Are people with mental disorders irrational? At first sight, this seems to be a trivial question: madness is the realm of non-sense. When someone tells you that her neighbour installed a tracking device in her abdomen, or that her internal organs are decomposing, you can’t help to wonder: how can she believe such impossible things? Where has her rationality gone? The common conceptualization of madness, which dates back to the Enlightenment, reflects this common-sense intuition: in people with mental disorders emotions are abnormal and unrestrained, and, on the other side, reason is severely affected. People with mental disorders can’t reason properly, healthy people can.
Yet, is it really so? We can bring it into question for several reasons. First, man is not a rational animal, after all. Irrationality is widespread in the general population. We are far from being rational, especially if by rationality we mean logic. When we reason, we make systematic errors, because our rationality is bounded (we don’t have enough cognitive resources to always apply correct strategies, see Simon, 1991), and because we use heuristics, i.e. quick mental shortcuts which reach satisfactory, even if not rational, solutions (Kahneman et al., 1982; Girotto, 1994).
On the other side, the clinical reality seems to show that madness is not the realm of non-sense. Even severe psychoses like schizophrenia don’t affect the ability to reason. Research in the area of both deductive and probabilistic reasoning showed that schizophrenic patients don’t break the formal rules of logic, and, surprisingly, they can also be more logical than non clinical subjects. For example, in the tasks where common sense truth and logical truth conflict, people with schizophrenia perform even better than normal controls. Asked to judge the validity of a syllogism, control subjects tend to consider it valid when its conclusion is commonsensical, while schizophrenic patients recognize the validity of the argument also when its conclusion goes against common sense.
Actually, common sense, rather than rationality, is dramatically impaired in schizophrenia. What would normally seem self-evident causes amazement and perplexity in schizophrenic subjects, who experience a detachment from the web of beliefs, attitudes and categories which represent the background of everyday life. To give an example, C.D.B., a schizophrenic patient, says that “nothing is obvious to him; everything can be uncanny. The world is complicated, difficult to understand. […] Facts are not self-evident.”
Furthermore, delusional subjects use logic and reason to protect and justify their beliefs. For instance, the patient who claims that his neighbour implanted a device in his abdomen, explains that the device has been atomized, i.e., dismantled into its component atoms to pass through the body, and reassembled again on the other side, otherwise there had been an incision. The subject who believes he’s decomposing, says he realized it because of the strong smell coming from his body. As observed by Jaspers in General Psychopathology (1963), delusions are frequently accompanied by the fully preserved capacity for reasoning and formal logic.
What lesson can we learn from this data? We can say that, even if we intuitively attribute irrationality to mental disorders, this attribution lacks any evidence, since both the experimental tasks and the autobiographical reports of single patients outline a different, even opposite, picture. Maybe we keep considering irrationality a crucial feature of mental disorder for different reasons, not grounded on evidence, but on historical, psychological and sociological factors. After all, it’s very easy to call irrational what we don’t understand.