Human intelligence is the intellectual capability of humans , characterized by sophisticated cognitive processes and a notable capacity for motivation and self-awareness . Through the application of their intelligence , individuals are equipped to learn , construct abstract concepts , achieve understanding , and engage in the application of logic and reason . Human intelligence is further understood to encompass the capacities for pattern recognition , strategic planning , creative innovation , effective problem-solving , judicious decision-making , the retention of information , and the adept use of language for complex communication .

The conceptualization and measurement of intelligence are subjects of considerable divergence. Within the field of psychometrics , human intelligence is frequently assessed via intelligence quotient (IQ) tests, though the inherent validity of these instruments remains a point of contention, often requiring citation needed . The proposition of distinct subcategories of intelligence, such as emotional intelligence and social intelligence , has been put forth, yet the debate persists regarding whether these represent truly separate forms of intelligence, as indicated by scholarly sources like Salovey & Mayer (1990) and Walker & Foley (1973). The very origins of an individual’s intelligence level are also debated, with perspectives ranging from a view of intelligence as an innate, fixed trait to one that is fluid and responsive to an individual’s mindset and dedicated efforts, as explored by Haimovitz & Dweck (2016).

History

The historical trajectory of understanding human intelligence is deeply intertwined with the study of our evolutionary past and the development of our species. Psychologists like Thomas Suddendorf propose that examining our closest relatives, such as primates, can illuminate the evolutionary path of human intelligence [3]. Furthermore, comparative analyses of the human brain with those of other organisms offer profound insights into the evolution of our cognitive capacities and, by extension, our intelligence. This section, however, requires considerable expansion to fully capture the rich history of this field, a task that could be undertaken by delving deeper into anthropological and archaeological evidence, as well as the philosophical underpinnings of early intelligence theories.

Correlates

Intelligence, both as a theoretical construct and as quantified by intelligence tests, stands as one of the most influential concepts within psychology . Its predictive power extends to a wide array of significant variables. For instance, intelligence levels correlate with the likelihood of experiencing accidents and with an individual’s earning potential [4]. Other notable correlations include:

Education

The relationship between education and intelligence is substantial. A comprehensive metastudy conducted in 2018 identified education as the “most consistent, robust, and durable method” for enhancing intelligence [5]. This suggests that structured learning environments not only impart knowledge but also foster the very cognitive abilities measured by intelligence assessments.

Personality

An extensive synthesis of thousands of studies, involving millions of individuals across over 50 countries, has revealed intricate connections between personality traits and cognitive abilities. Neuroticism-related traits tend to exhibit negative associations with intelligence, while traits such as activity, industriousness, compassion, and openness demonstrate positive correlations with various intellectual capacities [6]. This highlights a complex interplay where dispositional factors can influence, or be influenced by, cognitive functioning.

Myopia

Intriguing research has identified a correlation between IQ scores and the prevalence of myopia [7]. While some researchers posit environmental explanations—suggesting that individuals with higher IQs might engage in more reading, thus increasing their risk of myopia, or vice versa—others propose an underlying genetic link [8]. This area warrants further investigation to disentangle the contributing factors.

Aging

The process of aging is associated with a documented decline in certain cognitive functions. Cross-sectional studies have indicated that various cognitive abilities, including processing speed, working memory, and long-term memory, can decrease by approximately 0.8 standard deviations between the ages of 20 and 50 [9]. Understanding these changes is crucial for developing interventions to support cognitive health throughout the lifespan.

Genes

A growing body of research has identified specific single-nucleotide polymorphisms within human DNA that are associated with higher IQ scores [10]. This genetic component underscores the biological underpinnings of intelligence and opens avenues for exploring the genetic architecture of cognitive abilities.

Theories

The exploration of human intelligence has yielded a multitude of theoretical frameworks, each attempting to capture its multifaceted nature.

Relevance of IQ tests

In psychology , the intelligence quotient (IQ) serves as a common metric for assessing human intelligence, with higher scores often linked to favorable life outcomes [11]. While IQ tests generally exhibit strong inter-test reliability and possess predictive validity for specific achievements, their construct validity as a comprehensive measure of intelligence is subject to debate [12, 13]. Critics argue that IQ tests may not adequately capture broader definitions of intelligence, particularly aspects like creativity and social intelligence [13]. As psychologist Wayne Weiten notes, “IQ tests are valid measures of the kind of intelligence necessary to do well in academic work. But if the purpose is to assess intelligence in a broader sense, the validity of IQ tests is questionable” [13].

Theory of multiple intelligences

Howard Gardner ’s influential theory of multiple intelligences posits that intelligence is not a singular entity but rather comprises several distinct domains. Based on studies of diverse populations, including children, gifted individuals, those with brain damage, experts, and people from various cultures, Gardner initially identified seven intelligences: logical-mathematical, linguistic , spatial , musical, kinesthetic , interpersonal , and intrapersonal . He later added naturalist and existential intelligences. Gardner contends that psychometric (IQ) tests predominantly measure only linguistic, logical, and some aspects of spatial intelligence [15]. However, his theory has faced criticism for its lack of empirical testing and unfalsifiability [16]. Some scholars, like Locke (2005), suggest that advocating for numerous specific intelligences may stem from a political rather than scientific agenda, aiming to validate individual uniqueness over recognizing genuine differences in capacity. Conversely, researchers like Schmidt and Hunter propose that the predictive validity of specific aptitudes over general mental ability (“g”) lacks empirical support [18]. Nonetheless, Jerome Bruner lauded Gardner’s approach as a valuable conceptualization, stating it “deserves to be cheered.”

Triarchic theory of intelligence

Robert Sternberg introduced the triarchic theory of intelligence as a more holistic alternative to traditional theories. This framework identifies three core aspects of intelligence:

• Analytic intelligence: Encompasses the cognitive mechanisms underlying intelligent behavior.
• Creative intelligence: Essential for novel situations and the adaptation of learned skills to new contexts.
• Practical intelligence: The ability to adapt to, select, and shape one’s environment, deeply rooted in the sociocultural context.

Sternberg later refined this into the Theory of Successful Intelligence, defining intelligence as the capacity to achieve success as defined by an individual’s own standards within their specific sociocultural milieu. Success is achieved through a blend of analytical, creative, and practical intelligence, applied to adapting, shaping, and selecting environments, while leveraging strengths and compensating for weaknesses. Sternberg’s theories, while insightful, continue to provoke discussion within the scientific community [22].

PASS theory of intelligence

Drawing upon the foundational work of A. R. Luria (1966) on brain function modularity and supported by extensive neuroimaging research, the PASS Theory of Intelligence (Planning/Attention/Simultaneous/Successive) delineates cognition into four core processes mediated by distinct brain regions [23, 24]:

• Planning: Involves executive functions for behavioral control, strategy development, and performance monitoring, largely localized in the frontal lobe.
• Attention: Responsible for arousal and focusing on relevant stimuli, involving frontal and cortical regions, with parietal lobe involvement.
• Simultaneous processing: Engaged when understanding the relationships between elements and integrating them into a coherent whole, associated with the occipital and parietal lobes.
• Successive processing: Required for organizing items in a sequential order, linked to the frontal and temporal lobes.

The PASS theory is heavily indebted to Luria’s conceptualizations [23, 25] and integrates findings from cognitive psychology [26].

Piaget’s theory and Neo-Piagetian theories

Jean Piaget ’s seminal Piaget’s theory of cognitive development shifted the focus from mental abilities to the child’s evolving mental models of the world. As children mature, their models become more accurate, enabling more effective interaction with their environment. A key concept is object permanence , the understanding that objects continue to exist even when unseen. Piaget outlined four main developmental stages: sensorimotor (birth–2 years), pre-operational (2–7 years), concrete operational (7–11 years), and formal operations (11–16 years) [27]. While stage progression correlates with psychometric IQ, they are not identical [28]. Piaget viewed intelligence as an active process rather than a static capacity. His experiments, such as the one involving children’s perception of quantity in sweets, revealed that cognitive abilities develop and sometimes appear to regress before re-emerging, possibly due to an overreliance on perceptual cues [29]. Piaget’s theory has been critiqued for the influence of testing methodologies on age-related findings and the empirical challenges in verifying mental models as explanations for observed behavior [30]. Neo-Piagetian theories build upon Piaget’s work, incorporating psychometric factors like processing speed and working memory, alongside cognitive and domain-specific considerations [31].

Parieto-frontal integration theory of intelligence

Based on extensive neuroimaging studies, the Parieto-Frontal Integration Theory (P-FIT), proposed by Jung and Haier, suggests that intelligence arises from the efficient communication between the frontal and parietal regions of the brain [32]. Subsequent research largely supports this theory, with reviews indicating P-FIT as a leading explanation for individual differences in intelligence [33, 34].

Investment theory

Stemming from the Cattell–Horn–Carroll theory , the investment theory posits that fluid intelligence (Gf) is “invested” in the acquisition of skills and knowledge, leading to crystallized intelligence (Gc) [35, 36, 37]. This theory suggests that personality traits can influence “actual” ability, not just test performance [38]. Donald O. Hebb proposed a similar bifurcation: Intelligence A (physiological, akin to fluid intelligence) and Intelligence B (experiential, akin to crystallized intelligence) [39].

Intelligence compensation theory (ICT)

The intelligence compensation theory (ICT) proposes that individuals with lower intelligence compensate by exhibiting greater conscientiousness, working more diligently and methodically to achieve goals. Conversely, more intelligent individuals may not need to rely as heavily on these traits, given their cognitive strengths [40, 41]. This theory suggests a causal link where intelligence influences conscientiousness, rather than the reverse, with a stronger negative correlation expected between fluid intelligence and conscientiousness due to developmental timelines [42, 43]. However, recent findings have challenged ICT, suggesting that observed compensatory patterns might be an artifact of sample selection biases [44].

Bandura’s theory of self-efficacy and cognition

Albert Bandura’s work reframes cognitive ability not as a fixed attribute but as a dynamic capacity integrating cognitive, motivational, social, and behavioral elements. A critical, often overlooked, component is the management of emotions and adverse experiences that can impair thinking. Bandura links intelligence to success through the concept of self-efficacy , emphasizing the difference between possessing skills and effectively applying them under challenging circumstances. Individuals with similar skill levels can exhibit vastly different performance outcomes based on their self-efficacy beliefs. Cognition’s role is to enable prediction and effective coping with events, requiring the processing of ambiguous information and the application of learned knowledge. High self-efficacy is crucial for sustained effort in the face of adversity [45]. Bandura’s theory suggests that low self-efficacy leads to avoidance of challenges and slower recovery from failure, while high self-efficacy fosters effective performance through task-oriented goals [46].

Process, personality, intelligence and knowledge theory (PPIK)

Developed by Philip L. Ackerman, the PPIK theory elaborates on the work of Cattell, Hebb, and others, distinguishing between intelligence as a process (akin to “Intelligence A” or fluid intelligence) and intelligence as knowledge (akin to “Intelligence B” or crystallized intelligence) [47, 48]. This framework integrates these cognitive aspects with personality, motivation, and interests. Ackerman acknowledges the difficulty in separating process from knowledge, as content is inherent in ability tests [47, 48, 49]. Personality traits show minimal correlation with intelligence as process, except in cases of psychopathology. However, intelligence as knowledge is associated with traits like Openness and Typical Intellectual Engagement, which also correlate strongly with verbal abilities linked to crystallized intelligence [47, 51].

Latent inhibition

Research suggests a positive correlation between latent inhibition —the phenomenon where familiar stimuli elicit a delayed response compared to novel ones—and creativity [citation needed].

Improving

The pursuit of enhancing human intelligence is a multifaceted endeavor, exploring various avenues from genetic manipulation to educational interventions.

Genetic engineering

Given that intelligence is partly influenced by brain structure and the genes governing its development, genetic engineering has been proposed as a means to enhance cognitive abilities, a concept often explored in science fiction as biological uplift . Experiments with genetically modified mice have demonstrated improvements in learning and memory [52]. The ethical implications of such interventions, however, are profound and warrant careful consideration.

Education

Education plays a pivotal role not only in imparting knowledge but also in boosting intelligence. Studies indicate that higher IQ correlates with greater educational attainment [53], and conversely, education appears to raise IQ scores [54]. A meta-analysis from 2017 suggests that each year of education can increase IQ by 1–5 points, or at least enhance performance on IQ tests [55].

Nutrition and chemicals

Substances purported to enhance cognitive functions are known as nootropics . While meta-analyses show that omega-3 fatty acids may improve cognitive performance in individuals with deficits, their effect on healthy subjects is less clear [56]. Omega-3s have also been linked to mood improvement in patients with major depression, a condition often associated with cognitive deficits [57]. The exploration of nutraceuticals and other dietary interventions for cognitive enhancement continues to be an active area of research.

Activities and adult neural development

Engaging in mentally stimulating activities and maintaining a healthy lifestyle are crucial for cognitive function throughout adulthood.

• Regular exercise has been shown to enhance cognition in both healthy and unhealthy individuals [58].
• Intellectually demanding work and the manner in which one engages in work can positively influence cognitive abilities [59, 60].
• Adequate quality of sleep is fundamental for cognitive restoration and optimal functioning [61].

Digital tools

The proliferation of digital technology, particularly smartphones , has spurred research into its cognitive impacts. While these tools offer unprecedented access to information and facilitate communication, concerns have been raised by educators and experts regarding potential negative effects on critical thinking and academic performance [62].

Brain training

Programs designed to improve cognitive abilities, often referred to as brain training , have shown some success in enhancing specific task-related skills, such as working memory . However, the extent to which these improvements generalize to overall intelligence remains uncertain [63]. A notable study in 2008 suggested that practicing a dual n-back task could increase fluid intelligence (gf) [64], sparking widespread media attention [65]. Yet, subsequent methodological critiques have questioned the validity of these findings, citing issues with test standardization and time constraints [66].

Philosophy

The prospect of artificially enhancing human intelligence raises complex ethical questions. Neuroethics delves into the ethical, legal, and social implications of advancements in neuroscience, including the distinction between treating neurological disorders and enhancing cognitive abilities, as well as the role of socioeconomic factors in accessing neurotechnology . These issues are closely linked to the ethics surrounding human genetic engineering . Transhumanist thinkers explore the potential consequences of developing technologies to augment human capabilities. Historically, eugenics , a social philosophy advocating for the improvement of hereditary traits through intervention, has had a controversial past, falling into disrepute after World War II due to its association with Nazi Germany [67, 68].

Measuring

The predominant approach to assessing human intelligence relies on psychometric testing, a method with a long history and extensive research backing, widely used in practical applications [15]. Prominent intelligence quotient (IQ) tests include the Stanford-Binet , Raven’s Progressive Matrices , the Wechsler Adult Intelligence Scale , and the Kaufman Assessment Battery for Children . Additionally, various psychometric tests are designed to measure related constructs, such as scholastic aptitude, including the SSAT , SAT , ACT , GRE , MCAT , LSAT , and GMAT in the United States [15]. Regardless of the specific instrument, tests requiring complex reasoning and a range of difficulty levels typically yield intelligence scores that approximate a normal distribution within the general population [69, 70].

The efficacy of intelligence tests in predicting behavior has led to their widespread use in educational, business, and military settings. IQ scores and the general intelligence factor (g) demonstrate significant correlations with important life outcomes: lower IQ is associated with higher rates of divorce, out-of-wedlock births, incarceration, and reliance on welfare, while higher IQ correlates with greater educational attainment, higher-status occupations, and increased income [72]. Intelligence, as measured by psychometric tests, is also strongly correlated with success in training and job performance [73, 74, 75]. While IQ/g is considered the best single predictor of job performance, some researchers advise caution, citing statistical assumptions, historical inconsistencies in studies predating 1970, and ongoing debates about the validity of current measurement tools [76, 77].

General intelligence factor or g

The concept of a general intelligence factor (g), first proposed by Charles Spearman through factor analysis of correlations between various cognitive tasks, remains central to psychometric theory [78]. Spearman observed a positive manifold, indicating that most cognitive tests correlate positively with each other, which he attributed to a single underlying factor, g, influencing performance across all cognitive tasks. While this “common cause” interpretation is dominant, an alternative “mutualism” model suggests that intelligence emerges from interacting independent mechanisms that mutually support each other, thereby creating the positive manifold [79].

Intelligence tests can be ranked by their “g-loadings,” indicating how strongly they correlate with other tests. Raven’s Progressive Matrices , a nonverbal reasoning test involving abstract designs, consistently shows high g-loadings [80]. However, this test also reveals significant gender differences that are less apparent when g is measured directly from a broader battery of tests [81, 82]. Critics, such as Stephen Jay Gould , have challenged the concept of g, viewing it as a statistical artifact rather than a true representation of intelligence [83, 84]. Despite these criticisms, the American Psychological Association’s 1995 report “Intelligence: Knowns and Unknowns ” affirmed the correlations observed in IQ testing, characterizing the view of g as an artifact as a minority position.

General collective intelligence factor or c

Recent research has extended the study of intelligence to groups, exploring the concept of collective intelligence —a group’s general capacity to perform a wide range of tasks [85]. Employing methodologies similar to those used in individual intelligence research, this field seeks to identify a “c factor” that explains inter-group performance variations and their underlying causes [86, 87].

Historical psychometric theories

Historically, several theories have significantly shaped the field of psychometrics , often emphasizing factors beyond a singular general intelligence.

Cattell–Horn–Carroll theory

The Cattell–Horn–Carroll theory has profoundly influenced the design of modern intelligence tests. This hierarchical model places g at the apex, followed by ten broad abilities, each further subdivided into narrower abilities [88]. These broad abilities include:

• Fluid intelligence (Gf): The capacity for reasoning, concept formation, and problem-solving in novel situations.
• Crystallized intelligence (Gc): Acquired knowledge, communication skills, and the ability to use learned experiences.
• Quantitative reasoning (Gq): Comprehension and manipulation of quantitative concepts and symbols.
• Reading & writing ability (Grw): Fundamental literacy skills.
• Short-term memory (Gsm): Holding and manipulating information in immediate awareness.
• Long-term storage and retrieval (Glr): Storing and accessing information over time.
• Visual processing (Gv): Perceiving, analyzing, and synthesizing visual information.
• Auditory processing (Ga): Analyzing, synthesizing, and discriminating auditory stimuli.
• Processing speed (Gs): Performing routine cognitive tasks efficiently, especially under pressure.
• Decision/reaction time/speed (Gt): The immediacy of response to stimuli, distinct from Gs [88].

Modern tests may not encompass all these abilities, and some, like Gq and Grw, are sometimes viewed as measures of academic achievement rather than core intelligence. The distinction between Gf and Gc, once thought to align with performance and verbal subtests respectively, is now understood to be more complex [88].

Insufficiency of measurement via IQ

While IQ tests demonstrate strong reliability and predictive validity for academic and certain occupational outcomes, their capacity to measure the full spectrum of human intelligence is debated. Critics like Keith Stanovich and Robert Sternberg argue that focusing solely on IQ neglects crucial aspects of mental ability, such as creativity and social intelligence [90, 91, 92]. Psychologist Wayne Weiten cautions that the validity of IQ tests is “questionable” when assessing intelligence in a broader sense [89]. Despite these critiques, clinical psychologists often find IQ scores sufficiently valid for diagnostic purposes, including identifying intellectual disabilities and informing personnel decisions, due to their standardized nature and interpretable results [93, 94]. Emerging research suggests that intelligence may comprise distinct cognitive systems, potentially influenced by separate genetic factors, impacting abilities like reading and language differently from general intelligence [95, 96, 97, 98]. Measures beyond traditional IQ tests include cognitive flexibility , moral intelligence , prioritization abilities, and direct assessments of brain activity via neuroimaging [100, 100].

Intelligence across cultures

The definition and manifestation of intelligence are profoundly shaped by cultural context. Robert Sternberg emphasizes that a singular, Western-centric definition of intelligence risks an “egocentric view” and fails to capture diverse cultural interpretations [102]. For instance, in many East Asian cultures, intelligence is closely tied to social roles and responsibilities, while in some African communities, it is exemplified through social responsibilities and the ability to take on burdens. Within American culture, while IQ and problem-solving skills are often highlighted, other interpretations emphasize social conscience and the capacity for providing wisdom [103]. Sternberg’s concept of “successful intelligence” seeks to bridge these cultural divides by defining intelligence as the skills and knowledge needed for success according to an individual’s own definition within their sociocultural context.

Motivational intelligence

Motivational intelligence refers to an individual’s capacity to understand and leverage various motivations, such as the drive for achievement, affiliation, or power. It involves appreciating tacit knowledge related to these drives and recognizing diverse values, behaviors, and cultural differences with intrinsic interest. Research indicates a correlation between motivational intelligence, cross-cultural experiences, and leadership effectiveness [104, 105, 106]. Studies have also found motivational intelligence to be a significant predictor of job motivation, sometimes surpassing other forms of intelligence [107]. Therapeutic approaches rooted in motivational intelligence emphasize a collaborative relationship between therapist and client, fostering change through mutual understanding rather than imposition [108, 109, 110].

See also

• Evolution of human intelligence
• Flynn effect
• Genius
• Intellect
• Neuroscience and intelligence § Humans
• Outline of human intelligence
• Self-test of Intelligence
• Sex differences in intelligence
• Superintelligence
• Theory of multiple intelligences
• Volition (psychology)