How Low IQ Scores Are Determined

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

While you might often hear a lot of talk about high and low intelligence quotient (IQ) scores, many people aren't quite sure what these designations really mean. IQ is short for "intelligence quotient," which is a measurement of human intelligence and mental abilities derived from standardized tests.

IQ tests include questions that are designed to measure a range of different mental abilities and intellectual potential. These questions focus on things such as spatial recognition, analytical thinking, short-term memory, and mathematical ability. These tests are not meant to measure what a person has learned, but rather what they are capable of doing. 

Low IQ scores may be associated with deficits in mental abilities that may impact life areas including school performance and job performance. Low Iq may also affect an individual's ability to live and function independently.

What Is a Low IQ Score?

An IQ score of 70 or below is considered a low score. On most standardized tests of intelligence, the average score is set at 100. Anything over 140 is considered high or genius-level. Approximately 68% of all people score somewhere between 85 and 115, the range within 15 points of the average.

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 diagnosis include:

  • An IQ of 70 or below
  • Evidence that these cognitive limitations existed before the age of 18
  • Serious limitations in areas such as learning and reasoning
  • Severe limitations in adaptive areas such as communication and self-help skills.

Intellectual disability is the most common type of developmental disorder and affects approximately 0.05% to 1.55% of all people.

Low IQ Score Classifications
IQ Range Descriptive Classification
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 79 Borderline mental disability
80 to 89 Low average
Scores below 90.

History of Low IQ

IQ 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 identify students 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. However, 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 impairment in one adaptive domain. 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 term "mental retardation" 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
  • Injuries such as head trauma and illnesses such as meningitis and seizure disorders
  • Labor and delivery problems, such as not getting enough oxygen at birth
  • Problems during pregnancy that impact brain development such as drug and alcohol use

Effects of Low IQ

Low cognitive ability can present a range of challenges in many different areas of life. Research suggests that low IQ is associated with an increased risk of unsuccessful educational and occupational achievement. Because of this association, it is important to identify potential problems as soon as possible in order to provide effective interventions and assistance.

IQ testing is sometimes used as part of the job screening process. For example, the U.S. military has enlistment standards stipulating that applicants must score at or above the tenth percentile on the Armed Services Vocational Aptitude Battery (ASVAB). Such scores are roughly equivalent to an IQ score of 85. 

IQ scores below 70 may indicate the presence of some type of intellectual disability and may be accompanied by difficulties in functioning including learning, self-care, and independent living.

IQ testing is often offered in educational, healthcare, and psychological settings, often to diagnose intellectual disability in children. IQ tests are also available online, but many of these are informal assessments and should not be used for diagnosis purposes.

According to the American Academy of Pediatrics, around 85% of kids with an intellectual disability score between 55 and 70.

A Word From Verywell

IQ scores are just one way of measuring mental abilities. While a doctor or psychiatrist will consider IQ scores when diagnosing intellectual disability, they will also look at other factors including adaptive skills and overall functioning. While knowing your IQ score may help you better understand your needs, IQ alone does not determine success in life.

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Verywell Mind uses only high-quality sources, including peer-reviewed studies, to support the facts within our articles. Read our editorial process to learn more about how we fact-check and keep our content accurate, reliable, and trustworthy.
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Additional Reading

By Kendra Cherry
Kendra Cherry, MS, is an author and educational consultant focused on helping students learn about psychology.