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What Does the IQ Scale Measure?

What does the intelligence quotient scale really measure? What is an average score? How high can a person's IQ be? When did we first use this particular measurement of intelligence? We'll start with that last question.

The imperial examination system in China, established in 605 during the Sui Dynasty, may have been the first large-scale testing of mental abilities. It was used to choose the best candidates for administrative positions in the Chinese bureaucracy. In the 19th century testing of mental capacity became common in France. In 1904 psychologist Alfred Binet was commissioned by the French government to create a testing system to differentiate intellectually normal children from those who were mentally deficient.

From Binet's work the IQ scale called the "Binet Scale" was developed. With psychologists Victor Henri and Théodore Simon he developed the Binet-Simon test, which was first published in 1905. The term "Intelligence Quotient," came from the very similar German term Intelligenz-Quotient (or Intelligenzquotient), which was first used by psychologist William Stern. The acronym "IQ" is used most often now. Lewis M. Terman revised the Simon-Binet IQ Scale, and in 1916 published the Stanford Revision of the Binet-Simon Scale of Intelligence (also known as the Stanford-Binet). The following scale resulted for classifying IQ scores:

IQ Scale

Over 140 - Genius or almost genius
120 - 140 - Very superior intelligence
110 - 119 - Superior intelligence
90 - 109 - Average or normal intelligence
80 - 89 - Dullness
70 - 79 - Borderline deficiency in intelligence
Under 70 - Feeble-mindedness

Normal Distribution of IQ Scores

Modern tests are designed so that the average (mean) score is 100. The standard deviation is usually 15, and about 95% of scores fall within two standard deviations, which means that most of us have a score between 70 and 130. Here is a more detailed breakdown of the distribution of scores:

50% of IQ scores fall between 90 and 110
70% of IQ scores fall between 85 and 115
95% of IQ scores fall between 70 and 130
99.5% of IQ scores fall between 60 and 140

Low IQ and Mental Retardation

An IQ under 70 is considered as "mental retardation" or limited mental ability. 5% of the population falls below 70 on IQ tests. The severity of the mental retardation is commonly broken into 4 levels:

50-70 - Mild mental retardation (85%)
35-50 - Moderate mental retardation (10%)
20-35 - Severe mental retardation (4%)
IQ < 20 - Profound mental retardation (1%)

High IQ and Genius IQ

Genius or near-genius IQ is considered to start around 140 to 145. Less than 1/4 of 1 percent fall into this category. Here are some common designations on the IQ scale:

115-124 - Above average
125-134 - Gifted
135-144 - Very gifted
145-164 - Genius
165-179 - High genius
180-200 - Highest genius

The Average on an IQ Scale

The scoring of an IQ test is not the same for everyone, because age is used in determining a score. Intelligence quotient is determined by the "intelligence age" (IA) in relation to the chronological age of the person being tested. IQ scales can differ from each other (Americans use scales with IQ values above 200), but the mean value of most scales is an IQ of 100. This represents normal intelligence.

Think you are smart? Well, if your IQ is 130, that puts you ahead of 98% of people. Of course that means there are still 140 million people who are smarter than you (the other 2%). Also, recent research shows that if a person is anywhere within the range of normal intelligence, a person's level of self-discipline is more predictive of success than their specific intelligence quotient. In other words, don't take your score on an IQ test to mean too much.

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