People with schizophrenia do worse than others on many tasks. Indeed a global conclusion in the psychology of schizophrenia is that people with this disorder have somewhat lower IQ than those without – a conclusion that makes schizophrenia seem a disorder of cognition like dementia or learning disability. But this is an incomplete perspective. The phenomenology of schizophrenia also points in the direction of representational overactivity (1). Additionally, delusions, which people with schizophrenia often exhibit, are not explained by failures of formal inference such as inability to reason with modus ponens or modus tollens or inability to adhere to Bayesian updating norms. That is striking when one considers that delusion and illogically are often assumed to be the same.
To give a feel for the data, here are some contrasting responses to one of the syllogisms: If the sun rises then the sun is in the East, The sun is in the East, Therefore, the sun rises.
People with schizophrenia:
• “False. One can never predict out of habit what is the ultimate routine”
• “False. It doesn’t mean the sun rises in the east. It could be in the east and setting. It’s possible”.
• “True. Because the sun does rise in the east”
• “True. The sun does rise in the east. I’ve seen it”
In a recent study in a Danish sample (3), the superior logical performance I found was not replicated – implying the syllogisms are a somewhat unreliable way of exploring rationality in schizophrenia. But the methodological differences between the studies raise some interesting questions about how logic experiments should be conducted to capture phenomena of interest.
Another development is in schizophrenia and symbolic logic. Are people with schizophrenia operating with a different logic? Selesnick (4) has done some lovely work showing how a logic underpinning the mathematics of quantum mechanics can be used to derive observed thinking anomalies in people with schizophrenia including the classic case of a patient who abhorred war and took himself to be Switzerland. A logical judgment if shared attribute implies identity.
Schizophrenia and logic have an affinity and logicians have contributions to make in unraveling this most mysterious of disorders.
1) The Maudsley Reader in Phenomenological Psychiatry (2013) Eds. Broome, M., Harland, R., Owen, G., Stringaris, A. Cambridge University Press.
2) Owen, G., Cutting, J. & David, A. S. 2007. Are people with schizophrenia more logical than healthy volunteers? British Journal of Psychiatry 191; 453-454.
3) Revsbech, R. et al. Exploring Rationality in Schizophrenia. (in preparation)
4) Selesnick, S. A. & Owen, G. S. 2012. Quantum-like logics and schizophrenia. Journal of Applied Logic 10; 115-126.