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Understanding IQ

What Different IQ Scores Mean

Last updated:
12 Apr 2004

IQ Scores & Ratings

Cautions for Interpreting IQ Scores

Normal Distribution & IQ Scores

Low IQ & Mental Retardation

High IQ & Genius IQ


IQ Scores & Ratings

What is a good IQ score?  What is a high IQ score?  What is a low IQ score?  These are common questions, particularly after someone finds out their score from an IQ test.

Lewis Terman (1916) developed the original notion of IQ and proposed this scale for classifying IQ scores:

  • Over 140 - Genius or near genius

  • 120 - 140 - Very superior intelligence

  • 110 - 119 - Superior intelligence

  • 90 - 109 - Normal or average intelligence

  • 80 - 89 - Dullness

  • 70 - 79 - Borderline deficiency

  • Under 70 - Definite feeble-mindedness

Normal Distribution & IQ Scores

The properties of the normal distribution apply to 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

5% of people have an IQ under 70 and this is generally considered as the benchmark for "mental retardation", a condition of limited mental ability in that it produces difficulty in adapting to the demands of life.

Severity of mental retardation can be 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 IQ is generally considered to begin around 140 to 145, representing ~.25% of the population (1 in 400).  Here's a rough guide:

  • 115-124 - Above average (e.g., university students)
  • 125-134 - Gifted (e.g., post-graduate students)
  • 135-144 - Highly gifted (e.g., intellectuals)
  • 145-154 - Genius (e.g., professors)
  • 155-164 - Genius (e.g., Nobel Prize winners)
  • 165-179 - High genius
  • 180-200 - Highest genius
  • >200 - "Unmeasurable genius"

More notes on High IQ and Genius IQ:

  • Einstein was considered to "only" have an IQ of about 160.

  • Mensa is a society for people with high IQ, in the top 2% (1 in 50).

  • In 1926, psychologist Catherine Morris Cox published a study "of the most eminent men and women" who had lived between 1450 and 1850 to estimate what their IQs might have been.

  • Go to Estimated IQs of the Greatest Geniuses.