Intellectual Disability and IQ: Mild, Moderate, Severe Ranges

Intellectual disability is generally indicated by an IQ of roughly 70 or below on a standardized intelligence test. The IQ score alone does not decide it, though: there must also be difficulties adapting to everyday and social life, and the condition must appear during the developmental period (roughly up to age 18). This article sets out the IQ ranges for mild, moderate, severe, and profound intellectual disability, how DSM-5 and ICD-11 treat severity, how Japan's disability certificate grades relate to IQ, and where to seek advice, all based on official sources.

What intellectual disability is: IQ, daily living skills, and onset in development

Japan's Ministry of Health, Labour and Welfare (MHLW) defines intellectual disability in its national survey as a state in which impairment of intellectual functioning appears during the developmental period (roughly up to age 18) and causes difficulties in daily life that require some form of special support. There are two criteria. The first is an IQ of roughly 70 or below on a standardized intelligence test such as a Wechsler or Binet scale. The second is a level of daily living ability (self-care, motor function, communication, mobility, work, and so on) that falls into one of four defined levels (a to d). A person who meets both is regarded as having an intellectual disability (MHLW, glossary of the Survey on Persons with Intellectual Disabilities).

International definitions rest on the same three elements. The American Association on Intellectual and Developmental Disabilities (AAIDD) describes intellectual disability as significant limitations in both intellectual functioning and adaptive behavior that originate before the age of 22. Adaptive behavior is divided into conceptual skills such as literacy and concepts of money and time, social skills such as interpersonal relationships, and practical skills such as personal care and work (AAIDD definition of intellectual disability).

An IQ of 70 sits exactly two standard deviations below the mean on a normal distribution with a mean of 100 and a standard deviation of 15. In theory, about 2.3% of people (roughly 1 in 44) fall below it. You can check where it sits on the curve in what an IQ of 70 means and the IQ distribution data.

IQ ranges for mild, moderate, severe, and profound intellectual disability

Severity is usually described in four levels: mild, moderate, severe, and profound. In the intelligence bands used in the MHLW survey, IQ 51 to 70 corresponds to mild, 36 to 50 to moderate, 21 to 35 to severe, and 20 or below to profound. International diagnostic systems and local certificate schemes draw the lines at slightly different points.

The key point is that the IQ band does not translate directly into the severity level. In the MHLW survey, severity is determined by combining the intelligence band with the level of daily living ability (a to d), and the survey notes that daily living ability takes priority in that judgment. For example, a person in the IQ-20-or-below band whose daily living ability is at level d is classified as severe, one step milder than profound.

DSM-5 and ICD-11 do not set severity by IQ alone

DSM-5, the American Psychiatric Association's diagnostic manual, bases the severity of intellectual disability on adaptive functioning rather than on IQ scores. Adaptive functioning is assessed across three domains: conceptual, social, and practical. IQ remains in the text as a guide (approximately two standard deviations or more below the population mean, or an IQ of about 70 or below), but it was removed from the diagnostic criteria themselves, so that IQ is not overemphasized at the expense of how a person actually functions (American Psychiatric Association, DSM-5 Intellectual Disability Fact Sheet).

ICD-11, the World Health Organization's classification, requires that both intellectual functioning and adaptive behavior be approximately two or more standard deviations below the mean (roughly below the 2.3rd percentile). It defines mild as two to three standard deviations below the mean, moderate as three to four, and severe and profound as four or more. Severe and profound are distinguished only by differences in adaptive behavior, because existing intelligence tests cannot reliably tell people apart at that level (WHO, ICD-11, 6A00 Disorders of intellectual development). On a test with a standard deviation of 15, that corresponds to roughly IQ 55 to 70 for mild, 40 to 55 for moderate, and below 40 for severe and profound (our conversion).

An IQ of 70 or below does not automatically mean intellectual disability

An IQ below 70 is not, on its own, enough for a judgment of intellectual disability. Keep the following in mind when reading the number.

• Scores have measurement error: IQ varies by a few points from one testing to the next. AAIDD gives the guide for a significant limitation as an IQ of around 70 or as high as 75, so the line is not drawn mechanically at exactly 70
• The number depends on the test: the same person can score differently on the Tanaka-Binet Intelligence Scale and on Wechsler tests (how to read WISC results, what the WAIS is)
• It is judged together with adaptive behavior: a person with an IQ of 70 or below who has no difficulty in daily life is not classified as having an intellectual disability
• IQ 70 to 84 is not intellectual disability: this range is called borderline intellectual functioning and is not a diagnosis

Developmental disorders are also easily confused with intellectual disability. Autism spectrum disorder (ASD) and ADHD are separate conditions that may or may not occur together with intellectual disability. See autism and IQ and ADHD and IQ for details.

How common is intellectual disability? About 1% worldwide

A meta-analysis of 52 studies put the prevalence of intellectual disability at 10.37 per 1,000 people, or about 1% (Maulik and colleagues, 2011, Research in Developmental Disabilities). The American Psychiatric Association states that about 85% of people with intellectual disability have a mild form (American Psychiatric Association, What is Intellectual Disability?).

In Japan, the number of people with intellectual disabilities (including children) is estimated at about 1,268,000, or 10 per 1,000 people (Cabinet Office, 2026 White Paper on Persons with Disabilities, reference data). The number of ryoiku techo (intellectual disability certificate) holders was 1,321,350 at the end of fiscal 2024 (MHLW, Overview of the ryoiku techo system).

On a normal curve, about 2.3% of people would have an IQ below 70, yet only about 1% are identified as having an intellectual disability. Likely reasons for the gap include the additional requirement of difficulties in daily life and the fact that some people with a mild disability have never been tested or assessed.

Japan's ryoiku techo grades and IQ: criteria differ by municipality

The ryoiku techo is a certificate issued to people judged to have an intellectual disability by a child guidance center or an intellectual disability rehabilitation consultation office, and the criteria are set by each local government. The national guideline defines only two categories, severe (A) and other (B). Severe (A) covers people with an IQ of roughly 35 or below who need assistance with daily activities such as eating and dressing or who show behavioral problems such as pica or agitation, and people with an IQ of roughly 50 or below who also have blindness, deafness, or a physical disability (MHLW, Overview of the ryoiku techo system).

Many local governments subdivide this further. Tokyo's certificate, called Ai no Techo, has four grades from grade 1 (profound) to grade 4 (mild), with IQ guides of roughly 19 or below for grade 1, 20 to 34 for grade 2, 35 to 49 for grade 3, and 50 to 75 for grade 4. The assessment is made comprehensively from daily functioning as well as the test score (Tokyo Metropolitan Government, Ai no Techo Q&A). The certificate's name, the grading, and the upper IQ limit vary by municipality, so check with your local office.

Where to seek advice and get tested

Finding out whether someone has an intellectual disability requires a standardized intelligence test administered one-to-one by a professional, together with an interview about daily functioning. The WISC is used for children, the WAIS for ages 16 and up, and the Tanaka-Binet across a wide age range.

• Under 18: a child guidance center, the municipal office for children or disability welfare, a pediatrician or developmental clinic
• 18 and over: an intellectual disability rehabilitation consultation office, the municipal disability welfare office, a psychiatrist
• To apply for a ryoiku techo: start with the municipal disability welfare office

What an online IQ test can and cannot tell you

Online IQ tests, including BrainRank's, cannot diagnose or assess intellectual disability. If you have concerns, please contact one of the services above or a medical institution. If your aim is simply a guide to your own reasoning strengths and weaknesses, BrainRank's free IQ test scores 20 questions (about 10 minutes) across four domains, spatial reasoning, pattern recognition, logical reasoning, and classification, using item response theory (IRT), and returns an estimated IQ on a mean-100, SD-15 scale. Taking it is free with no signup; viewing the results, including the estimated IQ, is a one-time payment of ¥300 (tax included), with no subscription or auto-renewal (the result can be reopened for 24 hours after it is first shown, and saved as an image or PDF during that window). Results are estimates for entertainment and self-understanding, not a medical or diagnostic assessment.