The term "intellectual disability" is a specific type of disability. It is caused by limited mental capacity. Limited mental capacity makes it difficult to develop important mental abilities. This includes reasoning, planning, thinking, and judgment. This limited mental capacity makes it difficult to learn new things. The ability to learn is a very important mental ability. We learn new information and skills in school. We learn from our past mistakes. We learn how to do many things by watching others. When this ability to learn is lacking, it causes many problems in everyday life.
The Diagnostic and Statistical Manual of Mental Disorders of the American Psychiatric Association (APA, 2013) lists three main criteria for intellectual disabilities:
1. Significant limitations in intellectual functioning (mental abilities);
2. Significant limitations in adaptive functioning (conceptual skills, social skills, and practical life skills);
3. The problems begin before age 18.
Although the APA's diagnostic manual is called the Diagnostic and Statistical Manual of Mental Disorders (emphasis added), intellectual disabilities are not disorders in and of themselves. Instead, there are several causes of intellectual disabilities. For example, there are many genetic causes. Brain injuries can cause an intellectual disability. Some types of medical conditions can also affect the brain's development. Because intellectual disabilities are not disorders, there are no treatments. Instead, people are provided additional supports. These supports help people to have a satisfying life despite their disability.