At their extremes, both autism and schizophrenia are debilitating disorders. But what is the relationship between them? The Swiss psychiatrist Eugen Bleuler, who coined the terms "schizophrenia" and "autism" a century ago, viewed autism as a form of solitude and schizophrenia as an extreme form of autism representing withdrawal from reality. Ever since, there has been confusion as to the boundaries between these conditions. In particular, four main ideas regarding their relationship has emerged:
- Autism as a distinct subtype of schizophrenia in children ("childhood-onset schizophrenia")
- Autism and schizophrenia as completely separate disorders
- Autism and schizophrenia as completely opposite sets of conditions
- Autism and schizophrenia as partially overlapping conditions
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).
Based on these existing scales, what is the relationship between Asperger’s and Schizotypy in the general population? Two studies are of particular note. In one study, Ruth Hurst and her colleagues administered the AQ and the SPQ to a large non-clinical adult sample (607 college students). Consistent with the idea that AS and SPD vary in the general population, they found that the characteristics of both conditions were normally distributed. Looking at total scale scores, the AQ and SPQ were significantly positively correlated with one another, suggesting overlap among the two constructs. Results of the subscale correlations support the idea that both SPD and AD are characterized by social deficits: the SPQ Interpersonal factor was strongly related to the AQ Social Skills factor. In fact, this relation was stronger than any other relationship in their dataset. Further, the AQ Communication domain was positively related to the SPQ Disorganized factor. The SPQ Cognitive-Perceptual factor was much less related to the AQ factors, being unrelated to the AQ Social Skills domain. This suggests that autistic-like traits are much more related to the negative symptoms of schizotypy (interpersonal deficits) than the positive symptoms (cognitive-perceptual distortions).
A more recent study conducted by Suzanna Russell-Smith and her colleagues assessed the relationship between AD and SPD using the O-LIFE. Across two different studies, they found a strong positive correlation between the social dimensions of AQ and O-LIFE, again suggesting overlap among the two constructs. Also consistent with the Hurst and colleagues study, cognitive disorganization was associated with social and communicative deficits. Interestingly, they also found that the Unusual Experiences factor of the O-LIFE was positively related to the total AQ scale score. Unusual Experiences was most strongly related, though, to a preference for details/patterns and better imagination. Like the Hurst study, the correlation between cognitive-perceptual distortions (Unusual Experiences) and the Social Skills factor of the AQ was weak.
What can we make of these findings? What do they tell us about the relationship between Asperger’s and Schizotypy in the general population? I don’t think much. If you look at the specific items on these scales, the pattern of correlations is unsurprising. Here are some social skill items from the AQ: "I find it hard to make new friends", "I enjoy social occasions (reverse-coded)", and "I enjoy meeting new people (reverse-coded)". Here are some interpersonal items from the SPQ: "I prefer to keep to myself", "I am poor at returning social courtesies and gestures", and "I tend to keep in the background in social situations". These items are so general that they don’t differentiate at all the specific flavor of ‘social oddity’. All the strong correlation between these two scales tells us is that those with high levels of SPD and AD are odd in social situations. Not exactly that informative!
The significant relationship found between unusual experiences and the details/patterns factor is also not all that revealing. Here are some items from the details/patterns section of the AQ: "I often notice small sounds when others do not", "I tend to notice details that others do not", and "I notice patterns in things all the time". As my friend and colleague Marco Del Giudice pointed out to me, it’s very easy to imagine how a person with strong SPD traits would endorse these items, but for a completely different reason than why a person with strong AD traits would endorse the very same items! Perhaps those with schizotypal traits endorse these items because they see mystical numerological connections between them, while the person with strong autistic-like traits is fascinated by numbers and patterns in a mathematical, very domain-specific sense. Therefore, the attention to details section of the AQ can measure either autistic-like traits or positive schizotypal traits, depending on whether you have autistic-like traits or positive schizotypal traits!
The finding that cognitive disorganization is associated with interpersonal deficits in both AD and SPD is also not that informative or surprising. There are many reasons why a disorganized mind can cause someone to be socially awkward. For instance, if I’m having difficulty paying attention to someone telling me a story, and then they ask me a follow-up question about their story, my response is surely going to show my social ineptness.
One possibility is that executive functioning deficits (areas of the prefrontal cortex that coordinate and organize other areas of the brain) may exert their social deficit by reducing the ability to take another’s perspective (theory of mind). Deficits in theory of mind can be extremely detrimental to ‘normal’ interpersonal relations. As King and Lord note,
"Many social conventions logically flow from a theory of mind, and it is easy to imagine how someone could drift into idiosyncratic and seemingly bizarre behavior in the absence of the ability to see one’s own behavior from the vantage point of someone else."
Indeed, the development of theory of mind deficits found in autism is interdependent on the development of executive functioning, and both executive function and theory of mind deficits have also been found in schizophrenia. Still, these deficits seem to be linked to different behaviors. For autism, theory of mind deficits are linked to a lack of eye contact as well as a lack of showing, sharing, pointing, and comforting behaviors. For schizophrenia, theory of mind deficits are related to the presence and severity of delusions.
Also, while it appears that both autism and schizophrenia are linked to brain connectivity deficits, autism appears to be associated with an underactive theory of mind whereas schizophrenia appears to be associated with an overactive theory of mind. This may explain why positive symptoms of schizophrenia involve overinferring intentionality and seeing patterns where they don’t exist, whereas those with autistic-like traits show just the opposite characteristics, underinferring the intentions of others and focusing on the finer details of patterns. One could see how the former case could lead to delusions and paranoia, whereas the latter case could lead to a turning inward and social distance. But as it stands, none of the scales that currently exist for AD or SPD increase our understanding of how cognitive disorganization in AD and SPD is differentially related to their own particular social and communicative deficits.
In sum, the items that measure Asperger’s Syndrome (AQ) and the items that measure Schizotypal Personality Disorder (SPQ and O-LIFE) are way too vague to allow for fine discrimination between Asperger’s and Schizotypy. Significant correlations may not be all that revealing about the nature of the relationship between Asperger’s and Schizotypy, since specific manifestations of social behavior may differ quite a bit. Del Giudice’s proposed solution for the time being is to administer measures of both AD and SPD in every study, and use partial coefficients as predictors, thus controlling for the statistical overlap between AD and SPD traits. Clearly, this solution is not optimal. Before strong claims about the relationship between AD and SPD are made using self-report questionnaires, newer, better, and more nuanced questionnaires will have to be developed.
Multiple recent studies, from a variety of perspectives, suggest there are both overlapping and diametric causes of autism and schizophrenia. While it easier to demarcate these two disorders at the extremes, it’s becoming increasingly difficult dissociating milder forms of both disorders from each other. As I’ve argued, this is mostly an artifact of the current measures of Asperger’s and Schizotypy. New measures will have to be created if we want to further our understanding of these disorders. This research is extremely important for increasing our understanding of the causes of autism and schizophrenia, and also has relevance to the current discussions taking place about where to place AD and SPD in the DSM-V.