Intellectual Disability

Intellectual Disability

Intellectual disability is often poorly understood because its effects vary greatly among those who have it. Many people with intellectual disability are mildly affected and may not be identified until later during school years. Individuals with intellectual disability may also have physical, medical, behavioral, psychological, or emotional problems.

People with intellectual disability who have a very low intelligence quotient (IQ) have serious limitations in their ability to function. However, with early intervention and appropriate support, they can also lead satisfying lives. The goal of treating intellectual disability is to help children stay in the family and take part in community life. In the United States, they are guaranteed education and other services.

What Is an Intellectual Disability?

Intellectual disability usually begins in childhood and is characterized by limitations in both intelligence and adaptive skills. The following 3 criteria must be met for a diagnosis of intellectual disability:

  • IQ below 75
  • Significant limitations exist in adaptive behaviors, such as self-care, socializing, and communicating
  • The disability begins before age 18

What Are the Causes?

Any condition that impairs development of the brain before birth, during birth, or during childhood can cause intellectual disability. The main causes can be categorized as follows:

Genetic Conditions

Genetic abnormalities may be inherited from parents or may be caused by environmental factors. There are many genetic diseases are associated with intellectual disability. Examples include:

  • Phenylketonuria (PKU)—Children born with this rare genetic disorder cannot metabolize phenylalanine (PHE), which is an amino acid found in food. Without proper treatment, PKU can lead to intellectual disability.
  • Down syndrome—In a normal fertilized egg, chromosomes exist in pairs. But, in the case of Down syndrome, there are 3 of chromosome 21.
  • Fragile X syndrome—This is caused by mutations of the FMR1 gene, the leading cause of inherited intellectual disability.

Problems During Pregnancy

Use of alcohol or drugs by a pregnant mother can cause intellectual disability. Smoking can increase the risk, as well. Other risks during pregnancy include:

  • Malnutrition
  • Certain environmental toxins such as lead
  • Illnesses of a mother during pregnancy that can be passed on to her infant, such as:
  • Preeclampsia
  • Prescription medications such as isotretinoin and phenytoin

Problems at Birth

Prematurity and low birth weight may sometimes lead to intellectual disability. Other birth complications and conditions or physical stress in the newborn stage may also injure an infant's brain.

Problems After Birth

Other conditions that can damage a child's brain and possibly lead to intellectual disability include:

In addition, poisoning from lead, mercury, carbon monoxide, and other environmental toxins can cause permanent damage to a child's brain and nervous system.

How Is an Intellectual Disability Diagnosed?

The American Association on Intellectual and Developmental Disabilities (AAIDD) has a process for diagnosing and classifying a person with intellectual disability. This process involves assessing the person's IQ and adaptive skills. Adaptive skills fall into 3 categories:

  • Conceptual skills—For example, can the person read and write? Does he or she understand concepts like time and money?
  • Social skills—Does the person follow rules? Does he or she have interpersonal skills? Can he or she solve social problems?
  • Practical skills—Can the person take care of his daily needs? Can he or she work, use money, and stay safe?

What Are the Treatment Options?

The best assistance for people with intellectual disability begins with diagnosis and help early in life. Treatment includes:

  • Early intervention
  • Special education
  • Family support
  • Case management
  • Job training
  • Transition services
  • Housing services

With enough education and support, many people with intellectual disability can learn to take care of their basic needs and to live in the community.

Can Intellectual Disabilities Be Prevented?

Newborn screening followed by proper treatment can prevent intellectual disability resulting from certain conditions. Examples include:

  • Phenylketonuria (PKU)
  • Congenital hypothyroidism

Vaccines can prevent certain infectious diseases that may lead to intellectual disability. Women who plan to become pregnant should be current on all recommended vaccinations, such as:

Other interventions that can reduce the risk of intellectual disability include:

  • Early and comprehensive prenatal care, including prenatal vitamins containing folate
  • Abstaining from alcohol, tobacco, and drugs during pregnancy

Many parents also choose to have certain tests done during pregnancy, including ultrasound, amniocentesis, chorionic villus sampling, and blood tests. These tests cannot prevent intellectual disability, but they can give parents more time to prepare for a child with intellectual disability.


American Association on Intellectual and Developmental Disabilities
Centers for Disease Control and Prevention



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Last reviewed July 2016 by Michael Woods, MD

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