How Does Science Define Intelligence?


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Defining Intelligence: Image by Zaldy Icaonapo

What does it mean to be intelligent?  What are the characteristics, talents, achievements, or the degree of neuronal complexity that separates the average from the gifted?  Most of us could provide numerous examples of people we consider intelligent, but would we agree on the constructs of intelligence?  What about accomplished academics or wealthy, successful business tycoons that make apparently stupid, self-destructive decisions?  It is a complex question, but decisions are made daily that have a profound effect on children’s lives, based on tests designed to measure “intelligence.”

Historical Attempts to Quantify Intelligence

Scientific advancement relies on rigorous empirical research.  How can an abstract concept like intelligence be measured?  For 200 years philosophers, mathematicians, biologists, neurologists and more recently, psychologists have tried to define and measure human intellect, or intelligence.

Since Plato and Aristotle’s time, there has been interest in how people learn and advance.  It wasn’t until the early 19th century that the idea of measuring intelligence was considered.  Although psychology’s roots are in philosophy, early psychologists came from the ranks of medical doctors and neurologists, and they were anxious to have their evolving discipline accepted within the “hard” sciences.  There was a rush to the lab to produce empirical research that would explain human behavior from a cognitive or neurological perspective.

What Constructs Should be Measured?

James McKeen Cattell was a leader in the movement to measure intelligence.  He developed a series of tests that measured various aspects of sensation and perception and hoped that further experiential work would clarify the dilemma: does nature or nurture have the most influence on human intelligence and behavior?  This work continues today, with a general acknowledgement that there can never be a clear distinction between these two influences, however the attempts to do so have produced important advancements.

Edward Thorndike, a very influential psychologist and psychometrician believed that measuring intellect would entail measuring the complexity and variety of tasks attempted as well as the speed with which they were completed.  This, he believed would portray the level of intellect.

Thorndike initiated the idea of neuro bonds, meaning that the response to specific experience results in new connections between neurons, and learning occurs.  He believed that intellect was necessary in order for these bonds to form.  Today, neuro-psychologists use the phrase, “the neurons that fire together, wire together.”

Assessment for Inclusion or Exclusion?

Alfred Binet: Image by Skagedal

Alfred Binet, who studied law and then science at the Sorbonne, eventually becoming the director of the Laboratory of Experimental Psychology, was given the task of developing some means of determining which students were unable to benefit from regular schooling and who needed alternative education.  This meant determining how to measure children’s potential for learning.  Binet developed a scale consisting of a series of tasks that children at specific ages could typically perform, and this scale provided a  score representative of a student’s “mental age”, a measure of development or readiness for learning.  This scale was meant to be used as a means to advocate for educating all children, not to exclude children.   This mental age was called an intelligence quotient, or I.Q.

Binet insisted that culture, background and schooling influences intellect, therefore results from testing would change over time.  He was not a strong supporter of the idea that intelligence was entirely influenced by heredity, a common belief at the time.  Binet considered judgment to be the fundamental faculty of intelligence, or the ability to adapt to one’s environment and use common sense.

Binet intended his scale to be used to determine the readiness of children for school learning, but some researchers in the United States saw a relationship between “mental defectives” and social vices.  This precipitated a movement to identify and control these individuals.  While Binet continued his work for the benefit of children in France, Lewis Terman, in the United States, developed another version of Binet’s test and the manual specified its use:

“a) to allow for scientific diagnosis and classification of children to be placed in special classes

b)  bring thousands of high-grade defectives under the surveillance and protection of society”

Terman believed that through this identification and institutionalization process poverty and crime would be much reduced.  Others considered the “subnormal” to include runaways, spendthrifts, compulsive masturbators, liars, thieves, and a plethora of other apparently deviant types.  With this wide range of potential “subnormals”, the task of designing an instrument that would identify even some of these traits was very challenging.

When Binet became aware of how versions of his instrument were being used in North America, he called it “brutal pessimism” and embarked on a series of projects aimed at demonstrating that the environment, opportunity, and directed learning affect children’s intelligence, and that heredity is not the only influence on ability.

After years of study, Binet suggested three different ways of looking at intelligence: medical (physiological, anatomical influence), pedagogical (acquired knowledge), and psychological (observation/measurement).

Out of Disagreement Comes Progress

In the late sixties, a student of Arthur Jensen noticed that black children that had been identified as retarded appeared normal outside of their school program, whereas the other children did not.  The student asked if there was a culture-free I.Q. test that might be a better predictor of ability in black children than the Stanford-Binet.  This prompted Jensen to write the infamous article that was published in the Harvard Educational Review in February of 1969:  How Much Can We Boost I.Q. and Scholastic Achievement?

Jensen focused on racial differences and insisted that heredity determined intelligence, not social, nutritional and cultural influences, in contrast with Binet.  Jensen believed that all races were equally developed in terms of “Level I” abilities such as memory, or simple associative learning, but he believed that “Level II” abilities such as abstract reasoning were stronger in Caucasian and Asian races.   These beliefs were based on anatomical measurements and psychometric testing using instruments that are considered culturally biased today.

Robert Sternberg, of Yale University, believes that due to our traditionally limited definition of intelligence some children get disproportionate attention and stimulation, which results in a self-fulfilling prophecy whereby these children are encouraged to achieve and respond by achieving.  Other children, placed in less stimulating environments, predictably achieve less.  He developed the term “successful intelligence” :

“The ability to achieve success in life, given one’s personal standards, within one’s sociocultural context; in order to adapt to, shape, and select environments; via recognition of and capitalization on strengths and remediation of or compensation for weaknesses; through a balance of analytical, creative, and practical abilities.”This triarchic theory was more in keeping with Binet’s idea that assessing intelligence merely caught a snapshot in time, and abilities could evolve and be enhanced by experience and maturation.

Alan Kaufman has been one of the most prolific authors of texts for educational psychologists and has developed assessment tools that are as free of cultural bias as possible.  He acknowledges that we cannot measure every component of what we consider intelligence today, and agrees with Wechsler that intelligence is a complex interaction among abilities rather than a series of discrete abilities.  Everyone has strengths and weaknesses, and our strengths compensate to some degree for our weaknesses, resulting in the wonderfully rich individuality inherent in the human race.

In recent years there has been a movement to include a variety of skills and talents in the intelligence basket, which is a wonderful way to encourage children who excel in non-academic areas.  This can also enhance our appreciation of diversity.

But, to get back to those apparently brilliant people who do really stupid things….  Sternberg says that people can be intelligent, even successfully intelligent, and still make very foolish choices due to personality traits and beliefs such as egocentrism, omniscience, omnipotence, or invulnerability.


Indiana University. History of Influences in the Development of Intelligence Theory. (2009). Accessed July 15, 2011.

J. McKeen Cattell. Mental Tests and Measurements. Mind, 15, 373-381. (1890).

White, Sheldon H. Conceptual Foundations of I.Q. Testing. Psychology, Public Policy, and Law. (2000). Vol. 6, No. 1, 33-43.American Psychological Association.

Sternberg, R.J. Teaching for Successful Intelligence. POWER Point presentation. Yale University.

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