How much is 2 × 4? Who wrote Slaughterhouse Five? How do you tie a shoe? How are a pear and a chicken alike? Intelligence is said to separate humans from other animals (although more broadly defined, intelligences have been studied in plants and animals). There is no sound agreement on what actually constitutes intelligence; for example, are there many different types of intelligence or just one? This chapter covers some of the more popular theories of intelligence, how intelligence is expressed in the brain, and how consciousness relates to intelligence.
The conception of “intelligence” is actually a relatively new concept that was not defined until about a century ago. The word intelligence comes from the Latin word root inter, which means “between” or “within,” and the Latin root legere, meaning “to pick out, bring together, choose, catch with one’s eye,” or “to read.” Thus, intellegere can be interpreted as “to perceive,” “see into,” or “to understand.” Most people define intelligence by referencing examples of behavior. When one references a behavior in relation to intelligence, one is conceiving intelligence as being adaptive in nature, a common conceptualization among intelligence theorists. One of the most important aspects of intelligence is that it allows for predicting future behaviors. An intelligent person can predict, with good accuracy, what the consequences of his behavior will be (of course, this does not mean the person will always make the best choice). Likewise, others can predict that an intelligent person will behave in a manner consistent with being able to adapt to the environment and understand his situation. Formal theories of intelligence have defined intelligence in a number of different ways, ranging from abstract thought to reasoning abilities to understanding emotions. Some of the more common conceptualizations of intelligence from major theorists are discussed here.
English aristocrat and cousin to Charles Darwin, Francis Galton revived the term “intelligence” and made the first genuine attempt to measure intelligence. Galton was committed to the notion that there was a hereditary basis to intelligence, and he coined the popular term “nature versus nurture” to explain differences in innate abilities and learned ones. His intelligence-type tests were based on his notion that intelligence was a reflection of a person’s neurological efficiency and thus inherited. Galton believed that intelligence could be measured through physical attributes, such as sensory perception, grip strength, and reaction time. In the late 1800s, Galton attempted to correlate such psycho-physiological measures with a person’s rank in school or occupation. However, he found no such relationship.
Around the same time, James McKeen Cattell, a doctoral student from America, was performing reaction time experiments in William Wundt’s laboratory in Germany. He became aware of Galton’s work and began to communicate with him. Cattell developed his own series of psycho-physiological tests based on Galton’s earlier ideas. The early efforts of both Galton and Cattell were very influential in experimental psychology and psychological testing.
Alfred Binet, a French psychologist, was challenged by public school officials to develop a test that would be able to identify children who were at risk of falling behind in their academic achievement. As a result, Binet, along with collaborator Theodore Simon, developed the first bona fide intelligence test in 1905. Binet also developed the forerunner to the notion of the “Intelligence Quotient” or IQ, which was the concept of “mental age” or “mental ability.” However, his concept of how one’s intelligence was to be derived, as well as his overall concept of intelligence, has been modified.
The original Simon-Binet test was made up of an assortment of items, with the goal of measuring knowledge and skills that an “average” French schoolchild of a specific age would possess. Items consisted of such things as verbal knowledge of food, naming objects, placing weights in order, and other verbal and tactile tests. The test items were graded in terms of their difficulty according to one’s age. Thus, items that an average twelve-year-old could answer should be missed by a younger child of six. The difficulty level was determined by administering the tests to fifty normal children aged three to eleven years old and to some mentally challenged children and adults. By determining the average score on the test items for children of specific ages, Binet was able to determine what the average child of a certain age should be able to complete. He then could designate a specific child’s “mental age” by her test performance on the same items compared to the performance of his standardization group (Binet did not like the term “mental age,” instead preferring “mental level”). Children with mental level results significantly lower than their chronological age were identified as at risk of falling behind in school. No overall formulation or overall score was calculated as in current IQ tests.
Binet’s approach to the understanding of intelligence was functional; he wanted to predict something. One flaw that is often noted is that his original tests included many verbal items and only some physiological measures. This reflected Binet’s belief that intelligence was the capacity to be able to make judgments about relationships, comprehend meanings, and draw conclusions from relationships, all of which are primarily verbal in nature.
Charles Spearman was a student in Wundt’s psychology laboratory. Spearman believed that intelligence was a general ability involving mainly the ability to see relations and correlates. Like both Cattell and Galton, Spearman preferred the notion of a biologically based, single source of human intelligence, even though earlier research had not found statistically significant relationships (correlations) between mental tests and physical capabilities. Spearman demonstrated that uncorrected correlation coefficients underestimated the actual strength of the relationship between any set of variables, and that such an underestimation is severe if the tests have a restricted range of values, like Cattell’s tests had. (An intelligence test item score is restricted to a limited number of values.) Spearman developed a statistical correction formula and used it on Cattell’s data, resulting in significant positive correlations among the mental tests and physical attributes. Using his factor-analytic method, he was able to demonstrate that all intelligence test items loaded heavily on a single factor, which he named the “general factor” or as it is known commonly, g. This factor still forms the foundation for many current theories of intelligence.
During World War I there was a need for the U.S. Army to develop a quick-administration intelligence test for deciding the type of training that a recruit would receive. Psychologists Robert Yerkes, Lewis Terman, and others worked together and developed the Army Alpha and Army Beta tests. The Alpha test stressed verbal abilities, whereas the Beta test stressed nonverbal abilities. The Beta test was administered to those performing poorly on the Alpha test, and after the war, these tests became the models for future intelligence tests. After World War II, Terman, who was at Stanford University, translated the Binet-Simon tests into English and adapted them for American schools, renaming the test the Stanford-Binet Test. Terman later took the notion of mental age and one’s chronological age to compute a new metric called the IQ via a ratio method: (Mental Age/Chronological Age) × 100 = IQ. This new score was consistent with Spearman’s notion of g and Terman’s view of intelligence as the capacity to form concepts and to grasp their significance. The current derivation of an IQ score is not based on the concept of mental age, but is calculated by comparing how one scores in comparison to how one’s reference group performs on the test (a reference group is comprised of people with the same demographic characteristics).
Psychologist David Wechsler did not believe that available intelligence tests were accurate measures as they overemphasized verbal abilities (e.g., Binet’s tests). Wechsler devised probably the most widely used series of intelligence tests that are in some ways similar to the Stanford-Binet test. However, he also included a number of nonverbal or performance tasks based on the Army Beta tests. The original Wechsler tests provided three separate IQ scores: Verbal IQ, Performance IQ (based on nonverbal items), and a Full Scale or overall IQ based on the other two scores. The Full Scale IQ score was similar to Spearman’s g. Wechsler also used a different form of standardization that was based on the normal distribution and allows for the expression of IQ in terms of standard deviations from the mean or average IQ score. This allows the same IQ score of different age groups to have the same percentile rank, which was not the case in the other methods of calculating IQ scores, such as in Terman’s ratio method.
Some theorists have objected to the notion that intelligence represents a unitary construct. Perhaps the most well-known of these is Howard Gardner’s theory of multiple intelligences. Gardner believes that intelligence tests are too restrictive and has outlined at least ten different intelligences, ranging from spatial and linguistic to moral intelligences.
There have been many criticisms of Gardner’s approach. First, Gardner provides no empirical support for his designations, and there is none elsewhere to support them. Second, psychometric studies have constantly supported the notion of a unitary factor common to intelligence tests that is made up of many related functions. Third, modern intelligence tests measure many different abilities that reflect an overall underlying factor. What Gardner has done is to substitute “intelligence” for certain abilities and to try to demonstrate that these abilities are mutually exclusive forms of intelligence, when in fact, they most likely reflect different aspects of an underlying general construct.
Gardner purports nine separate intelligences: spatial, linguistic, logical-mathematical, bodily-kinesthetic (such as athletics), musical, interpersonal (social abilities), intrapersonal (self-knowledge), naturalistic (relating information to one’s natural surroundings), and existential (the ability to consider information transcending sensory inputs, such as infinity).
The Cattell-Horn-Carroll theory of cognitive abilities is a hierarchical model of intelligence that has received solid empirical support. The original theory proposed two broad categories of intelligence: crystallized intelligence, which involves knowledge that comes from prior learning and past experiences, and fluid intelligence, which is the ability to perceive relationships independent of previous practice. Carroll expanded on the Cattell-Horn theory and proposed a model that contains over seventy specific abilities that can be categorized under eight primary second-order abilities. The eight primary abilities are all part of an overall g ability (general intelligence).
Suppose that Jerry is a baseball player. Jerry can hit home runs, but he has a low batting average (for baseball people, suppose Jerry hits .250). In addition, Jerry can throw the ball hard, but his overall fielding skills are below average. One might say that Jerry can hit home runs, but he is an average hitter. Because Jerry’s overall baseball skills are not well developed, it would be wrong to say that Jerry is a good baseball player. Hitting is an aspect of the game of baseball, as is fielding. They are skills that comprise the concept of “baseball.” Likewise, intelligence is conceived of as an overall concept that is composed of many different skills. A person may be a good mechanic, but he may also score poorly on intelligence tests. Like Jerry, he may have a specific skill that he is very good at, but he may not be conceived of as an “intelligent person.”
One of the observations that should become apparent from discussion of the general theories of intelligence is that most of them refer to an overall or general intelligence factor (often called g as Charles Spearman referred to it) that is comprised of other specific abilities. This does not necessarily mean that there are different types of intelligences, as Howard Gardner labels them, but that the concept of intelligence is composed of many different types of abilities that are separate and yet related. Because there is much variation in behavior, most people will exhibit strengths in some abilities and relative weaknesses in others.
Decision making depends on a number of factors, and while it is aided by intelligence, it is not a direct product of it. Some very creative people have performed poorly on intelligence tests, and some very intelligent people have made some very questionable decisions. In general, intelligence is one of the best predictors of life outcomes, but it is not a perfect predictor.
A distinction made by laypeople is one of being “book smart” or having “common sense.” This certainly can be a distinction; however, most intelligence tests have a component that measures common-sense reasoning. Furthermore, research has indicated that people who score higher on intelligence tests generally do better in life, such as having better jobs, being more satisfied with their lives, and being better adjusted. The important thing to understand about this relationship is that it is not perfect; not everyone who scores well on intelligence tests does well in life or makes good decisions, but many more of them do.
One of the reasons to understand what intelligence is, and what the components of intelligence are, is to allow people to develop and use their intelligence constructively. When the concept of intelligence is better understood, researchers can identify what aspects of intelligence can be modified.
One interesting relationship regarding intelligence across all species of animals is the relationship between brain size, body size, and intelligence. The larger the ratio of brain size to body size, the more intelligent the species tends to be, although this is not always the case. There also seems to be a link between intelligence and the relative size of the prefrontal cortex in animals. Primates and dolphins have the largest prefrontal cortices and also are the most intelligent of animals. If this is the case, then it follows that there is a relationship between genetics and intelligence.
As is the case with almost every trait—even behavioral traits—intelligence, as measured by IQ scores, appears to have a significant genetic component. The estimates of the heritability of scores on IQ tests typically are around the 0.5 range, although some studies report stronger heritabilities and others weaker heritabilities. Heritability refers to the proportion of a trait that is expressed by a particular genotype (set of genes) in a particular environment, but it does not refer to how much of a trait is caused by the genotype or genes. There are two important things to take home from this finding: First, all traits most likely have some inherent factor that contributes to their expression; and second, all traits are expressed via the interaction of genes and the environment.
Still, the relationship between genes and intelligence is real and undeniable. A person’s genotype does determine in part how well that person will score on an IQ test. Probably the best way to view a genetic component to intelligence is that it helps to define the capacity of the person to express that particular trait. Genetic contributions define the upper limits of the trait (e.g., intelligence) that the individual can reach given a “perfect” environment. Moreover, very small differences in genotypes can be enhanced by experience and upbringing. Therefore, environment is the modifiable component of intelligence. Also, people who are inherently more gifted at learning will often seek to learn, thus furthering their mental abilities, whereas someone who inherently has trouble reading, for example, may not read as often. Thus, the genetic and environmental components of intelligence are interrelated and basically inseparable.
IQ tests were originally designed to measure a student’s potential to do well in the French public school system and then were applied in other countries. Over the years, and after many modifications, they have come to be identified with intelligence. A few years ago, a controversial book purported that certain ethnic groups demonstrated overall lower performances on IQ tests than other groups and that intelligence was inherited. As previously discussed, intelligence depends on experience and inherent factors. Of course when educational and socioeconomic backgrounds and opportunities are equalized between different groups, many of these aforementioned differences in IQ scores disappeared or were significantly reduced.
Studies of rodents in enriched environments have demonstrated how the brain is affected by the stimulation provided by its surroundings. Rodent brains from animals exposed to more enriched environments have demonstrated increased numbers of synapses and thicker cortices. This effect has been documented in rats placed in these environments that were the equivalent of eighty, and even ninety, years old!
The first thing to understand is that while IQ is a measure of intelligence, IQ is not the same thing as intelligence. Intelligence is a complex, abstract, psychological construct. Constructs are explanatory concepts that are not directly observable, such as things like thought, personality, feelings, the number three, pi, and intelligence. What are observed are the results of these “things”; how they affect the world, and not the construct itself (for instance, you can show someone three apples, three clocks, or three brains, but you can never show just three). Constructs are used to develop scientific theories, describe events, make predictions, and classify things. IQ tests are used to predict a person’s ability to function in environments such as school, or to determine if a person may have some deficit in functioning related to brain damage or a developmental issue. Intelligence, which is in part measured by IQ tests, is a broader construct.
People who are at the significantly low end of the IQ spectrum (IQ scores of 65 and below) were in the past designated “mentally retarded.” That term has lost favor, and the appropriate term is now having a “developmental disability.” As a group, these individuals often need some form of supervision, depending on the level of disability. At the other end of the spectrum, intellectually gifted individuals often do well in school and life, but not always, as IQ does not predict things like motivation or measure things like self-control or emotional stability. People who fall between these extremes are capable of quite a bit of variation in performance.
Mental retardation (MR) is defined by an IQ score of 65 or lower and significant developmental impairment. IQ levels for MR are: mild (50–69, can be educated to the level of an eight- to eleven-year-old); moderate (35–49, trainable to attain a four- to seven-year-old level); severe (20–34, requires lifelong supervision, but can learn some personal maintenance skills); and profound (below 20, requires custodial care).
There are some individuals with overall low IQ scores who are known as savants. This is not a common occurrence in individuals with severe developmental disabilities, such as autism, but when this phenomenon does occur, such people typically have an extraordinarily well-developed mental capacity that is often limited to one domain. For example, the person may be able to calculate the day of the week or the specific date for any particular historic event or display extremely well-developed arithmetic abilities that rival the fastest computers. There seems to be no reliable neuroanatomical feature in these cases, and even in cases where the savant is not autistic, these skills seem to be counterbalanced by poorly developed skills in other areas that are necessary for normal living. Such skills do not fit the conceptualization of intelligence, but instead appear to be isolated special abilities.
Philosophers and psychologists have debated the meaning of consciousness for centuries. At one point it was believed that consciousness could not be defined; however, with the advent of sophisticated neural imaging techniques and other new technologies, the understanding of what it means to be conscious has broadened greatly. Probably the most commonly accepted notion of consciousness divides the notions of pure consciousness and pure awareness.
Awareness includes a certain state of perception that results in behavior, and that can be experienced. The difference between pure awareness and consciousness is mostly due to language. Consciousness allows for one to use language to relate one’s experience and classify perceptions that are created by awareness. Consciousness is linked to memory such that, through language, one can link current experiences with prior concepts. Interestingly, a great deal of the research on intelligence has also associated intelligence with language abilities, despite the fact that different skills and abilities that contribute to intelligence are labeled as not being language-based.
The expression of intelligence in the brain is largely thought to be a function of the frontal and prefrontal cortex; however, the notion of intelligence requires the ability to use the entire brain, especially language-based abilities, which reside in the temporal lobes. Moreover, many severe injuries to the frontal lobes, such as a lobotomy, may not significantly affect IQ scores.
Recall that the human brain is composed of two hemispheres connected by commissures, the largest of which is the corpus callosum. Humans who have had the majority of their left hemisphere destroyed as adolescents or adults (recall that the left hemisphere is the dominant hemisphere for language) are often reputed to be profoundly mentally retarded and appear to respond to stimuli in an approach-avoidance manner. People with right hemisphere damage will often have poor visuospatial processing, but often appear to have relatively normal cognitive functioning. The famous neuroscientist Michael Gazzaniga developed the notion that the left side of the brain tries to make sense of the world by using language and contains an interpreter that is constantly making up a verbal story about experiences. Without the interpreter, awareness would exist, but it would be awareness without context, and behavior would be expressed in terms of stimulus-response associations. Thus, consciousness and intelligence are intimately related.
A number of neurobiological theories have attempted to relate conscious experience with neural activity, and there are many proposed neural correlates to conscious states. For instance, Crick’s theory proposes that consciousness results from the activity of neural assemblies, which are collections of neurons. Crick’s theory is represented by measuring the electrical activity of the brain and oscillations in the cortex, which form the basis of consciousness and are interconnected with different forms of sensory awareness. Binding theory refers to the process where separate pieces of information, about a single stimulus, are brought together and used for further processing later.
Unfortunately, Crick’s theory has never been able to explain the importance of the brain oscillations that allegedly give rise to conscious experience, and the binding theory has never been empirically demonstrated. Where do all of these isolated experiences come together in the brain? There is no good answer. One hypothesis proposes that consciousness requires the involvement of the sensory cortical areas that include the frontal lobe, parietal lobe, occipital lobe, and the thalamus. A coma refers to a state of unconsciousness that is not sleep and typically lasts more than six hours. A coma typically results from severe brain trauma or another insult to the brain. A vegetative state occurs when a person comes out of a coma (his eyes open) but he otherwise does not appear to be responsive to stimuli. Studies using brain imaging methods have been able to determine that the sensory areas of the brain in comatose and anesthetized patients respond to familiar music via input relayed from the thalamus to the frontal lobes. However, in truly vegetative patients, there is no feedback signal from the frontal lobes back to the sensory areas of the cortex, as occurs in conscious individuals. These findings seem to support this hypothesis of consciousness. Damage to other areas of the brain, such as to the temporal lobes and frontal lobes, can also disrupt consciousness as seen in post-traumatic amnesia and other types of amnesia.
The notion of an “unconscious mind” is the foundation of many psychological theories of behavior, especially those of Freud and his followers. However, Freud’s ideas have been replaced by a different notion of unconscious processing, such as the large component of memory that is implicit in nature, in that the person often cannot consciously recall where the memory came from. One cannot usually recall when and how one learned to tie one’s shoes, but one still can perform the procedure effortlessly. However, there is also empirical evidence to suggest that a fair amount of unconscious processing goes on in the brain directed at expressing attitudes and motivations. This has become referred to as implicit cognition.
Priming is an implicit effect where being exposed to a stimulus influences a later response. For instance, reading or seeing “brain” and later being asked to complete a word starting with br results in an increased probability that one will say “brain” rather than “bring” or “broke.” Priming is used to measure attitudes, understand brain processing, and judge the effects of brain damage.
Implicit attitudes are the positive or negative feelings, thoughts, or even actions that are directed at objects in the environment and that have developed due to past experiences. A person is either unaware of this attitude or cannot attribute it to a specific previous experience. These attitudes are often revealed in studies using specific measuring techniques of reaction time or choice preferences. For example, in a version of a test known as the Implicit Association Test, participants in an experiment make split-second decisions about whether or not to shoot intruders in a video game format. The decision to shoot or not to shoot is based on whether the video intruder is holding a gun or not. Many people exhibit an implicit bias toward blacks as measured by faster and more frequent decisions to shoot the intruder when he is depicted as a black man. Implicit cognition is studied in a number of contexts and relates to the automatic-processing mental component of the dual process model of cognition. It is fast and acts without conscious awareness.