DSM-5 Criteria for Identifying Intellectual Disabilities

Intellectual disability (ID) is a developmental condition that affects a person’s ability to think, learn, and perform everyday tasks. These challenges begin before age 18 and can range from mild to profound. Understanding intellectual disabilities helps clinicians, educators, and families recognize the condition early, provide appropriate interventions, and support individuals in leading meaningful, fulfilling lives.

What Is Intellectual Disability?

According to the Diagnostic and Statistical Manual of Mental Disorders, Fifth Edition (DSM-5), intellectual disability (ID) involves significant limitations in intellectual functioning, such as reasoning and problem-solving, and adaptive behavior, including communication and self-care. Individuals with ID may struggle with school, relationships, or daily routines. These deficits can impact an individual’s ability to reason, plan, solve problems, learn academically, think abstractly, make sound judgments, and manage everyday tasks ^[1].

The DSM-5 outlines three core criteria for diagnosing ID:

1. Onset must occur during the developmental period.
2. Another medical condition cannot fully explain deficits.
3. There must be measurable impairments in intellectual and adaptive functioning. 

 All three criteria must be met for a clinical diagnosis, guiding professionals in identifying and supporting individuals with intellectual disability.

Deficiencies in Intellectual Activities

Intellectual disability involves measurable impairments in intellectual functioning, typically identified through standardized intelligence tests and comprehensive clinical evaluation. An intelligence quotient (IQ) score below 70 is commonly used as a benchmark for significant intellectual impairment. However, diagnosis does not rely on IQ scores alone. Clinicians also consider cultural, social, and linguistic factors to ensure a fair and accurate assessment of an individual’s cognitive abilities ^[1].

Adaptive Functioning Deficit

In addition to intellectual impairments, individuals with intellectual disability experience deficits in adaptive functioning. This refers to how well a person manages everyday tasks and responsibilities, relative to age, community, and cultural expectations. The DSM-5 breaks adaptive functioning into three key domains:

• Conceptual (Academic): Skills related to reading, writing, math, time management, and self-direction.
• Social: The ability to communicate, understand social rules, form relationships, and interpret social cues.
• Practical: Competence in personal care, managing money, organizing tasks, and functioning at work or school.

When adaptive functioning is significantly impaired, individuals may struggle to live independently and meet the demands of daily life within their community or peer group. These challenges require lifelong support tailored to the individual’s needs and environment.

Severity Levels of Intellectual Disability

The DSM-5 categorizes intellectual disability into four severity levels: They are mild, moderate, severe, and profound, based on an individual’s level of adaptive functioning and need for support ^[1].

Mild Intellectual Disability

Individuals with mild intellectual disability may struggle with academic subjects such as reading, writing, and math. Although they often need money management and decision-making support, many can live independently and perform basic self-care tasks with minimal assistance. Social challenges may arise due to difficulties interpreting subtle cues or understanding complex social dynamics.

Moderate Intellectual Disability

Those with moderate intellectual disability generally reach an academic level equivalent to second grade. While they require ongoing support with daily living, they can acquire essential self-care abilities. They may experience difficulty understanding social norms and handling interpersonal challenges, leading to social isolation or conflict.

Severe Intellectual Disability

People with severe intellectual disability show minimal academic progress and may develop only basic skills like counting or letter recognition. Daily supervision is essential, particularly for personal care and communication, which is often limited to simple words or gestures.

Profound Intellectual Disability

Profound intellectual disability involves significant deficits in intellectual and adaptive functioning. Individuals with this level of disability require round-the-clock care for all daily needs. In many cases, they also live with physical disabilities or sensory impairments that further limit their independence and communication abilities.

Diagnosing Intellectual Disability

Diagnosing intellectual disability involves comprehensively evaluating intellectual functioning and adaptive behavior. This process goes beyond standardized testing—it requires clinical observation, interviews, and careful consideration of cultural, linguistic, and environmental contexts that may affect performance ^[2]. A team typically diagnoses professionals, including psychologists, physicians, and educational specialists.

A critical part of diagnosis includes assessing:

• Intellectual Functioning: Typically measured using standardized IQ tests. An IQ score below approximately 70–75 suggests a limited intellectual ability.
• Adaptive Functioning: Evaluated across three domains, conceptual, social, and practical, to determine how well an individual manages daily life compared to peers of the same age and cultural background.

Common Assessment Tools

• Wechsler Intelligence Scale for Children (WISC) and Wechsler Adult Intelligence Scale (WAIS): Widely used to assess intellectual functioning across age groups.
• Vineland Adaptive Behavior Scales: Measures personal and social skills needed for everyday living.
• Stanford-Binet Intelligence Scales: Another common IQ assessment used to evaluate cognitive abilities.

These assessments are combined with interviews, developmental history, and observations to ensure an accurate diagnosis, essential for creating an effective treatment or support plan tailored to the individual’s needs.

Differential Diagnosis

Several conditions can mimic the presentation of an intellectual disability, and it is essential to make differential diagnoses.

Global Developmental Delay (GDD)

This neurological disorder is characterized by slow psychomotor development, which leads to reduced activity and mental functioning.

GDD is a diagnosis for children under age five, whose developmental delay is across multiple domains, and who are too young for validated IQ testing. In some children with GDD, the child will later meet the criteria for ID; in others, the child may eventually ‘catch up’ developmentally ^[5].

Borderline Intellectual Functioning

People functionally marked as borderline intellectual functioning have IQ scores between 71 and 84, higher than those typical of ID but less than average. While they may struggle with some of the same issues that would qualify someone for ID in academic and occupational situations, they would not meet ID criteria from a clinical perspective ^[1].

Other Neurodevelopmental Disorders

ID can overlap with autism spectrum disorder (ASD), language disorders, and learning disabilities, but different symptom profiles identify the latter. For example, ASD is characterized by deficits in social communication and restricted behaviors, whereas ID involves global deficits in cognitive and adaptive domains ^[2].

An accurate diagnosis minimizes the risk of mislabeling or missing co-occurring conditions and provides clinicians with essential information to make interventions.

Common Comorbidities

Intellectual disability (ID) often occurs alongside other medical, genetic, and neurodevelopmental conditions, which can complicate the clinical picture but also provide valuable insight into its underlying causes.

One of the most frequently co-occurring conditions is Autism Spectrum Disorder (ASD). While social communication difficulties and repetitive behaviors characterize ASD, individuals with both ASD and ID often share overlapping challenges in social and adaptive functioning ^[1].

Attention-deficit/hyperactivity disorder (ADHD) is another common comorbidity. Difficulties with attention, impulse control, and executive functioning in individuals with ID can significantly interfere with learning and day-to-day skills ^[3].

Several genetic conditions are also strongly linked with ID, including Down syndrome, Fragile X, and Williams syndrome. Identifying a genetic cause informs treatment strategies and can help predict developmental outcomes.

Understanding and addressing these comorbidities is key to developing comprehensive, personalized care plans supporting the individual’s well-being ^[4].

Supporting Individuals with Intellectual Disability

The DSM-5 criteria for intellectual disability offer a clear and practical framework for identifying and understanding this complex neurodevelopmental condition. These guidelines enable accurate diagnosis and meaningful intervention by focusing on deficits in intellectual functioning and adaptive behavior, with an onset during the developmental period.

This approach helps clinicians tailor care to individual needs and equips educators, families, and support networks with the insight needed to advocate for and assist those affected. With early identification, compassionate support, and evidence-based care, individuals with intellectual disability can be empowered to reach their full potential and lead fulfilling lives.