Introduction to Intelligence

Defining Intelligence

Over the last century or so, intelligence has been defined in many different ways.

Learning Objectives

Trace the history of the study of human intelligence

Key Takeaways

Key Points

  • Intelligence involves the ability to adapt to one’s environment and the capacity to learn from experience.
  • Charles Spearman concluded that there was a common function across intellectual activities, including what he called ” g ” or general intelligence. Research has found “g” to be highly correlated with many important social outcomes and is the single best predictor of successful job performance.
  • The current American Psychological Association conceptualizes intelligence as a hierarchy of lower order intelligence factors with “g” at its apex.
  • Emotional intelligence and social intelligence have been positively associated with good leadership skills, good interpersonal skills, positive outcomes in classroom situations, and better functioning in the world.
  • In recent years, theorists such as Garder and Sternberg have proposed theories of multiple intelligence.

Key Terms

  • emotional intelligence: The ability, capacity, or skill to perceive, assess, and manage the emotions of oneself, of others, and of groups.
  • social intelligence: The ability to understand and manage men and women, boys and girls; to act wisely in human relations.
  • intelligence: Capacity of mind, especially to understand principles, truths, facts or meanings, acquire knowledge, and apply it to practice; the ability to learn and comprehend.

The meaning of the word “intelligence” has been hotly contested for many years. In today’s psychological landscape, intelligence can be very generally defined as the capacity to learn from experiences and adapt to one’s environment, but thanks to the many different theories of intelligence that have been developed over the last century or so, there are many different frames in which to discuss it.

“General” Intelligence

Francis Galton, influenced by his half-cousin Charles Darwin, was the first to propose a theory of intelligence. Galton believed intelligence was a real faculty with a biological basis that could be studied by measuring reaction times to certain cognitive tasks. Galton measured the head sizes of British scientists and ordinary citizens, but found no relationship between head size and his definition of intelligence.

A deeper search for understanding of human intelligence began in the early 1900s when Alfred Binet began administering intelligence tests to school-age children in France. His goal was to develop a measure that would help determine differences between normal and subnormal children. Binet’s research assistant, Theodore Simon, helped him develop a test for measuring intelligence. It became known at the Binet-Simon Scale, the predecessor for the modern IQ test.

In 1904, Charles Spearman published an article in the American Journal of Psychology titled “General Intelligence.” Based on the results of a series of studies collected in England, Spearman concluded that there was a common function across intellectual activities that he called g, or general intelligence. Since the article, research has found g to be highly correlated with many important social outcomes and the single best predictor of successful job performance. The current American Psychological Association definition of intelligence involves a three-level hierarchy of intelligence factors, with g at its apex.

In 1940, David Wechsler became a major critic of general intelligence and the Binet-Simon scale. He was a very influential advocate for the concept of non-intellective factors (variables that contribute to the overall score in intelligence, but are not made up of intelligence-related items, including lack of confidence, fear of failure, attitudes, etc.), and he felt that the Binet-Simon scale did not do a good job of incorporating these factors into intelligence. He suggested that these factors were necessary for predicting a person’s capability to be successful in life. Wechsler further defined intelligence as the capacity of an individual to act purposefully, to think rationally, and to deal effectively with his or her surroundings or situation.

A brief history of intelligence testing: In this video, Philip Zimbardo explores a brief history of intelligence testing, beginning with Binet’s original tests. The video concludes with a brief discussion of modern notions of IQ and intelligence testing.

Multiple Intelligence

An early theory of multiple intelligence is attributed to Edward Thorndike, who in 1920 theorized three types of intelligence: social, mechanical, and abstract. Thorndike defined social intelligence as the ability to manage and understand people. He focused on behavior rather than consciousness in his research; as such, his studies constituted the beginning of investigations related to social intelligence.

In the mid-20th century, Raymond B. Cattell proposed two types of intelligence rather than a single general intelligence. Fluid intelligence (Gf) is the capacity to think logically and solve problems in novel situations, independent of acquired knowledge. Crystallized intelligence (Gc) is the ability to use skills, knowledge, and experience. It does not equate to memory, but it does rely on accessing information from long-term memory. Cattell hypothesized that fluid intelligence increased until adolescence and then began to gradually decline, while crystallized intelligence increased gradually but remained relatively stable across most of adulthood until declining in late adulthood.

In more recent decades, many new theories of multiple intelligence have been proposed. In 1983, Howard Gardner published a book on multiple intelligence that breaks intelligence down into at least eight different modalities: logical, linguistic, spatial, musical, kinesthetic, naturalist, interpersonal, and intrapersonal intelligences. A few years later, Robert Sternberg proposed the Triarchic Theory of Intelligence, which proposes three fundamental types of cognitive ability: analytic intelligence, creative intelligence, and practical intelligence.

Emotional Intelligence

In 1990, Peter Salovey and John Mayer coined the term “emotional intelligence” and defined it as “the ability to monitor one’s own and others’ feelings, to discriminate among them, and to use this information to guide one’s thinking and actions. ” Hendrie Weisinger also worked with theories of emotional intelligence. He emphasized the significance of learning and making emotions work to improve oneself and others. He documented and illustrated the positive effect emotions could have in personal settings and work environments. Both emotional intelligence and social intelligence have been positively associated with good leadership skills, good interpersonal skills, positive outcomes in classroom situations, and better functioning in the world.

Theories of Multiple Intelligence

Theories of multiple intelligence contend that intelligence cannot be measured by a single factor.

Learning Objectives

Review the major theories of multiple intelligences

Key Takeaways

Key Points

  • Savant syndrome demonstrates how an individual who appears to be intellectually deficient, based on traditional definitions of intelligence, can display exceptional abilities in a specific area or areas.
  • Howard Gardner identified eight specific intelligences (including bodily-kinesthetic, linguistic, spatial, intrapersonal, interpersonal, musical, naturalist, and logical-mathematical) and two additional tentative ones (spiritual and existential).
  • Robert Sternberg’s Triarchic Theory divides intelligence into three dimensions that work together: componential, experiential, and contextual.

Key Terms

  • savant: An individual who has below-average IQ based on traditional measures, yet displays an exceptional ability in an area such as math, music, or art.

Today, the most widely accepted theory of intelligence is the “three stratum theory,” which recognizes that there are three different levels of intelligence, all governed by the top level, g , or general intelligence factor. However, there are alternate theories of multiple intelligence which are useful in their own way for delineating certain intellectual skill sets which vary between people. Additionally, certain individuals, such as those with savant syndrome, do not fit into traditional definitions of intelligence; multiple intelligence theory can offer a helpful way of understanding their situations.

Gardner’s Multiple Intelligence Theory

In 1983, Howard Gardner proposed a view of multiple intelligences from which our thoughts and behaviors develop. According to Gardner’s theory, these intelligences can emerge singularly or can mix in a variety of ways to achieve very diverse end results. Gardner identified eight specific intelligences and two additional tentative ones:

  • Bodily-kinesthetic intelligence: the control and use of one’s body (e.g.,  dance, sports, art, primitive hunting, etc.)
  • Linguistic intelligence: the use of language and communication
  • Spatial intelligence: visual perceptions and manipulations (e.g.,  packing items into a box, reading a map, etc.)
  • Intrapersonal intelligence: knowing oneself, emotional awareness, motivations, etc.
  • Interpersonal intelligence: discerning the emotions and motivations of others
  • Musical intelligence: competencies related to rhythm, pitch, tone, etc., and areas related to composing, playing, and appreciating music
  • Naturalist intelligence: discerning patterns in nature
  • Logical-mathematical intelligence: numerical abilities and logical thinking
  • Spiritual intelligence: (tentative) recognition of the spiritual
  • Existential intelligence: (tentative) concern with ultimate state of being

Sternberg’s Triarchic Theory of Intelligence

In 1986, Robert Sternberg proposed a Triarchic Theory of intelligence. His theory organizes intelligence into three dimensions that work together: componential, experiential, and contextual.

The componential dimension includes an individual’s mental mechanisms, and is composed of three parts:

  • Metacomponents: processes used in planning, monitoring, and evaluating the performance of a task. These direct all other mental activities
  • Performance components: strategies in executing a task
  • Knowledge acquisition components: processes involved in learning new things

The experiential dimension involves the way that individuals deal with the internal and external world. This dimension looks at how individuals deal with novelty and the eventual automation of processes. Finally, the contextual dimension examines how individuals adapt to, shape, and select the external world around them.

Savant Syndrome

Savant syndrome identifies individuals who are considered to be intellectually deficient, yet have extremely well-developed talents or skills in a specific area, often art, music, or math. For example, Kim Peek is a savant who was born with considerable brain damage including an enlarged head, a missing corpus callosum (the fibers that connect the two hemispheres of the brain), and a damaged cerebellum. Peek scored at below average intelligence when tested, and he had difficulty with gross and fine motor activities. But Peek’s savant abilities were demonstrated through his ability to read and memorize material extremely quickly. He was reported to read books two pages at a time, reading the right side with this right eye and the left side with his left eye. He was capable of memorizing the material as he read.


Kim Peek: Kim Peek, a savant, was the inspiration for the 1988 film “Rain Man.” Peek was able to read and memorize large amounts of information in a short amount of time, yet scored below average on IQ tests.

Savant syndrome demonstrates that an individual who appears to be intellectually deficient based on traditional definitions of intelligence can display exceptional abilities in a specific area or areas. If a savant such as Peek was measured by Gardner’s multiple intelligence theory, he would be considered to be very gifted in a subtype of intelligence, such as linguistics.

Genetic and Environmental Impacts on Intelligence

Human intelligence is shaped by both internal genetic factors and external environmental circumstances.

Learning Objectives

Relate human intelligence to both genetic and environmental factors

Key Takeaways

Key Points

  • Genetics and the environment are so intertwined in their influence on human intelligence that it remains difficult to determine which, if either, is most responsible for determining a person’s intelligence.
  • Up to 80% of the variation found in adult human intelligence is thought to be attributable to genetics, despite the fact that it is a complicated, polygenic trait.
  • Both sociocultural and biological influences in the environment affect the development of human intelligence.

Key Terms

  • perinatal: Of or pertaining to the time around birth
  • sociocultural: Of or relating to both society and culture
  • heritability: The ratio of the genetic variance of a population to its phenotypic variance; i.e., the proportion of variability that is genetic in origin
  • polygenic: controlled by the interaction of more than one gene

The Great Debate: Nature v. Nurture

The natural genetic make-up of the body interacts with environment from the moment of conception. While extreme genetic or environmental conditions can predominate behavior in some rare cases, these two factors usually work together to produce individual intelligence. There is much debate among researchers and scientists over which influence, genetics or environment, has the largest role in determining overall intelligence, because both have been scientifically established as having a significant impact on intelligence. Recent discoveries have further complicated this debate by proving that the relationship between internal predispositions (“nature”) and external circumstances (“nurture”) not only varies among populations, but also changes over time. Genetics and environment interact constantly, so the question of supremacy in the nature versus nurture debate in human intelligence will probably never be fully answered.


A gene is the unit of heredity by which a biological trait is passed down through generations of human beings. Heritability describes what percentage of the variation of a trait in a population is due to genetic differences in that population (as opposed to environmental factors). Some traits, like eye color, are highly heritable and can be easily traced. However, even highly heritable traits are subject to environmental influences during development. Intelligence is generally considered to be even more complicated to trace to one source because it is a polygenic trait, influenced by many interacting genes.

Twin studies in the western world have found the heritability of IQ to be between 0.7 and 0.8, meaning that the variance in intelligence among the population is 70%-80% due to genetics. Conventional twin studies reinforce this pattern: monozygotic (identical) twins raised separately are more similar in IQ than dizygotic (fraternal) twins raised together, and much more than adoptive siblings.


Heritability Correlations: This chart illustrates patterns in studies of heritability of traits in certain individuals. Even identical twins with shared family history fail to show 100% heritability, which helps explain the high amount of variance in intelligence among human beings.

However, the heritability of IQ in juvenile twins is much lower at 0.45. Heritability measures of IQ have a general upward trend with age (from as low as 0.2 in infancy to 0.8 in late adulthood), leading psychologists to believe that either we rely on or reinforce our genes as we age. This is thought to occur through human interaction with external circumstances, whereby people with different genes seek out different environments. Thus, despite the high heritability of IQ, we can determine that there is an environmental influence as well.

Genetics and Intellectual Disabilities

As mentioned, under normal circumstances intelligence involves multiple genes. However, certain single-gene genetic disorders can severely affect intelligence. Genetic causes for many learning disabilities, such as dyslexia, and neural disorders, such as Down syndrome, autism, and Alzheimer’s disease have been investigated by the field of cognitive genomics, the study of genes as they relate to human cognition. Down syndrome, for example, is a genetic syndrome marked by intellectual disability, and has implications for the ways in which children with Down syndrome learn. While experts believe the genetic cause for Down syndrome is a lack of genes in the 21st chromosome, the gene(s) responsible for the cognitive symptoms have yet to be discovered. And like most traits, the occurrence of neurobehavioral disorders is influenced by both genetic and non-genetic factors, and the genes directly associated with these disorders are often unknown.


Many different environmental influences have been found to shape intelligence. These influences generally fall into two main categories: biological and sociocultural. Biological influences act on the physical body, while sociocultural influences shape the mind and behavior of an individual.

Biological Influences

Biological influences include everything from nutrition to stress, and begin to shape intelligence from prenatal stages onward. Nutrition has been shown to affect intelligence throughout the human lifespan; malnutrition during critical early periods of growth (particularly the prenatal period and during the second year of life) can harm cognitive development. Inadequate nutrition can disrupt neural connections and pathways, and leave a person unable to recover mentally.

Stress also plays a part in the development of human intelligence: exposure to violence in childhood has been associated with lower school grades and lower IQ in children of all races. A group of largely African American, urban first-grade children and their caregivers were evaluated using self-report, interview, and standardized tests, including IQ tests. The study reported that exposure to violence and trauma-related distress in young children was associated with substantial decreases in IQ and reading achievement. Exposure to toxins and other perinatal factors have also been proven to affect intelligence, and in some cases, cause issues such as developmental delays.

Sociocultural Influences

The family unit is one of the most basic influences on child development, but it is difficult to untangle the genetic from the environmental factors in a family. For example, the quantity of books in a child’s home has been shown to positively correlate with intelligence… but is that due to the environmental impact of having parents who will read to their children, or is it an indicator of parental IQ, a highly heritable trait?

A child’s position in birth order has also been found to influence intelligence: firstborn children have been found in some studies to score higher, though criticism has been offered to these studies for not controlling for age or family size. Moving outside of the family unit, human beings are substantially shaped by their respective peer groups. Stereotype threat is the idea that people belonging to a specific group will perform in line with generalizations assigned to that group, regardless of their own aptitude; this threat has been known to affect IQ scores both positively and negatively. That is, if a person belongs to a group that is told they are intelligent, they will appear more intelligent on IQ tests; if they are told they belong to a group that is unintelligent, they will perform worse, even if these distinctions are random and fabricated (as in lab studies). People’s access to education, and specific training and intervention resources, also determines their life-long intelligence level.