What is the Most Studied Area in Psychology? Intelligence

What is intelligence? Intelligence is one of the most examined fields in the history of psychology (Hood, et. al). The Oxford American Dictionary defines intelligence as mental ability, the power of learning and understanding. In the early twentieth century, Alfred Binet developed a series of measures by which he could identify children in need of special education programs, using the concept that intelligence is a general ability to judge well, to comprehend well, and to reason well (1916). He showed mental processes grow and increase as a child gets older. Francis Galton and Herbert Spencer used the Latin word for intelligence to refer to individual differences in ability (Drummond, 2000). Cattell and others looked at Darwin’s theory of individual differences. Wechsler (1958) defined intelligence as, “the aggregate or global ability of the individual to act purposefully, to think rationally, and to deal effectively with his environment” (p.7). When looking at tests of intelligence, it is important to note the author’s definition of intelligence as tests measure different conceptual ideas of intelligence.

Just as there are myriad definitions of intelligence, there are many models that attempt to capture the many facets of intelligence. Spearman (1927), using factor analysis of test performance, identified two factors – general ability (g) and specific item ability (s). Thurstone (1938) identified seven factors, as follows:

1. Number ability: to perform basic mathematical processes accurately and rapidly

2. Verbal ability: to understand ideas expressed in word form

3. Word fluency: to speak and write fluently

4. Memory: to recognize and recall information such as numbers, letters, and words

5. Reasoning: to derive rules and solve problems inductively

6. Spatial ability: to visualize form relationships in three dimensions

7. Perception: to perceive things quickly, such as visual details and similarities and differences among pictured objects.

Cattell (1963) identified fluid and crystallized intelligence. Fluid intelligence refers to the intelligence used in tasks requiring adaptation and change to new situations. Heredity plays a role in fluid intelligence. Crystallized intelligence refers to those areas that are learned, such as in school, and are related to the environment.

IQ is the commonly used measurement of intelligence. IQ is a ratio of a child’s mental age (by testing) divided by the child’s chronological age, multiplied by 100. This original definition of IQ has a number of problems with limitations on age (e.g. answering all questions correctly on the Stanford-Binet Intelligence Scale yields a mental age of less than 20), thus rendering the usefulness of this scale to preteens (Hood & Johnson). Another problem with this measurement is that the general public has come to believe IQ is fixed and immutable, somewhat like eye color, rather than the particular score on a particular test at a particular time. A derived IQ standard score has replaced the ratio IQ to circumvent some of these problems.


Binet, A., & Simon, T. (1916). The development of intelligence in children (E.S. Kite, Trans.). Baltimore: Williams & Wilkins.

Drummond, R.J. (2000). Appraisal procedures for counselors and helping professionals (4th ed.). Upper Saddle River, NJ: Merrill/Prentice Hall.

Hood, A.B., & Johnson, R.W. (1997). Assessment in counseling, (2nd ed.). Alexandria, VA.: American Counseling Association.

Spearman, C. (1927). The abilities of man. London: MacMillan.

Wechsler, D. (1958). The measurement of adult intelligence. Baltimore: Williams & Wilkins.