Mental illness clearly implies serious deficits and deviations from a normal state of mind. An obvious consequence is that if you devise tests of various aspects of normal mentality, mentally ill people will be shown to perform poorly—or at least at odds with normality. Inevitably, they will have problems with relating to others and to themselves, and may well develop secondary symptoms, such as withdrawal or various kinds of avoidances.
The diametric model of mental illness proposes that although mental illnesses such as schizophrenia and autism often seem the same in featuring impaired social, inter-personal and communicative skills, they are in fact quite different with regard to the cause of these impairments. According to the model, autism spectrum disorders (ASDs) result from deficits in mentalism understood as an ability to interact with others and yourself on the mental level of meaning, emotion, belief etc. Psychotic spectrum disorders (PSDs), on the other hand, are the opposite: featuring hyper-mentalism understood as excessive sensitivity to mental factors manifested in things like compulsive thinking, delusions, or pathological swings of mood or sense of self-worth.
Clearly, there are three ways that ASDs and PSDs could relate to each other: they could overlap, be discreet, or could be opposites as the diametric model proposes. Past posts have documented previous attempts to tease out the truth, but now a new study has appeared which used the Autism Spectrum Quotient (AQ) and a version of the Schizotypal Personality Questionnaire (SPQ-BR) to test the prediction on a large non-clinical sample of students.
As the researchers comment, together, these analyses provide an unbiased assessment of autism and schizotypy as measured using two of the primary questionnaires employed in current research, and they allowed them to link neuro-psychological correlates of either spectrum with patterns of overlap between them. Additionally, this approach allowed them to determine in particular if an autistic versus schizotypy axis would emerge, as predicted by the diametric model.
Consistent with previous research, autistic features were positively associated with several schizotypal ones, with the most overlap occurring between measures of interpersonal schizotypy and social and communicative aspects of autism. The first component of a principal components analysis (PCA) of subscale scores reflected these positive correlations and suggested the presence of an axis representing general social interest and aptitude (PC1). By contrast, the second principal component (PC2) exhibited a pattern of positive and negative loadings indicating an axis from autism to positive schizotypy where positive schizotypal features loaded in the opposite direction to core autistic ones.
The diagram above shows PC2 as an axis from autism to positive schizotypy: (A) Plot of AQ scores versus SPQ-BR scores with point size indicating relative PC2 score (point size increases as PC2 score increases) and (B) a schematic representation of the relationships between AQ scores, SPQ-BR scores, and PC2 variation (doi:10.1371/journal.pone.0063316.g001).
These overall PCA patterns were replicated in a second data set from a Japanese population. Handedness and mental rotation ability are established correlates of psychosis and autism respectively, and so researchers measured these to evaluate the validity of their interpretation of the PCA results. As the researchers note, mixed handedness appears to be particularly predictive of positive schizotypal features such as magical thinking and delusional beliefs, and so it is not surprising that they found that PC2 scores were significantly associated with this, and that increasing schizotypal scores predicted reduced strength of handedness.
PC1 scores were positively related to performance on the mental rotation task, suggesting trade-offs between social skills and visual-spatial ability—or what the diametric model would describe as trade-offs between mentalistic versus mechanistic cognition. Indeed, only the diametric model can explain this finding, thanks to its implicit assumption that more cognitive resources devoted to mentalism mean less available for mechanistic cognition, and vice versa.
Overlapping or discreet systems do not predict this, and they certainly do not suggest a counter-intuitive therapeutic strategy for which there is already considerable evidence: that mechanistic skills training may be as effective for treating PSD as mentalistic skills training has been shown to be for ASD.
(With thanks to Bernard Crespi for bringing this to my attention and congratulations to him and the other authors of this study.)