Intellectual disability (also known as Intellectual Developmental Disorder) is a neurodevelopmental disorder that is characterized by deficits in cognitive and adaptive functioning. It is classified as a neurodevelopmental disorder because its onset occurs during the developmental period. In other words, this is a disorder that occurs during early development, as opposed to a disorder that is acquired due to an event or condition outside of normal development.
Cognitive deficits refer to deficits in intellectual functioning such as reasoning, problem solving, planning, abstract thinking, judgment, learning, and practical understanding. These intellectual functions are typically measured by intelligence tests. Common domains of intellectual functioning measured in these tests are verbal comprehension, working memory, perceptual reasoning, processing speed, and abstract reasoning. Moreover, an IQ score is generated based on the individual’s performance in these domains. The DSM-5 does not draw a hard line in the sand when it comes to determining which IQ score satisfies its cognitive deficit criteria. However, the DSM-5 provides a clinician with guidance regarding the clinical decision-making process used to determine if an individual has cognitive deficits. For example, it is common for many clinicians to set the IQ score cutoff point at 70 or 75 (meaning that the IQ score would have to be the same or lower than the cutoff point score).
In addition, the individual must have deficits in adaptive functioning. Deficits in adaptive functioning refer to how well a person meets cultural and age-related standards of independence and social responsibility. Adaptive functioning involves adaptive reasoning in the following domains: practical, conceptual, and social. The conceptual domain involves memory, language, reading, writing, math reasoning, acquisition of practical knowledge, problem solving, and judgment in novel situations. The social domain involves awareness of others’ thoughts, feelings, and experiences; empathy, communication skills, ability to make and maintain friendships, and social judgment. The practical domain involves learning and self-management across life settings, including personal care, job responsibilities, managing money, leisure activities, the ability to manage one’s behavior, and task organization. Adaptive functioning can be measured via a questionnaire given to a caregiver that inquires about an individual’s abilities in the above-mentioned domains; in addition, there are some available tests that actively measure an individual’s level of adaptive functioning via practical questions that require adaptive knowledge and tasks that the individual performs to demonstrate adaptive capability.
There are many conditions that can present with low IQ or impaired adaptive functioning (other than intellectual disability). Thus, I would recommend that an individual obtain a comprehensive evaluation to determine the causes of these deficits. This will help to ensure diagnostic accuracy, as well as to create an accurate and effective treatment plan.