How Low IQ Scores Are Classified and What They Mean

Low IQ distribution
Alessio Damato, Mikhail Ryazanov / Wikimedia Commons (CC BY-SA 3.0)

While we often hear a lot of talk about high and low IQ scores, many people aren't quite sure what these designations really mean.

What Is Considered a Low IQ Score?

An IQ score of 70 or below is considered a low score. Remember, on most standardized tests of intelligence, the average score is set at 100. Anything over 140 is considered high or genius-level. Sixty-eight percent of all scores fall within plus or minus 15 points of the mean (so between 85 and 115).

So what does it mean to have a score 70 or below? In the past, an IQ score below 70 was considered a benchmark for mental retardation, an intellectual disability characterized by significant cognitive impairments.

Today, however, IQ scores alone are not used to diagnose intellectual disability. Instead, the criteria for a diagnosis includes an IQ below 70, evidence that these cognitive limitations existed before the age of 18 and limitations in two or more adaptive areas such as communication and self-help skills.

Approximately 2.2 percent of all people have an IQ score below 70.

Classifying Low IQ Scores

The average score on most IQ tests is 100, and approximately 68 percent of all people score somewhere between 85 and 115. For those who fall below 85, scores are often classified in the following way:

1 to 24 - Profound mental disability

25 to 39 - Severe mental disability

40 to 54 - Moderate mental disability

55 to 69 - Mild mental disability

70 to 84 - Borderline mental disability

A Brief History of How Low IQ Is Identified

IQ, or intelligence quotient, is a score derived from a standardized test designed to measure intelligence. IQ tests formally emerged in the early 1900s with the introduction of the Binet-Simon test, which was later revised and became known as the Stanford-Binet.

Binet developed his initial test to help the French government to identify students who with cognitive impairments who needed additional assistance in school.

IQ tests have proven to be very popular both within psychology and with the general public, but there remains a great deal of controversy about exactly what IQ tests measure and how accurate they are.

Older conceptions of low IQ tended to focus purely on cognitive abilities, but more modern approaches also stress how well an individual functions mentally and in areas of everyday life. Individuals with an IQ score below 70 may be diagnosed with an intellectual disability if they also experience deficits in two or more areas that influence daily living. Examples of such adaptive behaviors include the ability to take care of oneself and the ability to communicate and interact with other people.

The term "mental retardation" was previously used to describe individuals diagnosed with low IQ, but this term is now viewed as an insult and has been largely replaced by the term "intellectual developmental disorder" or "intellectual disability." The previous term is still used in some settings including some diagnostic codes and for insurance purposes.

Common Causes of Intellectual Disability

The most common causes of intellectual disability include:

  • Genetic conditions such as Down syndrome
  • Problems during pregnancy that impact brain development such as drug and alcohol use
  • Injuries such as head trauma and illnesses such as meningitis

Intellectual disability is the most common type of developmental disorder and is thought to affect one to three percent of the U.S. population.


Boeree, C. G. (2003). Intelligence and IQ. 

Maulik, P.K., & Harbour, C.K. (2010). Epidemiology of intellectual disability. In J.H. Stone & M. Blouin, eds., International Encyclopedia of Rehabilitation.