App a Caldwell

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Human Intelligence

Human Intelligence

Historical Survey of Human Intelligence Research and Measurement

Allison Caldwell

Walden University

August 20th, 2009

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Abstract

The study of human intelligence and its measurement has been one of the mostconsistently successful areas of research and practice over the last century. Sinceits inception, a few lines of investigation have endured such as the nature of

intelligence and whether it is primarily based in biology or is more affected by

environmental and developmental dynamics. The structure of intelligence asmultivariate and hierarchical, and the single dominant general intelligenceg

factor, is examined within its historical context and in light of contemporary

research. A survey is provided of the major intelligence theorists and developersof cognitive testing batteries from Sir Frances Galton through to present day

psychologists and practitioners.


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Human Intelligence

Historical Survey of Human Intelligence Research and Measurement

The study and measurement of human intelligence has been one of the most consistently

important and successful areas of research and practice since the birth of psychology just over

a century ago (Geary, 2004). This success is largely due to the steady emphasis on statistics, and

the development of quantitative data that has been part of the evolution of research in this area of

study. A major line of investigation and debate has been the nature and origin of human

intelligence. The questions that have been foundational to the study of intelligence concern

whether it is constituted by heritable traits that are biologically based, or if it is more affected by an

environmental and developmental dynamic. Another line of inquiry is whether there is a single

dominant factor through which human intelligence can be defined, or are there multiple factors

through which human cognition is structured, and how are these factors measured (Kamphaus,

Winsor, Rowe, & Kim, 2005). The progression and refinement of these issues are seen through the

historical context of intelligence theories and methods of measuring human cognition. What

follows is a brief survey focusing on the major players involved in this work over the last 100

years.

Foundations of Research on Intelligence

The history behind the study of individual differences in intelligence dates back to the work

of Sir Francis Galton in the latter half of the 19th century. Galton contributed a great deal to the

psychology of intelligence; his work was foundational to intelligence theory, statistics, and

cognitive measurement (McIntosh & Dixon, 2005). The cousin of Charles Darwin, Galton was

greatly influenced by Darwinian Theory. Primary assumptions on which the theory of evolution

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was based included the concept of natural selection; that humankind was in a constant process of

adaptation and change, with dominant, ascendant characteristics being passed on to the next

generation through heredity (Jensen, 2002). In this regard, Darwin held that the strongest genes

promoting intelligent, survivalist behavior would be carried over to subsequent generations,

resulting in the evolution of human cognition. The effect of this heritable process is a great deal of

variation within the species of humankind. It is this disparity of differences in human intelligence

that became the focus of much of Galtons research (Jensen).

The nature versus nurture debate sparked by Darwins work was carried over to Galtons

research on mental abilities and has permeated theories of intelligence since that time. The

dispute between intelligence as an innate, inborn faculty, or whether it can be developed, has been

on-going. Galton believed in the genetic origin of human mental abilities, and put forward a

definition of fixed intelligence (McIntosh & Dixon, 2005, p.505) based on heredity. Much of

his research focused on this concept. Given his interest in statistics, Galton systematized studies

on heritable intelligence within families, measuring what he called general intellectual power

(Geary, 2004, p. 254). This led to the beginning of the eugenics movement, and the ranking of

human beings based on their abilities and characteristics, with the underlying idea of procreation of

humanitys brightest and best (Simonton, 2003).

It is worthy of note that as a result of his intelligence studies within families, Galton made

significant and lasting contributions to psychology through the discovery of measures of bivariate

correlations, analysis of variance, and the concept of regression (Jensen, 2002). He discovered

statistical correlations based on how family members were generationally similar to one another

(Jensen). He also found that excessively different heritable traits tended to move towards a more


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normal or average expression in subsequent generations, statistically moving closer to a central

means (Seligman, 2002).

Galton is credited with developing one of the first measures of intelligence (McIntosh &

Dixon, 2005). He theorized that intelligence was developed through sensory experience

(Wasserman & Talky, 2005). He, therefore, developed a measure of intelligence based on

quantifying sensory acuity, reasoning that the intellect would be able to gain more from experience

if a persons sensory acuity was high (Thorndike, 1997).

James McKeen Cattell was an American psychologist who studied under Wilhelm Wundt

in Germany, later collaborating with Galton at Cambridge before returning to the United States.

Highly influenced by Galton, Cattell brought home with him the idea of assessing intelligence

through the senses rather than through the higher mental processes (Wasserman & Tulsky, 2005,

p. 5). Like Galton, Cattell also held a firm belief in statistics and the quantitative scientific method

as a means of strengthening the science of psychology. He developed a mental testing battery

based on many of Galtons own tests and established a testing program for students at Columbia

University (Wasserman & Tulsky). This program was short lived, however, when experimental

results did not support correlations between Cattells mental tests (McIntosh & Dixon, 2005, p.

4) and levels of student achievement. The assumption held by Galton and Cattell as to the sensory

nature of human intelligence was incorrect but their belief in some underlying measure of intellect

would come to predominance in the study of human cognition.

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Development of Test Theory

In 1904, Charles Spearman, a graduate student, conducted research using a new statistical

technique called factor analysis. Using this mathematical formula, Spearman found correlations

between many different tests that measured intellectual ability, all of which were strongly related

to one central correlate; an individuals global intelligence capacity, which was termed theg factor

(Wasserman & Tulsky, 2005). Borrowing from the principles of physics, and in a flashback to the

explanations of Galton and Cattell, Spearman explained theg factoras an individuals amount of

general mental energy (Spearman, 1927, p. 137, as quoted in Wasserman & Tulsky, 2005).

Spearman published his research in a paper entitled, General Intelligence, Objectively Determined

and Measuredwhich theorized that all cognition has in common the foundationalg factor

(Wasserman & Tulsky); a central, defining measure of a persons intelligence. The fundamental

importance ofg, as opposed to a multivariate view of intelligence factors, immediately became a

topic of intense debate, and has remained so to the present day. Spearmansg factoris a

psychological discovery of phenomenal importance, and has guided much of the development of

human intelligence theories since its inception (Wasserman & Tulsky). A major query concerning

theg factorthat has remained is whether it just a psychometric, statistical occurrence, or are there

biological or psychological foundations to this finding (Johnson & Bouchard, 2005). A second

important discovery from Spearmans original work was that human intelligence factors could be

presented in a hierarchical fashion, withgas the overriding and dominant factor to which all else

correlated (Geary, 2004; Reynolds & Keith, 2007). This observation by Spearman was a

foreshadowing of the premise on which much contemporary intelligence theory is based (Geary).

A major influence on contemporary intelligence research occurred with the development of


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the Cattell-Horn Gf-Gc theory. A student of Spearman, Raymond B. Cattell was a psychologist

and scientist believing in a mostly heritable view of human cognition. Like his predecessors,

Cattell had a strong statistical bent, and held to empirically based factor-analytic studies in his

research. John Horn was a prominent student of Cattells. Together, Cattell and Horn developed a

theory that divided Spearmansg factorinto two distinct areas, each of which was made up of

multiple interacting factors of intelligence (McGrew, 2005). Fluid Intelligence (Gf) was theorized

to have a biological basis, free from the influences of learning and acculturation, but in essence

constituting an ability to learn (Geary, 2004, p. 256). Crystallized intelligence (Gc) is more

related to acquired skills such as language knowledge and vocabulary, having a direct relation to

experience, acculturation, and education (Geary). By its definition, Gf-Gc theory takes into

account biological and hereditary factors, as well as environmental influences such a learning and

experience. Gf-Gc theory is based on a hierarchical multi-factor view of intelligence, with

correlations evident between higher level factors of intelligence to lower stratum measures

(McGrew, 2005). Due to the correlations found in this hierarchy, Cattell and Horn minimized the

need and importance of theg factor. Indeed, Horn did not believe that ag factorwas necessary at

all in intelligence models or theories (Horn & Blankson, 2005; Kvist & Gustafsson, 2008).

A second major theoretical high point occurred in 1993 with the publication of John B.

Carrolls Human Cognitive Abilities: A Survey of Factor Analytic Studies (Carroll, 1993). Based

on an amalgamation of decades of research and over 460 factor-analytic studies of human

intelligence, Carroll put together a comprehensive framework of human cognitive ability (Johnson

& Bouchard, 2005). Carrolls Three-Stratum Theory presents three levels of cognitive ability, with

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theg factorat the top of the hierarchy due to its overriding influence and correlation with lower

stratum factors (Wasserman & Tulsky, 2005; Carroll, 2005). The second stratum contains 8 or

more factors, with greater than 65 lower levels abilities identified in the third stratum (Wasserman

& Tulsky).

The presentation of Carrolls Three-Stratum theory was strikingly similar to the Cattell-

Horn Gf-Gc hypothesis (McGrew, 2009), with the marked exception that Carroll included the g-

factor as a third level, and overarching element of human intelligence, believinggto be a

necessary component in the description of cognitive ability (McGrew). These two theories were

placed under one common umbrella because of their complementary strengths, to make what is

known as the Cattell-Horn-Carroll (CHC) theory of cognitive abilities. This amalgamated theory

represents an open-ended view of intelligence, and a model for the study of cognition that could be

built upon, and added to, with on-going research (McGrew). Recent research has examined the

hypothesis that Cattell-Horns fluid intelligence (Gf) is equivalent to theg factoras defined by

Carroll with positive, but not conclusive, results (Blair, 2006; Gignac, 2006; Kvist & Gustafsson,

2008). The concept of fluid intelligence however is clearly and strongly correlated with general

intelligence (Arendasy, Hergovich & Sommer, 2008).

Development of Testing Instruments

The bridge between theories of intelligence and practical application is seen in the

measures and testing instruments created to quantify human cognition. Two events in the early

20th century had a major impact on the development of intelligence testing batteries. The first was

the largely international requirement for the availability of general public education. The second

was the strong desire to establish psychology as a scientific discipline by emphasizing empirical


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research and quantitative data to support its growth and development (Thorndike, 1997).

A major breakthrough in intelligence testing came in 1904 when the French

government asked Alfred Binet to develop a test to identify children who may be in need of

alternative educational environments due to a lower level of cognitive functioning (McIntosh &

Dixon, 2005). Binet disagreed with the approach taken by Galton and James McKeen Cattell that

quantified sensory acuity, and instead focused on the creation of an assessment of higher mental

functions such as working memory, judgment and reasoning (Geary, 2004, p. 257), as well as

motor and perceptual skills. Binet defined intelligence largely in terms of an individuals capacity

for good judgment (Wasserman & Tulsky, 2005). His scales were more theoretically in line with

the later work of R. B. Cattell and Horn, since they were hierarchically organized. Binet

recognized three levels of intelligence, which was truly revolutionary at the time (Wasserman &

Tulsky, p. 3), but he also questioned the practical need for a focus on theg factorin explanations

of cognitive abilities (von Mayrhauser, 1989).

The major goal of Binets work was to measure childrens level of intelligence to

determine their probable success in school or their need for alternative education (Geary, 2004).

He did this by devising a system whereby a childs ability level was equated with age. The

problem with this system however was that a child of 7 and a child of 11 could both be determined

to be functioning at the level of a 9 year old, but it offered no further specification of their

cognitive performance. This difficulty was solved in 1914 however by Wilhelm Stern who

developed the Intelligence Quotient (IQ) which basically divided the mental age, proposed by

Binet, by the childs chronological age, multiplied by 100 (Boake, 2002).

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Binets work was cut short by his death in 1911 but other psychologists continued to build

on the firm foundation that had been laid (Boake, 2002). Foremost among them was Lewis

Terman. Along with a number of colleagues from Stanford University, Terman authored a U.S.

revision of the Binet-Simon scales, titled the Stanford-Binet. The revised battery met with wide-

spread and on-going acclaim, quickly becoming the new standard for intelligence tests

(Wasserman & Tulsky, 2005, p. 11). The solid reputation of this measure was largely the result of

Termans high scientific standards, and his methodically sound norm sampling (Wasserman &

Tulsky). A major advantage of the Stanford-Binet was its incorporation of Sterns 1914

Intelligence Quotient (IQ), which fell in line with the foundational belief in Galtons theory that a

central measure of intelligence provided an essential gauge of success in many different areas of

life (Simonton, 2003). Indeed, numerous studies have shown a significant correlation between IQ

and otherg factorscores, and levels of academic, job and career success (Kuncel, N. R., Hezlett,

S. A., & Ones, D. S., 2004; Schmidt, F. L., & Hunter, J., 2004).

Complimenting Binets pragmatic approach, intelligence testing in America took a

practical turn and gained a substantial foothold through the efforts of Robert Mearns Yerkes,

president of the American Psychological Association (APA), and his work with the U.S. Army

during World War I. Yerkes chaired a committee on the Psychological Examination of Recruits,

which included Lewis Terman and other prominent U.S. psychologists. The goal of the committee

was the development of group intelligence tests to screen army applicants regarding their fitness

for service in different levels of the military (Boake, 2002). Rather than focusing on a general

level of cognitive ability, such as would be measured by theg factor, these assessments were more

practical in nature, measuring intellectual aptitude (von Mayrhauser, 1989, p. 67) and specific


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abilities (p. 63) required for different jobs. Through the military testing program, mental testing

followed the functionalist approach, adding a pragmatic perspective that took into account

developmental and environmental aspects of intelligence, rather than embracing a predominantly

hereditarian view based on IQ and theg factor(von Mayrhauser).

A young doctoral student named David Wechsler worked with Yerkes as an army

psychological examiner during World War I, and later continued studies with James McKeen

Cattell and Charles Spearman (Wasserman & Tulsky, 2005). Wechsler developed one of the most

successful cognitive testing batteries of the last half century, the revisions of which constitute the

most widely used intelligence test today (Zhu & Weiss, 2005). A master at synthesizing existing

tests (Wasserman & Tulsky, p. 12), Wechlser used the Army tests, and other assessments, to

develop his Wechsler Scales. He defined intelligence as an individuals global capacity to act

purposefully and to think rationally (Wechsler, 1944, p. 3) in relation to their environment. Two

major innovations Wechsler made to intelligence testing included dividing subtests that measured

theg factorinto Verbal and Performance scales. This allowed for differential diagnoses based on

an individuals strengths and deficits in specific intelligence factors (Zhu & Weiss). He also

developed the Deviation IQ, which permits ranking of an individuals level performance relative to

their age group (Wechsler).

The Woodcock Johnson III Tests of Cognitive Abilities is one of the most recent

assessments to be developed for measuring the factors of human intelligence (Schrank, 2005). The

theoretical foundation for this battery of tests is the Cattell-Horn-Carroll (CHC) theory of cognitive

abilities, which is recognized as one of the most valid contemporary representations of a multi-

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factor theory of intelligence (McGrew, 2005). The marrying of the Cattell-Horn Gf and Gc theory

and the Carroll Three-Stratum hierarchical model of human intelligence in the Woodcock Johnson

III includes a measure of theg factorwhich is expressed as the general intellectual ability score.

When examined strictly on its own, theg factorholds limited meaning. The strength of this

assessment battery, and the significance of the correlatingg, is found in the examination of the

cluster scores indicative of the levels of cognitive skills; the broad abilities that make up the g

score (Schrank). It is also essential to consider the next level of narrow abilities that make up the

cluster scores to determine the cognitive strengths and needs (Davidson & Downing, 2000), and to

make practical recommendations that will be of assistance to the individual being tested, based on

the purpose for testing (Reeve, 2004). This assessment battery is representational of a new era in

bridging cognitive theory to the practice of applied measurement of intelligence (McGrew, p.

139).

Contemporary Research and Theory

Contemporary research and theory regarding intelligence and measurement of cognitive

abilities is very much a reflection of the issues embedded within its history. The seminal,

structural model of cognitive abilities proposed by J. B. Carroll in 1993, provided an empirically

based framework of intelligence, and the multi-factor and hierarchical nature of human cognition

has gained widespread acceptance (Alfonso, Flanagan & Radwan, 2005; McGrew, 2009). The idea

that intelligence is generated by a general unified brain capacity, commonly known as theg factor,

has been a matter of controversy since it was first proposed by Charles Spearman in 1904 (Geary,

2004). The existence of the g factor has been firmly established both from a theoretical

(Anderson, 2001,) and a statistical psychometric perspective through factor analysis studies


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(Johnson, Nijenhuis & Bouchard, 2008; Floyd, Shands, Rafael, Bergeron & McGrew, 2009), but

its practical relevance is still under examination. Some contemporary theorists, such as J. L. Horn,

whose combined work with Cattell and Carroll laid the foundation for the Carroll-Horn-Cattell

(CHC) model of cognitive abilities, fundamentally disagrees that the concept of ag factoris

necessary (Kvist & Gustafsson, 2008). Johnson & Bouchard (2005) challenged this position and

found that theg factorwas an important aspect to all areas of intelligence in their examination of

three prominent theories of intelligence, namely Carrolls three-strata, CHC and Vernons verbal-

perceptual model.

Johnson , Bouchard & Krueger (2004) provided support for the existence of theg factor,

and its strong correlation among 3 cognitive testing batteries, using heterogeneous populations. In

a successful second replication study, Johnson, Nijenhuis & Bouchard (2008) corroborated this

evidence, finding consistency in measurement of ag factorusing 5 different cognitive assessments

within a variety of populations. This demonstrated that the existence of theg factoris consistent

across the five testing batteries, being strongly generalized, and holding a significant place in

relation to the correlation of intellectual abilities.

As a psychological or biological construct (vanderMaas, Dolan, Grasman, Wicherts,

Huizinga & Raijmakers, 2006) the origin and nature ofgas a dominant, pervasive factor of general

intelligence is still debated in the realm of theoretical cognitive science (Johnson, Nijenhuis &

Bouchard, 2008). Two current explanations that attempt to hypothesize answers to this line of

inquiry are Cattells investment theory and a dynamical model of general intelligence

(vanderMass, et al; Kvist & Gustafsson, 2008). Investment theory proposes that a general

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intelligence exists, that it is developed in children through the process of maturation, affected by

such things as effort, experience, and the degree to which one applies themselves to learning (Kvist

& Gustafsson). This general intelligence, which Cattell theoretically equates with fluid

intelligence (Gf), is a causal agent accounting for variation in individual differences, because it has

a general involvement in the acquisition of a variety of knowledge, abilities and skills (Kvist &

Gustafsson). The dynamical model is similarly based on development, and the beneficial

interactions between cognitive processes (vanerMass et al., p. 842). This is explained as a

positive manifold effect (p. 855) whereby the strong correlation between intelligence factors is

produced through maturation and development in a dynamic process that takes into account

genetic, neurological, and environmental factors and their reciprocal causation (p. 857) in the

growth of intelligence (Hambrick, Pink, Meinz, Pettibone & Oswald, 2008). Next steps for

contemporary theoretical models of intelligence include making use of factor-analysis statistical

methods to both scrutinize their validity, as well as to enhance and widen theoretical frameworks

(Johnson & Bouchard, 2005; Keith, 2005; McGrew, 2009).

Individual differences in human cognition have been an area of study for over a century,

dating back to the time of Sir Frances Galton. The multivariate and hierarchical nature of factors

that make up intelligence has been well established, as has the psychometric, statistical notion of

an overriding general intelligence factor that correlates with other factors that are part of cognition.

Strong correlational evidence also exists regarding the connection between and among intelligence

factors at different levels of the hierarchy by making use of factor analysis statistics (Davidson &

Downing, 2000; Keith, 2005). The origin and nature of intelligence is still specifically undefined,

although contemporary theories reflect an approach to research that is open-ended, considering a


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variety of causal and dynamic factors in the development of intelligence, including biology,

neurology, environment, and affective influences. The evolution of mental testing is also on the

verge of a new era, whereby assessment is being developed on the foundation of intelligence

theory (McIntosh & Dixon, 2005). As we examine intelligence through a multidimensional lens,

and seek to measure it with instruments that reflect open multi-factor theory, we will gain greater

insights into its origin and nature, and more global benefits of this increase in knowledge,

translated into practical advantages.

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App a Caldwell