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Intelligence quotient, or IQ, is a number used to indicate a person's intelligence. A person's IQ is based on a comparison of his or her score on an intelligence test with the scores of others on the same test. 

Testing intelligence. 

Educators and psychologists use an intelligence test to determine a person's mental age (MA)--the level of understanding and performance that the person has reached. Such a test consists of a series of mental tasks arranged in order of difficulty. Most intelligence tests include tasks involving memory, reasoning, definitions, numerical ability, and recalling facts. 

Psychologists have worked out the age at which most people can correctly answer each question of an intelligence test. Suppose that a 10-year-old is asked to define certain words, to work out relationships of words and ideas, to solve simple arithmetic problems, and to remember certain facts. The child does as many of these tasks as possible. A child who completes the tasks expected of a 9-year-old but cannot complete those of a 10-year-old has a mental age of 9 years. 

Calculating IQ. 

A person's intelligence cannot be determined by mental age alone. A 6-year-old with an MA of 8 years is more intelligent than a 10-year-old who also has an MA of 8 years. 

The original IQ test divided the MA by the age in years and then multiplied the result by 100 to avoid fractions. Thus, a 4-year-old with an MA of 6 had an IQ of (6 divided by 4) X 100, or 150. A 10-year-old with an MA of 8 had an IQ of (8 divided by 10) X 100, or 80. In most IQ tests, 100 indicates an average score for a person's age level. 

Since 1960, all IQ tests have determined IQ by assigning a value of 100 to the average score of those tested. Then the testers assign values above and below 100 to the other scores, depending on how much above or below average the score is. Intelligence develops at a slower rate during adulthood, and so testers use this method to assign IQ scores to adults. 

( see Intelligence tests ratings )

Uses of IQ tests. 

Teachers in some countries use IQ scores to help judge whether children are progressing as well as their ability permits. If a child scores high on IQ tests but does poorly in class, the teacher may try to determine what circumstances keep the child from learning. If a child scores low on IQ tests and is doing poorly in class, the teacher may try to separate learning tasks into smaller or more familiar units. With such help, the child may progress more rapidly. IQ tests can also provide a means of grouping individuals with similar abilities. Classroom instruction can then be adapted to each group of students. Some school authorities use IQ scores to help determine whether to allow a student to be in special courses or programmes. 

Problems of intelligence testing. 

In designing an intelligence test, psychologists try to use questions on subjects to which every person to be tested has been equally exposed. But this cannot be done perfectly. As a result, every intelligence test measures experience to some degree. For example, a child who has always spoken only English is likely to score higher on a test given in English than a child who spoke only Spanish until learning English in school. Similarly, a child whose family reads and travels widely may score higher than a child who lacks these experiences. 

A home and school life that encourages learning may result in higher IQ test scores as a child grows older. On the other hand, deprived youngsters who go to an inferior school may score lower and lower through the years. Their falling scores could reflect the lack of many experiences that other children have had. Children may also score low because of malnutrition in their early years. IQ thus depends on both heredity and environment. 

Many people have tried to create culture-fair or culture-free tests. Such tests include only ideas, symbols, or words that are recognized by persons of different backgrounds. Sometimes they are called nonverbal (wordless) tests to contrast them with verbal tests, which use words. Some such tests consist only of pictures and diagrams that people of many cultures can understand. 

The IQ score of most persons does not change much from year to year. But scores of some individuals can vary from one test to another. People may score somewhat higher if they take a test feeling well, rested, and confident than if they feel ill or tired. 

Some educators and psychologists oppose using IQ tests for grouping students. These experts fear that a teacher may treat a child according to the "label" of the youngster's group. Thus, children labelled "slow" may think they are expected to learn slowly--and do so, even though they could progress more rapidly. 

History. 

Two French psychologists, Alfred Binet and Theodore Simon, developed the first modern intelligence tests in 1905. In later tests, Binet and Simon introduced the idea of measuring mental age. They wanted to separate children who probably would have difficulty with schoolwork from those who probably would succeed. Binet checked the accuracy of the tests against the children's actual classroom performance. He then dropped or revised parts of the tests in which an individual's schoolwork showed his or her abilities to be stronger or weaker than the test scores indicated. 

These tests have since been widely adopted in many countries.