IQ Scores and IQ Score Interpretation

human-brain-2IQ scores are often misunderstood. Learn the basics of IQ score interpretation in this article.

Intelligence testing began in earnest in France, when in 1904 psychologist Alfred Binet was commissioned by the French government to find a method to differentiate between children who were intellectually normal and those who were inferior. The purpose was to put the latter into special schools. There they would receive more individual attention and the disruption they caused in the education of intellectually normal children could be avoided.

This led to the development of the Binet Scale, also known as the Simon-Binet Scale in recognition of Theophile Simon’s assistance in its development. The test had children do tasks such as follow commands, copy patterns, name objects, and put things in order or arrange them properly. Binet gave the test to Paris schoolchildren and created a standard based on his data. Following Binet’s work, the phrase “intelligence quotient,” or “IQ,” entered the vocabulary.

Lewis M. Terman worked on revising the Simon-Binet Scale. His final product, published in 1916 as the Stanford Revision of the Binet-Simon Scale of Intelligence (also known as the Stanford-Binet), became the standard intelligence test in the United States for the next several decades. By the 1920s mass use of the Stanford-Binet Scale and other tests had created a multimillion-dollar testing industry.

Despite the fact that the IQ test industry is already a century old, IQ scores are still often misunderstood. Comments like, “What do you mean my child isn’t gifted — he got 99 on those tests! That’s nearly a perfect score, isn’t it?” or “The criteria you handed out says ‘a score in the 97th percentile or above.’ Jane got an IQ score of 97! That meets the requirement, doesn’t it?” are not unusual and indicate a complete misunderstanding of IQ scores.

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December 21, 2013

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Understanding IQ scores

IQ stands for intelligence quotient. Supposedly, it is a score that tells one how “bright” a person is compared to other people. The average IQ is by definition 100; scores above 100 indicate a higher than average IQ and scores below 100 indicate a lower that average IQ. Theoretically, scores can range any amount below or above 100, but in practice they do not meaningfully go much below 50 or above 150.

Half of the population have IQ’s of between 90 and 110, while 25% have higher IQ’s and 25% have lower IQ’s:

Descriptive Classifications of Intelligence Quotients



% of Population


Very superior






High average






Low average





Below 70

Extremely low


IQ expressed in percentiles

IQ is often expressed in percentiles, which is not the same as percentage scores, and a common reason for the misunderstanding of IQ test scores. Percentage refers to the number of items which a child answers correctly compared to the total number of items presented. If a child answers 25 questions correctly on a 50 question test he would earn a percentage score of 50. If he answers 40 questions on the same test his percentage score would be 80. Percentile, however, refers to the number of other test takers’ scores that an individual’s score equals or exceeds. If a child answered 25 questions and did better than 50% of the children taking the test he would score at the 50th percentile. However, if he answered 40 questions on the 50 item test and everyone else answered more than he did, he would fall at a very low percentile — even though he answered 80% of the questions correctly.

On most standardized tests, an IQ of 100 is at the 50th percentile. Most of our IQ tests are standardized with a mean score of 100 and a standard deviation of 15. What that means is that the following IQ scores will be roughly equivalent to the following percentiles:

































An IQ of 120 therefore implies that the testee is brighter than about 91% of the population, while 130 puts a person ahead of 98% of people. A person with an IQ of 80 is brighter than only 9% of people, and only a few score less than 60.

Be cautious!

It is necessary to be very cautious when using a descriptive classification of IQ’s. The IQ is, at best, a rough measure of academic intelligence. It certainly would be unscientific to say that an individual with an IQ of 110 is of high average intelligence, while an individual with an IQ of 109 is of only average intelligence. Such a strict classification of intellectual abilities would fail to take account of social elements such as home, school, and community. These elements are not adequately measured by present intelligence tests.

Add to this the fact that IQ tests are not very reliable, and it is understandable why IQ tests have been the focus of criticism for many years. The scores may vary as much as 15 points from one test to another, while emotional tension, anxiety, and unfamiliarity with the testing process can greatly affect test performance. In addition, Gould described the biasing effect that tester attitudes, qualifications, and instructions can have on testing. In one study, ninety-nine school psychologists independently scored an IQ test from identical records, and came up with IQ’s ranging from 63 to 117 for the same person!

Measures of intelligence may be valuable — although the value is often overrated — but much harm can be done by persons who try to classify individuals strictly on the basis of such measures alone. No one should be either alarmed or discouraged if he finds that his IQ is not as high as he might have hoped. Remember that many elements besides IQ contribute to success and happiness. Also note that IQ is not a fixed quantity, but can be increased by means of education. This was demonstrated by an experiment at the Glenwood State School as well as numerous other research studies.

The Glenwood State School

Research on the role of the environment in children’s intellectual development has shown that a stimulating environment can dramatically increase IQ, whereas a deprived environment can lead to a decrease in IQ. A particularly interesting project on early intellectual stimulation involved 25 children in an orphanage. These children were seriously environmentally deprived because the orphanage was crowded and understaffed. Thirteen babies of the average age of 19 months were transferred to the Glenwood State School for retarded adult women and each baby was put in the personal care of a woman. Skeels, who conducted the experiment, deliberately chose the most deficient of the orphans to be placed in the Glenwood School. Their average IQ was 64, while the average IQ of the 12 who stayed behind in the orphanage was 87.

In the Glenwood State School the children were placed in open, active wards with the older and relatively brighter women. Their substitute mothers overwhelmed them with love and cuddling. Toys were available, they were taken on outings and they were talked to a lot. The women were taught how to stimulate the babies intellectually and how to elicit language from them.

After 18 months, the dramatic findings were that the children who had been placed with substitute mothers, and had therefore received additional stimulation, on average showed an increase of 29 IQ points! A follow-up study was conducted two and a half years later. Eleven of the 13 children originally transferred to the Glenwood home had been adopted and their average IQ was now 101. The two children who had not been adopted were reinstitutionalized and lost their initial gain. The control group, the 12 children who had not been transferred to Glenwood, had remained in institution wards and now had an average IQ of 66 (an average decrease of 21 points).

More telling than the increase or decrease in IQ, however, is the difference in the quality of life these two groups enjoyed. When these children reached young adulthood, another follow-up study brought the following to light: “The experimental group had become productive, functioning adults, while the control group, for the most part, had been institutionalized as mentally retarded.”

Other examples of IQ increase through early enrichment projects can be found in Israel, where children with a European Jewish heritage have an average IQ of 105 while those with a Middle Eastern Jewish heritage have an average IQ of only 85. Yet when raised on a kibbutz, children from both groups have an average IQ of 115.

In another home-based early enrichment program, conducted in Nassua County, New York, an instructor made only two half-hour visits a week for only seven months over a period of two years. He spent time showing parents participating in the program how best to teach their children at home. The children in the program had initial IQ’s in the low 90s, but by the time they went to school they averaged IQ’s of 107 or 108. In addition, they have consistently demonstrated superior ability on school achievement tests.

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References and Bibliography:
  • Clark, B., Growing Up Gifted (3rd ed.), (Columbus: Merrill, 1988).
  • Dworetzky, J. P., Introduction to Child Development (St. Paul: West Publishing Company, 1981).
  • Engle, T. L., & Snellgrove, L., Psychology: Its Principles and Applications (6th ed.), (Harcourt Brace Jovanovich, Inc.: New York, 1974).
  • Gould, S. J., The Mismeasure of Man (New York: W. W. Norton, 1981), 151-152, cited in R. L. Osgood, “Intelligence testing and the field of learning disabilities: A historical and critical perspective,” Learning Disability Quarterly, 1984, vol. 7, 343-348.
  • Sattler, J., Assessment of Children’s Intelligences and Special Abilities (Boston: Allyn & Bacon, 1982), 60.
  • Skeels, H. M., et al., “A study of environmental stimulation: An orphanage preschool project,” University of Iowa Studies in Child Welfare, 1938, vol. 15(4).
  • Smith, C. R., Learning Disabilities: The Interaction of Learner, Task, and Setting (Boston: Allyn and Bacon, 1991), 63.
  • Swiegers, D. J., & Louw, D. A., “Intelligensie,” in D. A. Louw (ed.), Inleiding tot die Psigologie (2nd ed.), (Johannesburg: McGraw Hill, 1982).
  • “Test Score Interpretation,” Hampton City Schools, Psychological Services.
  • Tyler, cited in A. Anastasi, (ed.), Testing Problems in Perspective (Washington DC: American Council on Education, 1966).
  • Wilson, G., & Grylls, D., Know Your Child’s IQ (Futura Publications: London, 1977).
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