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Today’s ranking of IQ scores was developed by Lewis Terman in 1916 as a way of categorizing intelligence levels. The ranking of typical ranges of IQ scores and what they mean are summarized below:

• 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

The distribution of IQ scores in the world falls along a normal distribution curve, and can be summarized as:

• 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

Roughly 5% of the population has IQ scores under 70; this is considered the benchmark for mental retardation. On the other end of the scale, only 1% of the population falls in the 135 and above category, which is the benchmark for genius.

What is the relevance of these scores?  At best, they measure academic intelligence or the ability to succeed in an academic environment.  Many other kinds of intelligence have been discovered and categorized that are not measured by these tests, such as the physical intelligence demonstrated by athletes, and musical/artistic genius.  A rough ranking of what each IQ score means occupationally is shown below:

There is also considerable evidence that IQ scores can be improved by changes in ones environment, particularly for children.  Children who have been read to more often or have been subjected to variety in their environment tend to improve their scores over time.