Intellectual disability (also called mental retardation) is often poorly understood because its effects vary greatly among those who have it. Many people with intellectual disability are mildly affected. They are only a little slower than average in learning new information and skills. As children, their intellectual disability is not readily apparent and may not be identified until they enter school. As adults, many are able to lead somewhat independent lives.

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.

What Is an Intellectual Disability?

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

  • IQ below 70
  • Significant limitations exist in two or more adaptive skill areas. These include:
    • Communication
    • Community use
    • Functional academics (reading, writing, basic math)
    • Health and safety
    • Home living
    • Leisure
    • Self-care
    • Self-direction
    • Social skills
    • Work
  • The condition 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. More than 500 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 three 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
  • Illnesses of a mother during pregnancy that can be passed on to her infant, such as:
  • Maternal diseases (eg, high fever, toxemia of pregnancy)
  • Overexposure to x-rays or radiation—Typical diagnostic x-rays are considered acceptable in pregnancy if they are necessary for the mother’s well-being.
  • Prescription medicines (eg, isotretinoin [Accutane], phenytoin [Dilantin])

Problems at Birth

Prematurity and low birth weight may sometimes lead to intellectual disability. These conditions may be associated with bleeding in or around the brain. However, other birth conditions or physical stress in the newborn stage may 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.

Poverty and Cultural Deprivation

Children in poor families may become mentally retarded because of:

  • Malnutrition
  • Disease-producing conditions
  • Inadequate medical care
  • Environmental health hazards
  • Lack of stimulation to help the brain grow

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 three categories:

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

What Are the Treatment Options?

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

  • Family counseling
  • Training in daily living skills
  • Special education
  • Job training
  • 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, such as:

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

  • Removing lead from the environment
  • Using child safety seats and bicycle helmets
  • Early intervention programs with high-risk infants and children
  • Early and comprehensive prenatal care
  • Abstaining from alcohol, tobacco, and drugs during pregnancy