1

Intelligence Theory Through the Ages

Tara-Nicholle B. DeLouth

California State University, Bakersfield

Directory of Intelligence Theorists

• 500B.C. - 50 A.D.

Pythagoras (ca. 580-500 B.C.) Aristotle (384-322 B.C.)

Plato (ca. 427-347 B.C.) Philo (ca. 25 B.C. - A.D. 50)

• 900 - 1300

Avicenna (980-1037) St. Thomas Aquinas (1225-1274)

• 1600-1800

Rene Descartes (1596-1650) Franz Joseph Gall (1758-1828)

John Locke (1632-1704) Etienne Bonnot de Condillac (1715-1780)

John Stuart Mill (1806-1873) Gottfried Wilhelm von Leibniz (1646-1716)

James Mill (1773-1836) Immanuel Kant (1724-1804)

Julien de La Mettrie (1709-1751)

• 1800-present

Francis Galton’s (1822-1911) William Stern (1871-1938)

James McKeen Cattell (1860-1944) Charles Spearman (1863-1945)

Alfred Binet (1909-1975) Henry Herbert Goddard (1866-1957) Theodore Simon (1873-1961) Louis Terman (1877-1956)

Robert Yerkes (1876-1956) David Weschler (1896-1981)

• 500B.C. - 50 A.D.

2

One of the largest running, most complex debates in the history of psychology concerns the definition and structure of the construct we now call intelligence. The presently raging dialogue about the nature of human intellectual ability originated in Pythagoras’ (ca. 580-500 B.C.) theory of mind-body dualism. The early Greek cosmologist believed humans were comprised of both physical bodies and powers of reasoning that enabled comprehension of the abstract elements of the universe. This relatively simplistic analogue of intelligence was thought by Pythagoras to be located in the immortal soul.

Plato (ca. 427-347 B.C.) was perhaps the first to develop a theory recognizing individual differences in intelligence levels, which he distinguished on the basis of the extent to which an individual was able to understand purely abstract concepts, or Forms. On the lowest level of Plato’s intelligence taxonomy were the average citizens who were characterized by the passionate soul, capable only of conceiving intangible images (e.g., memories, reflections) of physical objects. These lowly souls had not even the capacity to confront the physical objects corresponding with these images. Above the citizens in this hierarchy lay the soldiers, who exemplified the spirited soul which dealt cognitively with the physical objects themselves. This, however, was still a suboptimal level of intelligence as the highest function allowed by direct cognition of physical objects was the formation of opinions, or beliefs.

The "states of mind" (read: levels of intelligence) characterized by the passionate and spirited souls (imagining and believing, respectively) were considered inferior due to their limited subject matter - the physical world. As such, Plato’s next intelligence level, thinking, was considered superior to these because it dealt with abstract mathematical relationships. Yet because these relationships could potentially be false, the highest level of intelligence (which Plato actually called "intelligence" or knowledge") required full comprehension of the abstract Forms or essences. To know the "Form of the good" was the ultimate in intelligence because it made one able to understand all other Forms and the relationship between them.

Plato also sparked the nature-nurture debate as it regards intelligence. He believed that each individual was naturally dominated by either the passionate, spirited, or rational soul; for those at the lower end of the intelligence spectrum, education held little benefit. For those destined to be philosopher-kings, however, education was necessary to allow one to attain his potential intelligence through the mastery of inner chaos (as infants), the external world (as children), and the forms (as adults). In this way, education was the medium through which the rational soul achieved control over the other souls.

Similarly, Aristotle (384-322 B.C.) postulated different levels of intelligent thought facilitated by the rational soul, the capacity for thought that is unique to humans. From the bottom of this classification scheme up, each level was a necessary but insufficient prerequisite for the next level of thought. For example, the ability to understand isolated sensations was prerequisite to the common sense’s ability to integrate sensations toward reaching a conclusion about their shared origin. Passive reason utilized these conclusions to develop practical applications thereof. The ability to abstract ideas from the passive mind was active reason, which Aristotle felt was the highest purpose of man, and the immortal component of his soul.

Philo (ca. 25 B.C. - A.D. 50) viewed intelligence not as a human ability at all, but rather as a divine grant. Philo postulated that the ability to know mandated freedom from fleshly distraction through meditation, dreams, and trances, as well as a close relationship with God.

3

God chose to reveal knowledge to souls prepared thusly. In Philo’s opinion, this path, not rational thought, was the only route to knowledge.

• 900 - 1300

The Islamic tradition of philosophy entered the fray with the contributions of Avicenna (980-1037), who essentially elaborated on the Aristotelian theme of hierarchical intelligences which he called interior senses. The common sense, as in Aristotle’s theory, integrated sensory information. The retentive imagination was a form of memory specific to the synthesized product from the common sense. Compositive imagination referred to approach/avoidance knowledge on the basis of pain and pleasure; the animal form was associative only, while the human form involved the novel integration of common sense and retentive imagination offerings. The estimative power referred to innate, reflexive approach and avoidance judgments. Next came the memory of all lower cognitions and recollection, the meaningful use of that memory.

Avicenna’s practical intellect referred to the application of information to everyday matters, while contemplative intellect was a passive cognition that could be actualized only through a supernatural, active intellect. This highest intellectual function was a relationship with and understanding of God. So it seems that Avicenna may have integrated the theories of Aristotle and Philo in postulating a hierarchical model of intelligence the highest level of which was divinely mediated.

St. Thomas Aquinas (1225-1274) retained a version of Aristotle’s Interior Senses that included common sense, estimation, memory, and imagination. Also essentially Aristotelian were Aquinas’ conceptions of passive and active minds as distinguished by their mere embodiment and true abstraction of universals (Forms), respectively. The real benefit of Aquinas’ work was in his reconciliation of Aristotelian thought (via the argument that rationality was simply an alternate route to knowledge of God’s truth) with Church teachings at a time when non-Christian thought was often stamped out; this may have preserved Aristotle’s work for posterity.

• 1600-1800

The end of the Dark Ages saw a shift in the overall emphasis of emerging intelligence theories from attempting to describe the structure of human intelligence to postulating the origins of intellectual abilities. The philosophy of Rene Descartes (1596-1650) represented the beginning of this trend. During intensive introspection, Descartes recognized that his thoughts included several ideas that were exceptionally clear and true, concepts such as geometric rules and God. Because the level of perfection represented by such concepts greatly exceeded that of the fallible human mind which entertained them, Descartes concluded that these important, perfect ideas were inborn mental components. There is a clear link between the Cartesian proposal that learned knowledge may be enhanced or supplemented by innate, extra-experiential elements of the intellect and the nature side of the nature-nurture debate over the origins of human intelligence.

John Locke (1632-1704) presented an opinion best described as diametrically opposed to Descartes’, the predecessor of the nurture side. Locke believed that the newborn human mind was a tabula rasa, or blank slate, free of any ideas. Ideas, if innate, should be shared by all

4

humans; since the moral and conceptual ideas Descartes professed to be innate are not held by all people, deduced Locke, then they cannot be innate. Locke believed that experience, whether the sensory experience of our external world or the mental experience of internal, psychological phenomena, is responsible for the development of each individual’s fund of knowledge.

John Stuart Mill (1806-1873) was reared by his father, James Mill (1773-1836) to be a living testament to the influence of environmental education on the intellect. Under his father’s close tutelage, young J.S. Mill had read the Greek classics by age 8, and read and taught the Latin classics and learned mathematics (through calculus) by 12 years of age. As J.S. Mill approached middle age, he generalized his view of himself as a product of an intensely educative environment into a psychology that saw the human mind as unlimited in the extent to which it can be shaped by environmental learning. Mill proposed that every mental impression, or perception, has a corresponding idea, which is represented to the conscious mind as a memory of the original impression. Mill’s psychology was primarily concerned with the principles which governed the development of the intellect, or the association of ideas. Four rules were presented to describe the functioning of this system:: similarity (similar ideas arouse one another), contiguity (ideas repeatedly experienced simultaneously or in rapid succession become associated), and intensity (the vividity of either or both ideas is directly proportional to the strength of the association between the ideas). Although J.S. Mill did leave room for the existence of some innate factors, similar to the instincts of a non-human animal, he saw even inborn qualities as being subject to significant modification by environmental factors.

Although French philosopher Julien de La Mettrie (1709-1751) was known as a Materialist, he advance one of the first truly moderate, integrationist positions in the nature-nurture debate. La Mettrie believed there to be three determinants of intelligence, a quality he did not consider uniquely human. In addition to brain size and complexity (innate, physiological characteristics), education also contributes to intelligence. (One generation later, Franz Joseph Gall (1758-1828) did find brain size and cortical complexity to be empirically implicated in intelligent behavior.) Indeed, La Mettrie believed education to be largely responsible for the difference in intelligence between humans and other primates. La Mettrie’s position strongly resembles today’s prevailing opinion of the origins of intelligence, that both genetic, innate factors and environmental and educational experiences contribute to human intellectual ability.

Etienne Bonnot de Condillac (1715-1780) proposed that all human mental ability was a product of sensation, memory and basic emotion (i.e. - pleasure and pain). Condillac disagreed with Descarte’s concept of innate ideas, arguing that the intellectual powers we now refer to collectively as ‘intelligence’ are consequential to sensation in some way. For example, the memory of sensations as they occurred represents retrieval, whereas the recollection thereof in a novel order is imagination, of which dreaming is one variant. The retrieval or imagination of hated or loved sensations is what we call fear or hope, respectively. The human tendency to categorize sensations on the basis of common attributes is abstraction. Although de Condillac’s view of human intellectual abilities might seem overly simplistic to sufficiently explain such a complex system, he attributed this complexity to man’s ability to sense in multiple modalities (e.g. - smell, taste, touch). Condillac’s position was located quite extremely on the nurture end of the nature-nurture continuum , as demonstrated by his belief that anything not experienced through sensation could never be known by the human mind.

5

Unusually, among the theorists discussed thus far, Gottfried Wilhelm von Leibniz (1646-1716) promoted his idea of the mind as quite active and intelligent without any contribution of experientially gained knowledge. Leibniz proposed that no physical event or entity (such as a sensation) could lead to a metaphysical event (such as an idea or thought). As the brain is a physical object, it could not lead to a nonphysical thought. Therefore, concluded Leibniz, each human is born with the potential to have an idea; experience is the mechanism thorough which this potential is realized. Leibniz went on to provide an explanation for individual differences in intelligence. He believed that the universe, and everything in it, was comprised of living, conscious, atom-like particles called monads, which vary only in their capacity for limpid cognition. Differences in intelligence amongst humans and other animals represent differences in the clarity of thought capable of by the most intelligent monads the organism possesses. So, while plants and animals are characterized by rather dull monads, the awareness of the most advanced human monads was second only to that of God’s. That is, humans are second in intelligence only to God. However, the potential represented by the monad’s clarity must be actualized by experience; the varying degrees to which this occurs explained individual differences in human intelligence.

Immanuel Kant (1724-1804) attempted to establish the existence of innate categories of thought by illustrating several universal constructs that are never actually experienced, including time, space, causality, and reality. Allowing that sensory input is necessary for the development of human knowledge, Kant believed that these sensations are only intelligible and coherent after interpretation through the filter of the innate mental concepts. By integrating Descarte’s notion of innate ideas with the empiricist concept of the importance of experience, Kant provided another remarkably modern, moderate opinion in the nature-nurture debate.

• 1800-present

Perhaps the best known of Francis Galton’s (1822-1911) qualities was his somewhat obsessive penchant for quantification; this, coupled with the heavy influence of the theory of evolution proposed by his first cousin, Charles Darwin (!809-1882), resulted in Galton’s theory of intelligence as being inherited and his lifelong mission to measure human intellectual ability. As such, Galton incited the pervasive concern with the measurement of intelligence around which all attempts to define the construct and discussions of the relative influence of genetics and the environment since have revolved.

Galton echoed the sensationalist view that data are input to the human mind only through the senses. Accordingly, he believed, an individual’s level of intelligence must be directly proportional to their sensory acuity, a constellation of inherited characteristics. As such, intelligence must be inherited, reasoned Galton.

As a natural product of this conclusion and his fixation on measurement, Galton first sought to assess the occurrence of eminence (as an indicator of genius) in the children of eminent parents as compared to the frequency of eminence in children of lay people. As a matter of fact, it was Galton who first used the phrase "nature and nurture" to describe the opposing viewpoints in the debate over the origins of intelligence. Galton admitted, however, that any genetic predisposition was not a sufficient cause of eminence; motivation was the variable he believed to mediate the genetic/eminence relationship.

6

Another implication of intelligence being a product of sensory information, according to Galton, was that an individual’s intellectual ability ought be able to be assessed using tests of the senses. Galton assembled a battery of such tests and the instruments necessary for their administration in his "Anthropometric Laboratory" in 1884. Here, Galton made various measures of sensory acuities, which were intended as tests of intelligence, as well as other physiological characteristics (included so Galton could study individual differences).

Galton’s belief in the heritability of intelligence was the basis for his proposal of a governmentally sanctioned system of selective human breeding called eugenics. If children of highly accomplished individuals were more likely to be highly intelligent than other children, Galton reasoned, then governments should materially encourage the eminent to procreate with one another in hopes of causing a proliferation of intelligent individuals in the species. Galton’s eugenic platform was posthumously misinterpreted as supportive of the genocidal rhetoric of various hate groups, including the Nazi party of World War II Germany. This, perhaps, has contributed to his having been maligned or, at the very least, underappreciated in the annals of psychology until recently. For even though Galton’s anthropometry eventually proved invalid, his work sparked an era of mental testing in psychology which continues today. As well, Galton made numerous profound contributions in the areas of research methods and statistics, including the concepts of correlation and regression toward the mean, the twin study, the adoption study, the control group, the word association method, and the questionnaire.

In a strange twist of fate, it was the work of Galton’s most ardent disciple, James McKeen Cattell (1860-1944), that both popularized Galtonian intelligence theory, anthropometry, and eugenics in America and discredited them in the scientific community. Cattell, the first American to receive a doctoral degree from Wilhelm Wundt, sought Galton out because of their shared interest in the objective measurement of individual differences. After studying with Galton for two years at Cambridge, Cattell returned to America and a teaching post at the University of Pennsylvania.

Cattell immediately established a psychology laboratory (the first for undergraduates) and training and research programs based on his set of "mental tests and measurements" (a term Cattell coined). Cattell had essentially integrated Galton’s measures of sensory acuity with Wundt’ psychophysical methods to develop a battery of mental anthropometric tests of intelligence which Cattell believed should be administered to all university students. A couple of years after moving to Columbia University in 1891, Cattell actually implemented a large scale program of administering his test battery to every student entering one of Columbia’s schools. Among Cattell’s goals for this testing program (which lasted for over ten years) were determining the concurrent validity (correlation between results on the various tests) and criterion validity (correlation between test results and academic success) of his tests. His mathematical illiteracy prevented Cattell from performing such data analyses until he was joined by an assistant, Clark Wissler, who’d been trained in Galton’s new correlational methods. After subjecting Cattell’s decade of work to his analyses, Wissler failed to uncover correlations among the tests and between any of the tests and academic success in college. These well publicized, inarguable findings constituted the death knell for mental anthropometry, as well as for Cattell’s career as a researcher. Yet Cattell would go on to created a very respected position for himself in the history of mental testing as the founder of the Psychological Corporation, which, worldwide, is the largest publisher of psychological testing instruments today.

7

Even before Wissler, Cattell’s mental anthropometry had been criticized on the basis that it focused excessively on sensory and motor (physiological processes, effectively excluding higher cognitive (psychological) functions). In rejecting Cattell’s defense that such complex cognitive abilities were unquantifiable and immeasurable, his opponents pointed to the work of a French psychologist, Alfred Binet (1909-1975), who was developing an instrument for detecting mental retardation in schoolchildren for special educational purposes.

In 1905, Binet and his student, Theodore Simon (1873-1961), published the Binet-Simon scale of intelligence, a battery of 30 tests primarily assessing cognitive abilities which the authors believed comprised intelligence and had found to discriminate well between normal and developmentally disabled children. In addition to being the first direct test of intelligence, the Binet-Simon scale may have introduced the concept of the norm group. That is, a determination of the existence and degree of mental retardation was made based on the comparison of the subject’s performance with the average performance on the same items by norm groups of children with slight, moderate, and severe mental retardation.

The 1908 revision of the Binet-Simon was an expansion of the original aim of the scale to discriminate levels of intelligence for both retarded and non-retarded children. The authors selected tasks which were able to distinguish between children of different ages. For example, "it was found that only a minority of 3-year-olds could copy a square, a majority of 4-year-olds (75% or more) could copy a square, and essentially all 5-year-olds could do so (p. 280)." If at least 75% of children a certain age could pass a test, then the test was designated with that age level. This allowed the examiner, by comparing a child’s performance with the norm for her age, to determine whether even a non-developmentally disabled child’s intelligence was average, below average, or above average.

In 1911, the scale was revised to raise the ceiling of the level of item difficulty (to one appropriate for individuals aged 15 years) and to give 5 tests for each age level. This allowed for a more precise computation of a child’s "intellectual age" (in fifths of a year) which could be greater or less than, or approximately equal to the child’s chronological age, as determined by how the child’s performance compared to children younger, the same age, and older than him or her.

Binet recognized the danger of assigning a child an intelligence age label lower than their chronological age, noting the potential for self-fulfilling prophecy in the classroom. In addition, Binet asserted the inconclusiveness of poor test performance, citing lack of motivation, physical illness, and lack of cultural knowledge as potential confounds. While Binet believed in an heritable ceiling on intellectual ability, he also believed that the majority of humans do not function at their intellectual potential, and that, therefore, education could enhance almost anyone’s intellectual ability. Due to its ease of use, effectiveness, and relative time efficiency, the Binet-Simon was being used worldwide by the start of the first World War.

Despite Binet’s objections, in 1911 German psychologist William Stern (1871-1938) substituted the term mental age (MA) for intellectual age, and proposed that this number be divided by the child’s chronological age (CA) to derive an intelligence quotient: IQ = MA/CA x 100. (Both the abbreviation of intelligence quotient to IQ and the multiplication of the result by 100 to eliminate the decimal point were suggestions of Lewis Terman, who we’ll discuss later). Binet argued that the distillation of the complex skills and abilities comprising intellectual function into a construct represented by a single number was a gross and improper

8

oversimplification. Nevertheless, the idea of a single indicator of overall intelligence has historically been popular and endures in today’s leading intelligence tests.

During a hiatus from his studies with Kulpe, Charles Spearman (1863-1945), inspired by his recent reading of Galton’s work and unaware of Clark Wissler’s recent results discrediting anthropometric testing for purposes of assessing intelligence, conducted several correlational studies of sensory acuity and teacher/peer ratings of "cleverness in school". Spearman’s results included moderately high correlations amongst the sensory measures, and more importantly, between the sensory acuities and intelligence ratings, which, when he "corrected" them for attenuation due to the imperfect reliability of his measures, became perfect correlations of +1.0. (Later in his career, Spearman would dismiss Wissler’s results a having been the artifact of excessively unreliable sensory measures.) Based on these perfect correlations, Spearman concluded that his sensory measures were indices of an overall faculty of intellectual ability, or a ‘general intelligence’ (g) which he believed to be wholly heritable. Despite the popularity of Binet’s scale, it was Spearman’s concept of a unitary, inherited intelligence that became the most widely accepted definition of intelligence in America at the time.

Henry Herbert Goddard (1866-1957) became the research director at the Vineland, New Jersey Training School for the Feebleminded after receiving his doctorate at Clark University under G. Stanley Hall. Goddard translated the Binet-Simon scale (as well as the other works of Binet and Simon) into English, tested it extensively on Vineland’s students and New Jersey public school students, and became an outspoken advocate of the effectiveness of Binet’s methods for distinguishing between children displaying various degrees of mental retardation. As did many of his contemporaries who accepted Binet’s methods, Goddard rejected Binet’s conception of intelligence as greatly malleable and adopted, instead, Galton’s, Cattell’s, and Spearman’s view of intelligence as inherited,

Inspired by this conviction and his finding that many of the children in his public school sample scored below their age norms, Goddard proceeded to study a family he gave the fictitious name of Kallikak (Greek for good/bad). The essence of the study was that the patriarch had a wartime fling with a ‘feebleminded’ barmaid which produced a child who eventually had 10 offspring, all of whom were undesirables as a result of their lack of intelligence (according to Goddard). They were mentally retarded, alcoholics, prostitutes, criminals, etc. When the war was over, the patriarch married a "worthy girl"; the descendants of this union were almost all upstanding and even prestigious citizens.

These findings led Goddard to loudly vocalize, along with several other prominent scientists, a call for the mandatory sterilization and/or segregation of ‘undesirables’ (as described above) to prevent their procreation. In response to this call, at least 20 states and several other countries passed statutes mandating the sterilization of such individuals, some of which were enforced until the 1970’s. Goddard’s assertion that approximately 50% of immigrants were ‘feebleminded’ (based on the results of poorly translated Binet-Simon scales administered under adverse testing conditions) directly effected severe increases in the deportation rates of immigrants from Ellis Island from 1912 to 1914. During the latter part of his career, Goddard’s views regarding the nature of intelligence changed such that he agreed with Binet’s much broader conception of the genetic limits on intelligence and his belief in the educability of many children who had tested as retarded. Despite the damage Goddard had done, his work constituted the first large scale use of the Binet tests and the first major applications of mental tests to social policy.

9

Louis Terman (1877-1956), a professor of education and , later, the chair of the Psychology Department at Stanford University, discovered that Goddard’s translation of the Binet-Simon was not accurate for U.S. populations as it was. The norms were somewhat skewed in different directions for different age groups, probably based on cultural differences (e.g. the 5-year-old questions were too easy and the 12-year-old questions were too hard for their respective age groups). Terman deleted offending items, added appropriate items, and set the mean score at 100 so that the average MA and the CA of the groups would be approximately equal. The results, the Stanford-Binet has been revised several times and is still one of the most widely used tests of childhood intelligence.

Terman agreed with Goddard that intelligence was genetically transmitted and that a lack thereof predisposed an individual to an immoral life. He argued, however, with the popular view of childhood genius as a negative aberration that led to an unsuccessful adult life. In his search for support of his belief that child geniuses had a greater likelihood of success as adults than other children, Terman initiated the longest running longitudinal study in the history of psychology. He began by administering the Simon-Binet to thousands of students in California schools and comprised his sample of 1,528 kids who were assigned IQ scores above 135 (his genius cutoff point). These individuals have been (and continue to be) studied repeatedly throughout their life span and have been found to achieve higher rates of education, prominence, and overall success and adjustment than non-gifted individuals, contrary to popular belief at the time the study began.

Robert Yerkes (1876-1956) advocated the administration of all items on the Binet-Simon to each subject (as opposed to attempting to customize a range of questions appropriate for each subject) to derive a point score (as opposed to only an IQ score). This would increase the ease of administering the test and facilitate the statistical analysis of the results, making the test much more appropriate for group administration. As President of the American Psychological Association during World War I, Yerkes spearheaded an effort to aid the military by developing a group test of ‘innate’ intelligence that was easy to administer and score, which was called the Army Alpha. The illiteracy of 40% of recruits mandated the development of a performance based test called the Army Beta. Although the Army discontinued the testing program shortly after the War ended, 1.75 million recruits had been assessed by that time. Nevertheless, the program was not the great success it is often thought to have been; apparently, poor test performance only resulted in a recommendation of discharge in .005% of individuals tested, and even in these instances, the Army often disregarded the recommendation. However, the widespread publicited of the Army intelligence testing program cemented the intelligence testing movement as a lasting force in psychology.

This same publicity was likely responsible for sparking David Weschler’s (1896-1981) interest in intelligence testing to the extent that, as a graduate student in psychology at Columbia University, he volunteered to score the Army Alpha tests. Later, as an Army officer, Wechsler was given the duty of further examining individuals who’d scored poorly on the Alpha or the Beta using the Stanford-Binet, the most popular individual test of intelligence availbale at the time. Wechsler criticized the Stanford-Binet as being inappropriate for evaluating adult intelligence. During his military career, Wechsler was sent to England to study with Spearman, whose ‘g’ he felt was impressive, but underestimated the role of motivation and personality in intelligence and failed to recognize the many component abilities which comprise intelligence. Out of all the popular intelligence tests of the time, Wechsler had found the Army Alpha to be the most useful because it used the same type of

10

items (and measured the same abilities) at all levels of difficulty, alloweing for a comparison of relatively strong and weak skill areas, and because the point-scale scoring system did not use a mental age/chronological age relationship, which would be inappropriate for use with adult populations. Wechsler set out to improve upoon the Army tests when hecreated the Wechsler Bellevue in 1939 and its revision, the Wechsler Adult Intelligence Scale in 1955. The WAIS is divided into Verbal and Performance subtests which Wechsler derived from the Army Alpha and Beta, respectively. The results on the different subtests can be compared for a multifaceted view of the subjects intellectual abilities (or disabilities). In deference to the utility of Spearman’s single index of intelligence, however, Wechsler also included a general deviation IQ score based on the location of the individual’s score on the normal distribution of scores for his or her age peers. This deviation score method has now been incorporated into most populat intelligence tests, and the WAIS is now the most popular instrument for the assessment of adult intelligence worldwide.

The prevailing view of the origins of intelligence today is a pretty moderate view that closely echoes that expressed by Alfred Binet. Most psychologists believe that there are some genetic ceilings on one’s level of intellectual ability, but that almost everyone’s intelligence can be improved by educational intervention. The widely held construct of intelligence is similar to Wechsler’s idea that an overall ‘g’ probably exists and influences all of our intellectual abilities, but that there are a number of specific abilities that are required by different tasks to different extents. Modern discussions of intelligence theorists have expanded to include debates over the existence of an ‘emotional intelligence’, arguments over the merits and dangers of sophisticated artifical intelligence, and a variety of controversies over the applications of intelligence test results. Nevertheless, the age-old polemics regarding the relative importance of nature and nurture and an exact definiton of intelligence remain alive and are can be traced in the manuals of intelligence assessment instruments.