The history of IQ tests; from Galton to Wechsler

Intelligence testing, or as we know it, IQ Tests have a long and controversial history. Its development began with Galton’s statistical innovations and went through several stages in which earlier approaches were shown to be inadequate or invalid. While many researchers regarded these methods as accurate and reliable, others raised serious concerns about their validity.

In this Blogpost, we trace the history of psychological testing, IQ tests, the concept of the intelligence quotient, and the major developments associated with them.

Francis Galton and the Pursuit of Measurement

Francis Galton, Charles Darwin’s cousin and Erasmus Darwin’s grandson, was a pioneer in psychological assessment. He was born into a wealthy family with a strong scientific background and close connections to the British aristocracy. Historical sources report that Galton learned to read and write from his mother by the age of three and was reading Shakespeare for entertainment by the age of seven.

School, however, was a very different experience. He frequently got into physical fights with his classmates, argued with his teachers, and was punished almost every day. At the age of 16, following his family’s wishes, Galton entered Birmingham General Hospital to study medicine. He later continued his medical training at King’s College London. When his father died, however, he abandoned his studies and never obtained a formal medical degree. Because of his family’s wealth, he was able to pursue another path: traveling to different parts of the world in search of new experiences.

He initially spent several years in Egypt, Sudan, and the Middle East. After returning home, he devoted himself for a time to sports and leisure. During this period, he became closely acquainted with a friend who was a phrenologist. He later traveled to South Africa for two years with financial support from the Royal Geographical Society.

During this expedition, Galton produced the first official map of the region now known as Namibia and received the Society’s medal of honor for his work. After this achievement, one phrase came to characterize much of Francis Galton’s life: the desire to measure.

Throughout his professional career, Galton conducted and recorded an enormous number of measurements. More important than the measurements themselves was his effort to standardize measurement procedures and develop statistical methods for analyzing them.

Some of these methods sound absurd to us, but some remain influential today. His early measurement projects included:

* Measuring weather patterns to predict future weather conditions

* Measuring fingerprints and proposing their use for personal identification

* Measuring the physical attractiveness of women from different countries

* Measuring audience boredom during lectures

* Investigating the effectiveness of prayer

* Measuring physical characteristics and intelligence

After his cousin Charles Darwin proposed the theory of evolution, Galton’s enthusiasm for measurement increasingly turned toward mental and physical characteristics. Galton argued that natural selection operated on the basis of traits and that, logically, these traits should therefore be measurable.

This seemingly simple idea would have both highly productive and deeply harmful consequences. Initially, Galton proposed that intelligence should be understood in terms of sensory acuity. He argued that humans experience the world through their senses and respond on the basis of those experiences. Therefore, someone with sharper sensory abilities should, in his view, be more intelligent. He further argued that sensory acuity reflected a biological predisposition (what we would now describe in terms of genetics).

From this, he concluded that intelligence was hereditary. To investigate this hypothesis, Galton proposed comparing the number of offspring's of people in high social or official positions in prominent families, with the number of offspring's in high offices from general public (comparison between how many of current high offices are born from people who were in high offices). His underlying assumption was that attaining a high position was directly related to intelligence; If intelligence was hereditary, he reasoned, parents who had achieved high status should be more likely to have children who also achieved high status.

Today, the flaws in this reasoning are easy to identify. Nevertheless, as one of the earliest attempts to investigate human characteristics statistically, the study was highly influential and helped shape the development of later research on the matter, challenging Galton's hypothesis.

Galton found, unsurprisingly, that families with high social and official status tended to have more children who themselves attained high positions. He published his findings in "Hereditary Genius: An Inquiry into Its Laws and Consequences" in 1869. The more troubling consequences of Galton’s ideas emerged after the publication of this book. He asked whether, if intelligence was hereditary, the overall intelligence of individuals and society could be increased by encouraging intelligent people to marry one another. He subsequently proposed that governments provide financial incentives to intelligent people to marry and have children and even subsidize their children’s education.

Conversely, he proposed restricting the reproductive capacity of people he considered socially inferior, arguing that they would otherwise produce less intelligent children and lower the average intelligence of society.

These ideas became associated with eugenics. Although eugenic policies were not implemented on a large scale in Galton’s native England, they were adopted in various forms in the United States and Nazi Germany, with devastating consequences for human rights.

The First Psychological Questionnaire

Galton’s strong emphasis on heredity did not go unchallenged. In one of his works, Alphonse de Candolle criticized Galton’s proposals and discussed the potential influence of numerous environmental factors, including religious beliefs, the degree of democracy within a government, climate, and economic conditions. He concluded that environmental factors contributed at least as much as hereditary factors to intelligence and the attainment of social status.

Galton’s subsequent book was, in part, a response to this work. He developed a wide-ranging questionnaire covering topics from political beliefs and economic behavior to seemingly trivial characteristics such as hat and trouser measurements. He sent the questionnaire to 200 scientists who were members of the Royal Society. His aim was to investigate whether these individuals’ interest in science and knowledge was innate or shaped by their environment.

The questionnaire was extremely long and contained many personal questions. As expected, not all of the scientists completed it, but Galton reported that a substantial number returned their questionnaires.

One of his more interesting observations was that many respondents were Scottish scientists. Compared with the English scientists who responded, the Scottish participants were more likely to speak positively about their educational experiences and emphasize the role of kind and intelligent teachers. English respondents, by contrast, were often highly critical of their educational experiences. Based on these observations, one of Galton’s major recommendations was to reform the English educational system and draw lessons from the Scottish system.

In summarizing his observations, Galton acknowledged the positive influence of the environment but largely overlooked the possibility of negative environmental effects. He maintained that a substantial portion of intelligence was hereditary and that the role of the environment was to cultivate and nurture this inherited potential.

Galton’s next major methodological contribution appeared in "Inquiries into Human Faculty and Its Development" (1883). Among other things, he used twin studies to investigate the heritability of intelligence-related characteristics. He observed that identical twins showed many similarities even when they had not lived together or had not been raised together. He also reported that this degree of similarity was less evident among fraternal twins. At the same time, fraternal twins appeared more similar to one another than ordinary siblings, while siblings were more similar to one another than unrelated individuals. Galton interpreted these patterns as evidence for the heritability of human characteristics.

Galton later developed what is generally regarded as the first word-association test. He wrote 75 words on separate pieces of paper and presented them to himself under four different conditions. He observed that his responses to particular stimulus words remained remarkably stable over time. A particular word would repeatedly come to mind in response to the same stimulus, even though he had not consciously retained the association.

From this observation, he drew several conclusions. First, these stable responses were related to experience. Second, in many cases, the responses appeared to be closely connected to childhood experiences or memories. Third, he speculated that these otherwise inaccessible associations between words and memories must be stored somewhere outside conscious memory, because he could not consciously recall the connections without the test.

Wundt and Jung later acknowledged Galton’s work as an influence on their own research, whereas Freud did not indicate that he had either drawn on or been aware of Galton’s work. Another fascinating observation concerned individual differences in "mental imagery". Galton initially assumed that the ability to form mental images would be directly related to intelligence. To his surprise, he discovered that many of his scientist friends were unable to form even a simple abstract image in their minds. His observations suggested that mental imagery varied substantially across individuals and appeared to follow a roughly normal distribution. He found no obvious relationship between this ability and any particular characteristic. Perhaps even more interestingly, regardless of their own level of imagery ability, his participants tended to assume that other people experienced mental imagery in essentially the same way they did.

Anthropometry (Physical Measurement of human body)

After his investigations into mental and psychological characteristics and his development of new research methods and statistical techniques, Galton turned increasingly toward physical measurement. In 1884, he established an anthropometric laboratory. Over approximately one year, he measured 9,337 people.

He measured almost anything he could think of: height and weight, sitting and standing height, arm span, head circumference, respiratory capacity, wrist strength, visual and auditory acuity, and much more.

The primary purpose of the laboratory was to collect measurements that could be used to investigate relationships between sensory acuity, physical characteristics, and intelligence. In 1888, Galton established a similar laboratory at the Kensington Museum. Visitors could have themselves measured for three to five pence, receive information about their sensory abilities and physical development, and obtain a copy of their measurements. Another copy was sent to Galton’s office. Because of the enormous volume of data he collected, Galton never analyzed the entire dataset himself.

Decades later, however, with the development of increasingly powerful computers and statistical techniques, researchers were able to process and analyze substantial portions of these data. Galton’s Contributions to Statistics Perhaps Galton’s most important contribution to statistics and psychological research was his work on correlation.

His 1888 paper, "Co-relations and Their Measurement", and his 1889 book, "Natural Inheritance", introduced and developed the concepts of correlation and regression. As Galton explained, two variables can be considered related when changes in one tend to be accompanied by changes in the other. For example, hand size and foot size are related because people with larger hands tend, on average, to have larger feet.

The concept of "tendency" is crucial here. Galton emphasized that a relationship between two variables does not mean that one always predicts the other perfectly. People with larger hands tend to have larger feet, but this is not universally true.

For example, when examining the relationship between children's height and their parents’ height, Galton observed that a child’s height was not necessarily equal to the average height of the parents. Instead, children’s heights tended to be closer to the population average than their parents’ heights were. He referred to this phenomenon as "regression toward the mean".

Galton initially conceptualized this as the result of an unknown factor drawing children’s height toward the population average. He interpreted the finding as evidence against Lamarckian inheritance and in support of Darwinian inheritance. To illustrate the strength of relationships between variables, Galton plotted data using scatterplots and visually assessed whether the relationship appeared weak or strong.

Karl Pearson later developed a mathematical formula for quantifying this relationship, which became known as the "Pearson correlation coefficient". Galton also recognized that the "median" could sometimes provide more useful information than the mean. He observed that the mean was strongly influenced by extreme values, whereas the median could provide a more robust indication of central tendency. Beyond these contributions, Galton was among the first researchers to examine the nature–nurture debate using statistical methods. His use of questionnaires for psychological and mental assessment, the word-association test, his proposal to use twin studies to investigate heredity, his work on mental imagery, and his attempts to measure intelligence all became influential research traditions.

Galton’s American Disciple: James McKeen Cattell

James McKeen Cattell entered university at the age of 16 and consistently ranked at the top of his class with relatively little effort. After graduating in mathematics and physics, he traveled from the United States to Germany to study physiology under Lutz, a physiologist associated with Wundt. Cattell became closely acquainted with Lutz and was deeply affected by his death less than a year after Cattell began his studies.

He subsequently returned to the United States. Cattell temporarily abandoned his studies, but an essay he wrote on philosophy won him a scholarship at Johns Hopkins University. There, Stanley Hall encouraged him to pursue psychology. After completing his work under Hall, Cattell returned to Germany and began studying directly under Wundt.

Many of Cattell’s studies with Wundt focused on reaction time to sensory stimuli. Cattell observed substantial individual differences in reaction time and argued that results obtained from one group could not simply be generalized to other individuals. He suggested that these individual differences deserved greater attention, but Wundt rejected the proposal.

After receiving his doctorate and returning to the United States, Cattell learned about Galton’s anthropometric laboratory and began corresponding with him. His enthusiasm for measurement led Galton to recommend him for a two-year research fellowship at Cambridge University, where he worked alongside Galton. After completing the fellowship, Cattell returned to the United States and began teaching at the University of Pennsylvania.

He introduced a new course largely based on Galtonian measurement. He administered 10 of Galton’s tests and 50 additional tests to his students and published the results in a book. This was the first work to use the term "mental test". Like Galton, Cattell believed that intelligence and sensory acuity were essentially the same thing. He therefore expected sensory measures to be measurable and to correlate with outcomes such as academic achievement.

In 1901, Clark Wissler used the statistical methods developed by Galton and Pearson to examine the relationship between Cattell’s tests and academic achievement. Contrary to Cattell’s expectations, the correlations between sensory acuity and intellectual ability were close to zero. These findings directly challenged the central assumptions of both Cattell and Galton. Wissler subsequently moved into anthropology, while Cattell turned his attention toward theories of personality, and the early tradition of Galtonian psychometrics quickly lost momentum.

The First ever IQ Test

The first major development of IQ tests resembling their modern counterparts took place in France. A growing view within French educational psychology was that, if intelligence had measurable components, these components should involve "mental operations" rather than merely sensory acuity or reaction time.

Alfred Binet

Alfred Binet was born into a family of physicians. He initially studied law and later medicine, but his professional interests increasingly turned toward psychology. Binet’s family wealth allowed him to pursue intellectual interests independently of formal university coursework. He carefully studied major works in empiricism, the writings of Darwin and Galton, and the works of John Stuart Mill.

Early in his career, he worked with Jean-Martin Charcot at the Salpêtrière. Like Charcot, Binet was deeply interested in hypnosis and conducted extensive research in this area. In his notes, he claimed that moving a powerful magnet could alter patients’ physical symptoms following hypnosis.

He also claimed that hypnosis could transform an intense fear of snakes into an intense affection for them, among other effects. Binet believed these findings could have important medical applications. However, other physicians and hypnotists were unable to replicate his findings, and the results were never formally published.

In 1890, Binet acknowledged that his findings were the result of suggestion under hypnosis rather than genuine changes in his patients’ medical conditions. He resigned from the Salpêtrière and openly acknowledged his mistakes. Nevertheless, this episode remained an important part of his professional history. One of Binet’s well-known statements captures the lesson he drew from such experiences:

"Tell me what you are looking for, and I will tell you what you will find.”

These early failures pushed Binet toward greater methodological rigor. He became more cautious and systematic in formulating hypotheses and interpreting findings, and this shift contributed to the success of the second phase of his career. His renewed scientific work began partly with his interest in his daughters. He attempted to document and investigate their cognitive development.

Many of the questions and tasks he used in studying his daughters later influenced Piaget’s work. Three years later, Binet published his observations in a series of articles. Thirteen years after that, he published a book summarizing these findings and adding further information about the intellectual development of his two daughters through adolescence.

Around the same time, Binet began working at the Laboratory of Physiological Psychology at the Sorbonne, where he assessed children across a wide range of domains, including memory, fears, creativity, and psychophysics. One of his notable experiments involved an inkblot perception task that was later further developed by Rorschach.

Unlike Galton and Cattell, who focused primarily on similarities among individuals, Binet became increasingly interested in "individual differences". In 1896, Binet and his assistant published a paper entitled "Individual Psychology", in which they discussed numerous nonphysical characteristics on which people differ and proposed variables that could be used to study these differences. Following the problems encountered by Galton and Cattell, Binet suggested that their lack of meaningful correlations might have resulted from an excessive focus on objective physical variables.

He therefore began investigating alternative psychological variables. Although Binet had addressed an important theoretical problem, his methods still had serious practical limitations. His tests could take hours to administer, leaving both the participant and examiner exhausted.

The results were largely qualitative, and interpreting them required additional time and effort. A particularly damaging finding came from Stella Sharp, who statistically examined Binet’s results and found, much like the earlier work of Galton and Cattell, that the measures showed no substantial correlations.

She summarized the problem as follows:

“These tests measure something; but that something is not intelligence!”

This failure led Binet to abandon that particular line of work. At the same time, the experience provided valuable lessons that influenced the design of his later intelligence scale.

The First Formal Intelligence Test: Binet–Simon, 1905

In 1899, Theodore Simon, a medical intern at an institution for children with intellectual disabilities, asked Binet to supervise his doctoral research. Binet accepted, partly because the position gave him access to a larger population of children for psychological assessment.

Through Simon, Binet became a member of the Free Society for the Psychological Study of Children and, less than three years later, was elected president of the organization. In 1903, the French government asked the organization to conduct a large-scale investigation into the education of children with intellectual disabilities within the French educational system. Binet and Simon became the principal investigators.

They initially argued that children with special educational needs required specialized instruction and should receive it separately from other children. Their immediate challenge was therefore to develop a reliable way of identifying which children required such support. They initially used Galtonian measures but quickly realized that these measures were not sufficiently accurate. They therefore decided to develop a new scale.

These efforts resulted in the publication of the "Binet–Simon Scale" in 1905, regarded as the first formal intelligence test. The scale consisted of 30 short tasks covering abilities ranging from the speed of pupil movement to the ability to define abstract concepts. Each task was assigned a level of difficulty, and children who successfully completed more tasks received higher scores. At this stage, the scores did not yet have a standardized numerical interpretation. Nevertheless, the scale provided a clearer means of distinguishing between children who were developing typically and those who appeared to require additional educational support. It was designed for children between the ages of two and twelve.

The test reflected several of Binet’s views about intelligence:

1. Intelligence is not entirely hereditary; environmental influences are highly important.

2. Intelligence is not a single, unitary ability but consists of multiple and diverse abilities.

3. Children’s performance can improve or decline over time, and educational effort can contribute to these changes.

In 1908, Binet and Simon revised the scale so that the scores would have a more specific meaning. They administered the scale to large numbers of children at different ages and established age-based norms. The level of performance expected from children of a particular age was then used to interpret individual performance. In 1911, Binet and Simon expanded the scale so that it could be used with children up to age 15.

Binet’s findings were incorporated into changes in the French educational system, and the tests were administered regularly to help guide teachers. Shortly afterward, German psychologist William Stern proposed a simple formula for standardizing these scores. He introduced the concept of "mental age", which could be derived from the Binet–Simon scale. By dividing mental age by chronological age and multiplying the result by 100, one could obtain the Intelligence quotient (IQ).

Binet’s most important legacy was perhaps his recognition that children’s intelligence-test performance could change considerably over time.

He believed that educational intervention could play an important role in these changes. He therefore warned teachers and educational administrators that a low score should not be interpreted as a reason to give up on a child’s education. Instead, it should indicate the need for greater educational support.

Charles Edward Spearman; General and Specific Intelligence

Charles Spearman was a British Army officer who entered psychology at the age of 34 out of intellectual curiosity. He repeatedly returned to military service because of wars and military conflicts, but nevertheless made substantial contributions to psychology.

He studied under Wundt and Külpe and, influenced by Galton, developed a strong interest in psychological measurement. In 1904, Spearman succeeded in identifying aspects of Galtonian sensory measures that were associated with intelligence and academic achievement.

Publication of these findings attracted considerable attention, and he later succeeded William McDougall at the University of London. Spearman proposed that intellectual performance could be understood in terms of a "general factor" (g) and "specific factors" (s). The general factor represented a broad underlying component of cognitive performance, while specific factors reflected abilities relevant to particular tasks or domains. Spearman associated the general factor primarily with heredity and specific factors more closely with experience and education. He also responded to Wissler’s findings showing that Galtonian and Cattellian sensory measures did not correlate meaningfully with academic achievement or intelligence.

The method Spearman developed for distinguishing between general and specific factors eventually became the foundation for "factor analysis", which was subsequently developed further by other researchers, including Burt.

Henry Herbert Goddard and the Kallikak Family

Henry Goddard was the first person to translate the Binet–Simon scale into English and administer it to children under his institution’s care. He also introduced specific labels for different ranges of intelligence, including terms such as "moron" and "borderline intelligence". These terms subsequently became heavily stigmatized and are no longer used in contemporary medical or psychological terminology.

Although Goddard used the Binet–Simon scale, his theoretical position was closer to that of Galton, Cattell, and Spearman: he believed that intelligence was substantially hereditary.

He examined this assumption by administering the scale to 2,000 schoolchildren who had no previous records of behavioral or cognitive difficulties. Surprisingly, a large proportion of these children scored below their chronological age according to the Binet–Simon criteria.

This finding was difficult to reconcile with a strictly hereditary interpretation because the children’s parents did not appear to have intelligence levels that were inappropriate for their age. Goddard subsequently decided to investigate the relationship between intelligence and family background in greater detail. He selected a young woman and her family and assigned her the pseudonym "Deborah Kallikak". Deborah was 22 years old, but the Binet–Simon scale placed her mental age at approximately nine. Goddard traced her family history back several generations using a variety of sources. Deborah’s father, whom Goddard called Martin 2, was the result of Martin 1’s sexual relationship with a household servant. Some accounts described the servant as an adult who displayed childlike behavior and difficulties. Goddard reported that none of Martin 1’s children from his marriage had significant intellectual or cognitive difficulties. Martin 2 likewise did not appear to have obvious intellectual problems.

Deborah, however, displayed cognitive difficulties two generations later that Goddard interpreted as resembling those attributed to her biological ancestry. One belief that was relatively widespread during Goddard’s time was that people with low intelligence were particularly likely to engage in criminal behavior.

Religious leaders, physicians, and others argued that moral judgment depended on intelligence and that low intelligence could lead to criminality. These assumptions contributed to harmful acts on people with intellectual and developmental disabilities. Goddard reported that although Martin 2 did not display obvious intellectual difficulties, he had a history that included criminal behavior, theft, alcohol misuse, sexual behavior that contemporary sources described as deviant, and relationships with prostitutes.

Goddard interpreted these observations as supporting both the hereditary nature of intelligence and an association between low intelligence and criminality. He also compared this information with reports concerning Martin 1’s other children from his marriage, some of whom became physicians, lawyers, and businesspeople. Goddard regarded this contrast as further evidence supporting his interpretation.

However, historical sources have questioned the reliability of many details in Goddard’s account. Some reports indicate that a substantial proportion of the information was unsupported by documentation, and some researchers have argued that Goddard omitted or downplayed details that contradicted his interpretation.

Goddard, along with other physicians who held similar views, eventually advocated the sterilization of people with low "intelligence". Their arguments were partly based on the supposed relationship between intelligence and criminality. Inspired by eugenic ideas associated with Galton, Goddard and others argued that imprisoning, sterilizing, or even detaining people who had committed no crime could improve society by preventing the transmission of supposedly hereditary intellectual deficiencies.

Eventually, approximately 20 U.S. states enacted laws permitting the sterilization or detention of people classified as “undesirable.” The definition of an undesirable person was often unclear, intelligence tests had substantial limitations, and the application of these laws could depend heavily on the discretion of individual authorities.

Some of these laws remained in effect and continued to be enforced into the early 1970s.

Lewis Terman: Standardizing the Binet Scale and Studying Gifted Children

When Terman was nine years old, his father invited a phrenologist to assess the family. The phrenologist considered Terman unusually intelligent compared with the other members of the family. As a result, his farming family placed considerable emphasis on his education. Between the ages of nine and twelve, he progressed rapidly through school. At 15, he entered Indiana State College, and at 17, he was teaching at a rural school near his home. During the following three years, while continuing to teach, he married, became a father, earned two master’s degrees, and began doctoral studies on a government scholarship.

Terman completed his doctoral studies under Stanley Hall but decided to focus his dissertation on psychological testing. He had become aware of teachers’ need for methods to distinguish between children considered intellectually disabled and exceptionally gifted children, and he decided to develop tests for this purpose.

According to the historical records available, Terman was unaware that Binet and Simon had already developed a similar approach. His dissertation examined seven children considered intellectually disabled and seven children considered gifted. Before he could formally complete the dissertation, however, Terman contracted tuberculosis and became seriously ill. On his physicians’ advice, he moved to California in the hope that the warmer climate would improve his health.

Several years later, Terman learned about the Binet–Simon scale. He quickly recognized that the scale would not necessarily function appropriately with American children. When he tested it, he found that the age norms developed within the French educational system did not correspond well to the American educational system. As a result, the scale produced highly inconsistent results. In particular, it appeared to assign unusually high intelligence to American preschool children and unusually low intelligence to children aged 11–12. Terman therefore began revising the scale and published a standardized version in 1911.

Five years later, the test was revised again and eventually became one of the most widely used intelligence tests in the world. One important feature of Terman’s version was its use of "100 as the normative reference point" rather than relying exclusively on an exact mental-age interpretation. Scores below 100 indicated performance below the average for the relevant reference group, while scores above 100 indicated performance above that average.

Terman’s revised test, which formally incorporated the terms "IQ" and standardized cutoffs, became known as the "Stanford–Binet Intelligence Scale", reflecting Terman’s position at Stanford University. The test continues to be used today in revised forms.

Theoretically, Terman was broadly aligned with Goddard. His revision of the Binet scale, however, improved its standardization and made its administration and interpretation more systematic. Terman repeatedly expressed views concerning the hereditary nature of intelligence, criminality among people with low intelligence, and related ideas.

These views attracted substantial criticism. One of the most prominent critics was journalist Walter Lippmann, who wrote:

“I hate the presumption that you [Terman and his colleagues] can judge and classify a man’s predestined competence in fifteen minutes. I hate the claim. I detest the abuses inherent in this method. I hate the sense of superiority it creates and the sense of inferiority it imposes."

(Lippmann, The New Republic, p. 46, 1923)

Terman also sought to validate the Stanford–Binet by comparing its results with teacher evaluations, academic performance, and school grades. These measures showed high correlations, which Terman interpreted as evidence of the scale’s validity. From a modern perspective, however, this interpretation raises an important methodological issue: teacher evaluations, academic performance, and school grades may themselves reflect abilities that overlap substantially with those assessed by the Stanford–Binet. Thus, the scale correlated with other measures, but this correlation alone did not establish that it was measuring intelligence in a broader or theoretically independent sense.

Terman also devoted considerable attention to exceptionally gifted children. He described them as "precocious" and believed that if a child’s intellectual development occurred unusually early, there might be less room for further intellectual growth later in life. In one of the most influential longitudinal studies in psychology, Terman and his colleagues identified 1,528 elementary-school children with IQ scores above 135, with a mean IQ of 151, and followed their educational and personal development over many years.

The first major findings were published approximately nine years after the beginning of the study in "Genetic Studies of Genius" (1926). (Terman used the term "genetic" in the sense of development; modern genetics as a scientific field was still in its early stages.)

When the participants reached an average age of 29, Terman and his colleagues conducted their final formal assessment. Seventy percent of the men and women had completed college, while 56 percent of the men and 33 percent of the women had obtained advanced degrees at the master’s or doctoral level. The participants also tended to occupy high percentiles on many other indicators of educational and occupational achievement.

Twelve years later, Terman published additional information about the group, primarily in descriptive form. He reported that the participants, who were then at least 40 years old, continued to perform at high levels relative to the general population. They worked in fields including medicine, law, management, writing, science, and business, and some had become highly prominent. Other researchers continued to follow the group after Terman’s death.

Members continued to be studied into their 60s, 70s, and 80s, and their average life expectancy was also reported to differ from that of the general population. The final formal assessments were conducted in the 1990s, when some participants were in their late 90s. Of the original 1,528 participants, 1,069 were still alive at that point. Because the participants’ names and identifying information are not publicly available, later reports have relied on historical records and aggregate information.

World War I and IQ Test in the U.S. Army

Robert M. Yerkes, a member of the Harvard University faculty, took a second position because of financial difficulties. He worked at the Boston Psychopathic Hospital as director of psychological research and quickly noticed problems with Terman’s proposed scoring system. The hospital used intelligence testing as an auxiliary assessment tool, but nurses were not sufficiently familiar with the tests and there was not enough time to assess patients individually.

As a result, the measurements were often inaccurate and were not given much attention. Yerkes believed that changes in administration and scoring could address these problems. After approximately two years of trial and error, Yerkes developed a simplified procedure. Instead of administering the complete test, items were administered according to age-based standards, and the result was essentially a "pass-or-fail judgment": whether the individual could successfully answer all the questions at a particular level.

The procedure resembled Binet’s original approach in some respects, but it did not produce a conventional numerical score. For example, if a child was nine years old, the examiner would first administer the items corresponding to nine-year-olds. If the child answered all of them correctly, mental age and chronological age were considered equivalent. If not, items corresponding to eight-year-olds were administered. If all of those were answered correctly, the child’s mental age was considered one year below chronological age.

The process continued until the child successfully completed all items associated with a particular age level. This procedure was faster, required fewer statistical calculations, and produced less emphasis on a stigmatizing numerical label. One year later, the United States entered World War I.

Yerkes participated in discussions at Harvard concerning possible psychological contributions to the military and the government. He and his colleagues concluded that psychologists, particularly those experienced in psychological testing, could assist the government in evaluating new military recruits.

Their proposal was accepted, and research began to identify tests that could be used for military selection. Their objectives included identifying individuals with serious intellectual difficulties, identifying individuals who might pose significant risks to themselves or others, and identifying people with particular physical or psychological characteristics who might benefit from specialized military training.

They soon encountered a major practical problem: approximately half of the men being assessed for military service were unable to read or write. To address this, the researchers developed two tests; one for literate recruits and another for individuals who were illiterate or had limited proficiency in English. By the time Yerkes and his colleagues finalized the tests, almost a year had passed since the end of the war.

Over the two and a half years during which Yerkes worked on the two tests, approximately 1.5 million people were assessed. Later evidence, however, indicated that the Army followed the recommendation to reject an applicant in only 0.005% of cases.

Subsequent research conducted before World War II suggested that intelligence-test performance at military entry was associated with successful completion of training and with some indicators of risk. Based on these findings, some researchers argued that psychological testing could help reduce the inefficient allocation of human and financial resources and mitigate certain risks.

Modern Intelligence Tests

Louis L. Thurstone was an important figure in the development of modern theories of intelligence. He organized approximately 100 characteristics that had been identified by different researchers and proposed seven primary mental abilities associated with intelligence:

1. Verbal comprehension

2. Word fluency

3. Number facility

4. Spatial visualization

5. Associative memory

6. Perceptual speed

7. Reasoning

These abilities became influential in subsequent theories of intelligence and also influenced later alternative models, including Sternberg’s triarchic theory. Major changes in the administration and scoring of intelligence tests that made them more practical and clinically useful were introduced by David Wechsler, particularly around the period of World War II.

Wechsler had previously worked as an evaluator with Yerkes’s team during World War I. In the years that followed, he developed a new intelligence test specifically designed to assess adults. Until this point, intelligence testing had focused primarily on children and school-age populations. The work of Yerkes and his colleagues helped increase attention to adult intelligence assessment. While working as an examiner at Bellevue Hospital, Wechsler recognized a need for an intelligence assessment that could be used with adult patients and contribute to clinical evaluation, prognosis, and treatment planning.

After developing the initial version of his test, Wechsler evaluated and standardized it using data collected during the assessment of soldiers in World War II. Shortly after the war, the "Wechsler–Bellevue Intelligence Scale" was published for adults. It was later followed by the "Wechsler Adult Intelligence Scale (WAIS)".

A few years later, Wechsler developed the "Wechsler Intelligence Scale for Children (WISC)", which used a similar framework for assessing children. The popularity of the Wechsler scales led to further revisions of the Stanford–Binet so that its scoring and interpretation could better reflect developments in contemporary testing.

Over the decades since Wechsler’s work, numerous intelligence tests and theoretical models have been proposed and investigated. Nevertheless, the Wechsler scales for children and adults, along with the Stanford–Binet, remain among the most widely used standardized measures of cognitive ability.

Final Words

The history of psychological testing, and intelligence testing in particular, begins in large part with Galton’s efforts to measure human characteristics. His interest in quantifying traits and examining relationships among them contributed to the development of statistical methods that remain fundamental to psychological research today.

The increasing sophistication of these statistical methods encouraged researchers to believe that intelligence could be measured systematically. This belief led to a long series of fascinating and important studies. At the same time, however, the history of intelligence testing also demonstrates how psychological measurement can be misused.

In the United States, Nazi Germany, and parts of Eastern Europe, thousands of people were classified as "socially undesirable" on the basis of intelligence tests that had serious methodological and validity limitations. In some cases, these classifications contributed to forced sterilization, institutionalization, detention, and other severe violations of human rights.