Each of us encounters the word every day. We say someone did something stupid. We curse the idiots in traffic. We shake our heads at stupid politics. But what do we actually mean when we label something or someone as "stupid"? And is it even something psychology works with?
Stupidity in Language and Thought
The word "stupid" and its derivatives are among the oldest and most universal expressions in any language. As early as antiquity, the Greek philosopher Theophrastus (ca. 371–287 BCE) described the "stupid man" in his study of character types, defining stupidity as slowness of mind in both speech and action. The Latin word stupidus, from which the English stupid is derived, originally denoted a state of astonishment, dullness, or stupor. In Roman theater, stupidus was a stock character of the mime — the bumbling fool whose role was to play the dupe.
In everyday speech, "stupidity" is a remarkably multivalent concept. Sometimes we mean a lack of intelligence; at other times inattention, an inability to learn from experience, impulsive behavior without forethought, or even systematic disregard for available information. James F. Welles, in his book Understanding Stupidity, distinguishes stupidity from mere ignorance: stupidity, in his view, requires that a person know they are acting against their own interests and yet do so anyway. It must involve conscious and maladaptive behavior, not an accident or a lack of information.
An interesting empirical study was conducted by Aczel, Palfi, and Kekecs (2015), who analyzed real cases in which people labeled certain actions as stupid. They found that people use the label "stupid" in three types of situations: first, in cases of a mismatch between self-confidence and actual abilities (for example, when someone overestimates their own capacities); second, in failures of attention; and third, in lapses of self-control. The perceived degree of stupidity grew with the degree of responsibility attributed to the individual and with the severity of the consequences of their actions.
When Psychology Measured the "Stupid": Idiocy, Imbecility, and Debility
Although contemporary psychology does not systematically work with the term "stupidity," there was a period in the history of our field when concepts close to the everyday understanding of stupidity were used as official diagnostic categories. In the early 20th century, the American psychologist Henry H. Goddard introduced a classification system for intellectual deficit based on the Binet–Simon scale. He originally defined these categories in terms of mental age, which was later commonly converted into approximate IQ bands. In the American context, he distinguished three levels: "idiots" (IQ roughly up to 25), "imbeciles" (IQ roughly 26–50), and "morons" (IQ roughly 51–70) — the last term he coined himself from the Greek moros. It is important to note, however, that Czech and broader Central European psychiatric tradition never adopted the term moron. In this tradition, the classification used was: idiocy (IQ approximately up to 20–34), imbecility (IQ 35–49), and debility (IQ 50–69). Debility thus corresponded to what Goddard, in the American context, designated as a "moron."
Goddard's work was closely tied to the eugenics movement. He needed a term that would sound scientifically legitimate and at the same time alert the public to the "danger" that individuals with mild intellectual deficits supposedly posed, according to the (now entirely discredited) views of the time. He believed that "feeblemindedness" was hereditary and advocated the isolation or sterilization of those affected. These approaches, including the use of IQ tests to screen immigrants at Ellis Island, belong to the darkest chapters in the history of psychology.
The older terms — debility, imbecility, and idiocy — were used in clinical practice until roughly the 1990s, when they were replaced by the ICD-10 classification: mild, moderate, severe, and profound mental retardation. Today, the DSM-5 uses the term intellectual disability (intellectual developmental disorder), and the ICD-11 uses disorders of intellectual development. Ironically, each new "neutral" term gradually acquired pejorative connotations — the same fate befell the word "retarded," which itself was originally meant to be an improvement over its predecessors.
Stupidity Is Not Just Low IQ
Here we arrive at the crux of the problem. In everyday life, we frequently regard as "stupid" the actions of people who certainly do not have low IQ. The Harvard-trained banker who invests clients' money in obviously dubious projects. The politician with a law degree who lies repeatedly despite knowing they will be exposed. The physician who ignores clear evidence because it does not fit their preconceived view.
Psychoanalysts reflected on this problem long ago. Otto Fenichel pointed out that a considerable proportion of apparent feeblemindedness is in fact "pseudo-debility" conditioned by inhibition — people become stupid ad hoc whenever understanding would provoke anxiety or guilt, or whenever it would threaten their neurotic equilibrium. Wilfred Bion, in turn, described how psychological projection erects a barrier to learning anything new and thereby produces its own form of pseudo-stupidity. Eric Berne, in his transactional analysis, described the game "Stupid," in which the player deliberately lowers others' expectations in order to avoid responsibility.
These observations lead to a fundamental distinction: stupidity in the everyday sense is not the same as low intelligence. It is, rather, a multidimensional phenomenon that encompasses deficits in rationality, judgment, self-reflection, critical thinking, and emotional regulation. And it is precisely these components that we can now measure.
So What Can We Measure?
Intelligence: A Foundation That Is Not Enough
The most obvious candidate for measuring stupidity is, of course, intelligence. The Wechsler scales, Raven's Progressive Matrices, and the Stanford–Binet scale enable us to quantify cognitive abilities with good reliability and validity. The IQ score is standardized to a mean of 100 and a standard deviation of 15 points. A score below 70 (two standard deviations below the mean) has traditionally indicated intellectual deficit.
However — and this is the key point — a high IQ does not protect against stupid behavior. Keith Stanovich, professor of psychology at the University of Toronto, wrote a foundational book on this topic, What Intelligence Tests Miss (2009), for which he received the Grawemeyer Award. Stanovich convincingly argues that IQ tests measure only a narrow segment of cognitive abilities and overlook precisely those competencies that most people associate with a "well-functioning mind": judgment, decision-making, critical evaluation of evidence, and the assessment of risks. To describe this, he coined the term dysrationalia — the inability to think and act rationally despite adequate intelligence.
Rationality: What IQ Tests Miss
Stanovich, together with West and Toplak, took this approach to its logical conclusion and produced the CART (Comprehensive Assessment of Rational Thinking) — the first comprehensive instrument for measuring rational thought, analogous to IQ tests. The CART measures probabilistic and statistical reasoning, scientific thinking, the capacity to avoid the cognitive miser approach (miserly information processing), and contaminated mindware. The concept of a rationality quotient (RQ) thus complements traditional IQ with a dimension that better corresponds to what we ordinarily call "stupidity" in practice.
Stanovich draws on Daniel Kahneman's dual-process theory of thought: System 1 (fast, automatic, intuitive) and System 2 (slow, controlled, analytical). Many forms of "stupid" behavior stem from relying too heavily on System 1 in situations where System 2 should be activated. The correlation between IQ and the degree of rationality, moreover, is only small to moderate, which means that highly intelligent people may be markedly irrational and vice versa.
Cognitive Biases and the Dunning–Kruger Effect
Closely related to stupidity are cognitive biases — systematic errors in thinking that lead to irrational conclusions and decisions. Kahneman and Tversky identified dozens of such biases: the anchoring effect, confirmation bias, the availability heuristic, the illusion of control, and many others. All of them can lead to their own kind of "stupid" behavior, and all of them are measurable by experimental methods.
Particularly noteworthy is the Dunning–Kruger effect (1999): the tendency of people with low competence in a given domain to overestimate their abilities, while those with high competence — in the original studies — tended to underestimate their relative standing in comparison with others. This metacognitive failure — the inability to recognize one's own incompetence — is perhaps what comes closest to the lay concept of "stupidity": a person not only does not know, but does not know that they do not know. The effect is measured by comparing an individual's objective performance with their subjective assessment of their own abilities.
Emotional Intelligence and Social Cognition
Another dimension of "stupidity" is failure in the social and emotional domain. A person may be a brilliant mathematician and at the same time tragically inept in interpersonal relationships. Emotional intelligence, as conceptualized by Mayer and Salovey and later popularized by Goleman, encompasses the ability to perceive, understand, regulate, and effectively use emotions. Instruments such as the MSCEIT (Mayer–Salovey–Caruso Emotional Intelligence Test) measure it, testing, for example, the ability to recognize emotions in faces, to understand emotional dynamics, or to use emotions to facilitate thinking.
Executive Functions and Self-Regulation
Many manifestations of "stupidity" are linked to deficits in executive functions: the inability to plan, to inhibit impulsive responses, to switch flexibly between tasks, or to monitor one's own behavior. These functions are measured by neuropsychological tests — the Wisconsin Card Sorting Test (WCST), the Stroop Test, the Trail Making Test, the Tower of London, and others. Deficits in executive functions can exist even with intact intelligence and lead to behavior that observers readily label as stupid.
Cipolla's Laws of Stupidity: An Economic Perspective
It is also worth mentioning the unorthodox but influential approach of the Italian economic historian Carlo M. Cipolla, who in 1976 published the essay The Basic Laws of Human Stupidity. Cipolla defined a stupid person as someone who causes losses to others without gaining any benefit themselves — and possibly even harming themselves in the process. Unlike "bandits," who harm others to their own advantage, the stupid person acts irrationally and unpredictably, and is therefore particularly dangerous.
Although Cipolla wrote with a healthy dose of irony and his "laws" do not constitute a scientific model in the strict sense, his definition of stupidity as irrational harm without personal gain corresponds remarkably to what psychological research finds: stupidity is not simply a lack of intelligence, but rather a failure of rationality — behavior that runs counter to the interests of all parties involved, including the individual themselves.
Stupidity as a Psychological Mosaic
Can stupidity, then, be measured? The answer depends on what we understand by the term. If we conceive of stupidity narrowly as low intelligence, then yes — we have at our disposal many valid and reliable intelligence tests. If, however, we understand stupidity more broadly — as it is used in everyday speech — as a complex of irrational behavior, failures of judgment, cognitive biases, deficits of self-regulation, and lack of self-reflection, then no single test for "stupidity" exists. There is, however, an entire battery of instruments capable of measuring its individual components.
Stupidity, as we understand it in everyday life, is a psychological mosaic. It is not a single construct that could be captured by a single number. It is the intersection of deficits in intelligence, rationality, emotional maturity, metacognition, and executive functions — and each of these components has its own tools of measurement. No "SQ" (Stupidity Quotient) is likely ever to emerge — but perhaps that is precisely a good thing. It compels us to reflect on what exactly we mean when we call someone a fool. And that, in itself, is already a step toward wisdom.
This article serves educational purposes and does not constitute a diagnostic tool. The assessment of cognitive abilities and personality traits requires a comprehensive clinical evaluation including standardized diagnostic instruments.
Selected Sources and Recommended Reading
Aczel, B., Palfi, B., & Kekecs, Z. (2015). What is stupid? Intelligence, 53, 51–58.
Berne, E. (1964). Games people play: The psychology of human relationships. Grove Press.
Bion, W. R. (1962). Learning from experience. Heinemann.
Cipolla, C. M. (2019). The basic laws of human stupidity. Doubleday. (Original work published 1976)
Fenichel, O. (1945). The psychoanalytic theory of neurosis. W. W. Norton.
Kahneman, D. (2011). Thinking, fast and slow. Farrar, Straus and Giroux.
Kruger, J., & Dunning, D. (1999). Unskilled and unaware of it: How difficulties in recognizing one's own incompetence lead to inflated self-assessments. Journal of Personality and Social Psychology, 77(6), 1121–1134.
Marmion, J.-F. (Ed.) (2018). The psychology of stupidity. Penguin.
Stanovich, K. E. (2009). What intelligence tests miss: The psychology of rational thought. Yale University Press.
Stanovich, K. E., West, R. F., & Toplak, M. E. (2016). The rationality quotient: Toward a test of rational thinking. MIT Press.
Welles, J. F. (2015). Understanding stupidity: An analysis of the premaladaptive beliefs and behavior of institutions and organizations.
