We like to think of ourselves as special because we can reason and we like to think that this ability expresses the essence of what it is to be human. In many ways this belief has formed our civilization; throughout history, we have used supposed differences in rationality to justify moral and political distinctions between different races, genders, and species, as well as between “healthy” and “diseased” individuals. Even to this day, people often associate mental disorder with irrationality and this has very real effects on people living with mental disorders.

But are we really that rational? And is rationality really what distinguishes people who live with mental illness from those who do not? It seems not. After decades of research, there is compelling evidence that we are not as rational as we think we are and that, rather than irrationality being the exception, it is part of who we normally are.

So what does it mean to be rational? We usually distinguish between two kinds of rationality.  Epistemic rationality, which is involved in acquiring true beliefs about the world and which sets the standard for what we ought to believe, and instrumental rationality which is involved in decision-making and behavior and is the standard for how we ought to act.

We are epistemically rational if we believe things for which we have good evidence and if we would change our beliefs in light of evidence against those beliefs. For example, it is epistemically rational for me to believe there is some wine in the fridge since it was left over from last night and I put it there myself. But that is not enough; I should also be willing to change my belief, for example if I am told that someone drank it in the meantime and maybe if I saw for myself that the wine in the fridge is no more. To still believe, for no other reason, that there is wine in the fridge despite this new evidence would be epistemically irrational.

We are instrumentally rational when we act in ways that are appropriate for achieving our goals – for instance when we go on a diet when we want to lose weight, or we study hard to do well in an exam.

In contrast, one is irrational when one’s beliefs or actions are not in accord with the requirements of rationality. For instance, if one wants to achieve a certain goal but acts in ways that do not lead to that goal; when one forms beliefs for which there is no evidence, or that fly in the face of available evidence; when one reasons faultily and so on. And, because we value rationality and hold it up as a standard we should aspire to, irrationality is understood to be a bad thing.

One does not have to look far to find irrational beliefs. Many racist and sexist beliefs are not supported by evidence and are notoriously resistant to contrary evidence. And, as behavioral economist Dan Ariely has observed, most people’s financial decision-making and eating habits fall short of rationality, in the sense that our intentions and our behavior are often not in agreement. We also – and practically everyone is included in this - routinely have inconsistent beliefs and preferences and we habitually make mistakes in reasoning.

So it seems that we are not really that rational. But there is more.

It is common to believe that rationality is needed for making good decisions. But evidence from psychology and cognitive science indicates that this is not quite right.

In the 1990’s, neuroscientist Antonio Damasio introduced the Somatic Marker Hypothesis according to which when we make decisions, especially complex or important ones, before we come up with an answer, and without our being consciously aware of it, physiological changes take place in our bodies. These changes are associated with positive or negative emotions that signal to us which options to prefer and which to rule out. One can think of these automated emotional signals as gut feelings that have resulted by our associating in the past certain situations with positive or negative outcomes. Just like when you need to lose weight and the idea of going on a diet is unpleasant to you: the automatic positive emotion elicited by the prospect of the advantages of future weight loss helps you make a decision to endure all the unpleasantness that a diet involves. According to Damasio, these automatic emotional responses can improve the accuracy and efficiency of decision-making.

Recently, philosopher and head of the PERFECT project Lisa Bortolotti has offered evidence that though we tend to rationalize our decisions after we have made them, we do not make choices primarily by rational deliberation. Instead, most of our decision-making involves emotions and intuition and, often, these processes lead to better results than those achieved by reasoning through our choices.

These are not just fun facts to know, though, they are also important because giving so much significance to rationality while also having wrong beliefs about it can have undesirable consequences, as is the case in the area of mental health.

The World Health Organization’s International Classification of Diseases and the American Psychiatric Association’s Diagnostic and Statistical Manual that set the criteria for psychiatric diagnosis of mental disorders describe many mental disorders as involving deviations from rationality.  This means that rationality plays a big role in what counts as a mental disorder and, hence, in who we judged to have one and how we treat them.

In recent work Jillian Craigie and Lisa Bortolotti addressed the legal and ethical significance of the association of irrationality with mental illness and questioned the idea that being irrational is what separates mental health from mental illness as well as the idea that someone who is irrational is unable to make good decisions. 

As we have seen, though it is often believed that what distinguishes mentally healthy individuals from the mentally sick is that the latter are epistemically irrational, it seems that the average person – and that means most of us – is epistemically irrational. So that is not enough for someone to be mentally ill. More importantly though, it seems that mental disorders are not even always characterized by epistemic irrationality. For instance, people who suffer from anxiety disorders, depression and personality disorders are not always epistemically irrational. 

How about instrumental irrationality? In order to question whether instrumental irrationality is a characteristic of mental disorders, Lisa Bortolotti, in her book on irrationality, presents counter-examples of individuals living with mental disorders who apparently flourish. For instance, she cites “successful psychotics” (well-functioning individuals who find positive meaning in their psychotic experiences) and “successful psychopaths” (people who display the core traits of psychopathic personality but who are successful in life). 

This is important because our tendency to identify irrationality with mental disorders and to believe that mental disorders compromise people’s ability to make rational choices has important implications for decisions concerning a patient’s rights and responsibilities, as well as for the social stigma associated with a psychiatric diagnosis. 

In psychiatry a patient’s rights can be overridden if it is deemed necessary to disregard them. For example, one can disregard the patient’s wishes concerning some aspect of their medical care in order to protect them. Also, a psychiatric diagnosis is still sometimes considered enough by itself to permit us to disregard a patient’s wishes, or for a person not to be held accountable for his actions. But if irrationality is not necessarily part of mental disorders and if many patients diagnosed with psychiatric disorders are capable of making decisions for themselves, then it is ethically and legally problematic to use a psychiatric diagnosis as a criterion for the lack of decision-making abilities. 

Also, distinguishing between mental health and mental disorder based on irrationality can contribute to mental health stigma. By distinguishing one part of the population from the rest and labeling it “irrational”, one sustains the attitude that patients suffering from mental disorders are somehow essentially different from the rest of the population in terms of rationality and thus are less capable, to be avoided, not to be trusted with responsibility etc. This kind of “othering” helps increases the separation of “us” and “them”, “healthy” and “diseased”, in the eyes of both the public and the people diagnosed and feeds a mentality which leads to marginalization and stigmatization.

Of course, if a person’s contact with reality or decision-making abilities are severely compromised we can legitimately raise the question of responsibility or of restrictions to autonomy. And, of course, some forms of irrationality are more difficult to comprehend than others. Still, given what we currently know, our persistent belief that we are primarily rational could itself be an irrational belief.