Intellectual disability is a specific type of disability. It is caused by limited mental capacity (intelligence). These limitations make it difficult for someone to care for themselves without additional support. An estimated 1-3% of the population have intellectual disabilities.
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.
When people have limited mental abilities, it makes it difficult to acquire the skills needed for independent living. Without these skills, it is hard to live in a safe and socially responsible manner. Children with intellectual disabilities develop more slowly than other children. They usually sit, walk, and talk much later than other children. The delayed development means they do not act their age.
The Diagnostic and Statistical Manual of Mental Disorders of the American Psychiatric Association (APA, 2013) lists the three main criteria for intellectual disabilities:
1. Significant limitations in cognitive capacity (mental abilities); 2. Significant limitations in adaptive functioning (conceptual skills, social skills, and practical life skills); 3. The problems started 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, intellectual disabilities can be caused by several things. 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.
Intellectual disabilities have many causes. For this reason, no single set of characteristics can describe an intellectual disability. Nonetheless, it may be useful to discuss some of the most common signs.
Some medical conditions that cause intellectual disabilities are easy to identify at birth. Other causes of intellectual disabilities make them more difficult to spot early on. In these cases, the child's caregivers or teachers are usually the first notice something unusual about a child's development. When caregivers are concerned about their child's development they should talk with the child's doctor. The early identification of an intellectual disability helps both the child and the entire family.
Some of the most obvious signs are physical features of certain medical conditions that cause intellectual disabilities. These physical signs are so noticeable they are easily identified at birth. These include the unique facial features. For instance, fetal alcohol syndrome (FAS) is a leading cause of intellectual disabilities. The facial features of FAS include small eyes, small head, flattened face, and thin upper lip. However, physical features vary according to the cause of the disability. Some people with intellectual disabilities have a perfectly normal physical appearance.
Another early sign of an intellectual disability is developmental delay. Children with intellectual disabilities sit, walk and talk later than other children do. As children grow older, caregivers may notice functional problems. They may have difficulty with ordinary self-care. They might have trouble dressing or feeding themselves. Likewise, they may not understand and avoid dangerous situations.
Caregivers may also notice a slower development of social skills. Children with intellectual disabilities have trouble following social rules and customs. This includes commonly accepted practices such as taking turns or waiting in line.
Many times intellectual disabilities are not detected until a child starts school. Education challenges children to develop and expand their cognitive, social, and emotional skills. Children with intellectual disabilities cannot easily meet these challenges. It is hard for them to learn new information as quickly as other children do. They also have trouble retaining that information. It is not always easy to spot these mental limitations outside an academic setting.
More complete and detailed information can be found in our Intellectual Disabilities Topic Center.