IQ scale

IQ scale

What does the IQ scale measure? What is an average score? How high can an IQ be? Here are some answers to these and other questions.

First of all, the concept of IQ was developed by either the German psychologist and philosopher Wilhelm Stern in 1912, or by Lewis Terman in 1916, depending on which sources you consult. Intelligence testing was first done on a large scale before either of these dates. 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 inferior.

From Binet's work the IQ scale called the "Binet Scale," (and later the "Simon-Binet Scale") was developed. Sometime later, "intelligence quotient," or "IQ," entered our vocabulary. 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

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 & 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 & 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