas the focus in AD is on stereotypic and inflexible patterns of non-verbal behavior. While all of these criteria represent ‘odd social behavior’, the particular manifestations are very different.

Perhaps the clearest and most stark demarcation between AD and SPD is in the cognitive-perceptual domain. The diagnostic criteria for SPD includes mild forms of “positive symptoms” of schizophrenia, including ideas of reference (excessive self-reference), odd beliefs or magical thinking that is inconsistent with subcultural norms (e.g., belief in telepathy), unusual perceptual experiences, and suspiciousness. The AD checklist does not mention any of these positive symptoms of schizophrenia. It seems this is in fact warranted: autistic symptoms in adolescence do not appear to be predictive of later conversion to a psychotic disorder. Therefore, the most striking difference between AD and SPD is that those high in SPD tend to display mild positive symptoms of schizophrenia, whereas those high in AD do not. Indeed, the absence of positive symptoms of schizophrenia seems to be the most reliable factor distinguishing Asperger’s from Schizotypy.

Measuring Asperger’s and Schizotypy

To further clarify the relationship between Asperger’s and Schizotypy in the general population, researchers have devised self-report questionnaires that tap into the various diagnostic criteria of both conditions. These questionnaires are built on the assumption that both the autism and schizophrenia spectrums include a wide range of symptom severity. The Autism Spectrum Quotient (AQ) is a self-report measure developed to assess mild autistic-like traits in the general population. The two main measures of SPD are the Schizotypal Personality Questionnaire (SPQ) and the Oxford-Liverpool Inventory of Feelings and Experiences (O-LIFE).