Schizophrenia Daily News Blog

Schizophrenia and Logic Versus Commonsense Reasoning

A new study, which appears in the Nov. 2007 issue of the British Journal of Psychiatry, examined reasoning capabilities in people with schizophrenia. The study compared healthy individuals to people with schizophrenia "under conditions where common sense and logic conflict(ed)." The results suggested that people with schizophrenia reasoned more logically than healthy individuals, but lacked commonsense reasoning skills.

"The researchers were testing a hypothesis that in schizophrenia there is an enhancement of theoretical over practical reasoning. They looked at whether tasks that are correct from a theoretical (or formal logical) point of view, but depart from practical knowledge (common sense), were performed better by people with schizophrenia than by healthy controls. They selected syllogisms testing theoretical reasoning that were deductively valid or invalid, and syllogisms testing common sense that strongly conformed to, or departed from, practical knowledge.

17 people with schizophrenia and 19 healthy controls took part in the study...All participating patients were taking antipsychotic medication. There was no difference between the groups in IQ or years of education. 53% of the control group and 65% of the schizophrenia group were men. 2 types of syllogism were constructed, in each of which there was a conflict between deductive truth and commonsense truth. The first type were non-commonsense syllogisms that were valid (NCS) e.g. all buildings speak loudly; a hospital does not speak loudly; therefore, a hospital is not a building. The second type were commonsense syllogisms that were invalid (CS) e.g. if the sun rises, then the sun is in the east; the sun is in the east; therefore, the sun rises.

Participants were asked to accept the first 2 sentences of each syllogism as true, and then to decide on the truth or falsity of the third sentence. Syllogisms were scored as correct if they were answered logically. It was found that people with schizophrenia significantly outperformed controls. They comment that the results of the study suggest that on a straightforward interpretation, people with schizophrenia reason more logically than healthy controls either because they are better at logic, or because they are worse at common sense.

They hypothesize that it is because they are worse at common sense, but caution that the question remains open. The results are intriguing, they say, because they shed light on reasoning in schizophrenia, but also have significance beyond schizophrenia research. They suggest that in situations where commonsense knowledge is at stake, normal rationality is violated by people with schizophrenia to a lesser extent than by healthy individuals.

People with schizophrenia seem to have a bias towards theoretical rationality over and above practical rationality. Given that schizophrenia is at its core a pathological state of thinking, the results suggest that concepts of rationality that prioritize theoretical reason over practical reason might apply more accurately in a pathological example of human thinking than in healthy human thinking."

Are people with schizophrenia more logical than healthy volunteers? British Journal of Psychiatry, 191, 453-454.