Children and adults with intellectual disabilities inspire us every day.

What Is an Intellectual Disability?

Intellectual disability (or ID) is a term used when a person has certain limitations in cognitive functioning and skills, including communication, social and self-care skills. These limitations can cause a child to develop and learn more slowly or differently than a typically developing child. Intellectual disability can happen any time before a child turns 18 years old, even before birth.

Intellectual disability is the most common developmental disability.

According to the American Association of Intellectual and Developmental Disabilities, an individual has intellectual disability if he or she meets three criteria:

  1. IQ is below 70-75
  2. There are significant limitations in two or more adaptive areas (skills that are needed to live, work, and play in the community, such as communication or self-care)
  3. The condition manifests itself before the age of 18

How Does an Intellectual Disability Happen?

Intellectual disability—formerly known as mental retardation—can be caused by injury, disease, or a problem in the brain. For many children, the cause of their intellectual disability is unknown.

Some causes of intellectual disability—such as Down syndrome, Fetal Alcohol Syndrome, Fragile X syndrome, birth defects, and infections—can happen before birth. Some happen while a baby is being born or soon after birth.

Other causes of intellectual disability do not occur until a child is older; these might include severe head injury, infections or stroke.

What Are the Most Common Causes?

The most common causes of intellectual disabilities are:

Genetic conditions. Sometimes an intellectual disability is caused by abnormal genes inherited from parents, errors when genes combine, or other reasons. Examples of genetic conditions are Down syndrome, Fragile X syndrome, and phenylketonuria (PKU).

Complications during pregnancy. An intellectual disability can result when the baby does not develop inside the mother properly. For example, there may be a problem with the way the baby’s cells divide. A woman who drinks alcohol or gets an infection like rubella during pregnancy may also have a baby with an intellectual disability.

Problems during birth. If there are complications during labor and birth, such as a baby not getting enough oxygen, he or she may have an intellectual disability.

Diseases or toxic exposure. Diseases like whooping cough, the measles, or meningitis can cause intellectual disabilities. They can also be caused by extreme malnutrition, not getting appropriate medical care, or by being exposed to poisons like lead or mercury.

We know that intellectual disability is not contagious: you can’t catch an intellectual disability from anyone else. We also know it’s not a type of mental illness, like depression. There are no cures for intellectual disability. However, children with intellectual disabilities can learn to do many things. They may just need take more time or learn differently than other children.

How Common Are Intellectual Disabilities?

Approximately 6.5 million people in the United States have an intellectual disability. Approximately 1 – 3 percent of the global population has an intellectual disability—as many as 200 million people.

Intellectual disability is significantly more common in low-income countries—16.41 in every 1,000 people. Disabilities overall are more common in low-income countries.

The United Nations Development Program estimates that 80 percent of all people with disabilities live in low-income countries. While people with disabilities represent approximately one in 10 people worldwide, they are one in every five of the world’s poorest people.

Sources: American Association of Intellectual and Developmental Disabilities; National Center on Birth Defects and Developmental Disabilities; the United Nations Development Program; and the Centers for Disease Control and Prevention.

  • Down Syndrome

    What Is Down Syndrome?

    Down syndrome is the term for a set of cognitive and physical symptoms that can result from having an extra copy or part of a copy of chromosome 21.

    Down syndrome is the most frequent chromosomal cause of mild to moderate intellectual disability, and it occurs in all ethnic and economic groups.

    The degree of intellectual disability in people with Down syndrome varies but is usually mild to moderate. Generally, children with Down syndrome reach key developmental milestones later than other children.

    According to the Centers for Disease Control and Prevention, approximately 6,000 babies are born in the United States each year with Down syndrome, or approximately 1 out of every 691 live births.

    In recent decades, thanks to appropriate support and treatment, life expectancy for people with Down syndrome has increased dramatically: from 25 years in 1983 to more than 60 years today.

    What Causes Down Syndrome?

    Down syndrome is caused by a random error in cell division that results in the presence of an extra copy of chromosome 21.

    Usually, when one cell divides in two, all pairs of chromosomes are split so that one of the pair goes to one cell, and the other goes to the other cell. But in rare cases, both chromosomes from a pair go together into one cell.

    Most of the time, the error occurs at random during the formation of an egg or sperm. No action by parents or environmental factor is known to cause Down syndrome.

    After much research on these cell division errors, researchers know that:

    • In more than 90 percent of cases, the extra copy of chromosome 21 comes from the mother in the egg.
    • In about 4 percent of cases, the father provides the extra copy of chromosome 21 through the sperm.
    • In the remaining cases, the error occurs after fertilization, as the embryo grows.

    Researchers also know some, but not all, of the risk factors for Down syndrome. For example, parents who have a child with Down syndrome or another chromosomal disorder, or who have a chromosomal disorder themselves, are more likely to have a child with Down syndrome.

    In addition, the likelihood that an egg will contain an extra copy of chromosome 21 increases significantly as a woman ages. Therefore, older women are much more likely than younger women to give birth to an infant with Down syndrome.

    Still, about 60 percent of babies with Down syndrome are born to women under age 35.

    What Are Some Symptoms Related to Down Syndrome?

    Symptoms of Down syndrome vary from person to person. Common physical symptoms include:

    • Decreased or poor muscle tone
    • Short neck, with excess skin at the back of the neck
    • Flattened facial profile and nose
    • Small head, ears and mouth
    • Upward slanting eyes, often with a skin fold that comes out from the upper eyelid and covers the inner corner of the eye
    • Single crease across the palm of the hand
    • Deep groove between the first and second toes

    Physical development in children with Down syndrome is often slower than development of children without Down syndrome. For example, because of poor muscle tone, a child with Down syndrome may be slow to learn to turn over, sit, stand, and walk.

    Despite these delays, children with Down syndrome can learn to participate in physical exercise and similar activities like other children. It may take children with Down syndrome longer than other children to reach developmental milestones, but they will eventually meet all or many of them.

    Intellectual and Developmental Symptoms

    Cognitive impairment—which means problems with thinking and learning—is common in people with Down syndrome and usually ranges from mild to moderate. Down syndrome is only rarely associated with severe cognitive impairment.

    Common cognitive and behavioral problems may include:

    • Short attention span
    • Poor judgment
    • Impulsive behavior
    • Slow learning
    • Delayed language and speech development

    In addition, people with Down syndrome are at increased risk for a range of other health conditions, including Autism Spectrum Disorders, problems with hormones and glands, hearing loss, vision problems, and heart abnormalities.

    Sources: The Eunice Kennedy Shriver National Institute of Child Health and Human Development/National Institutes of Health, and the Centers for Disease Control and Prevention, and Special Olympics.

  • Fragile X Syndrome

    What Is Fragile X Syndrome?

    Fragile X syndrome is a genetic disorder that affects a person’s development, especially that person’s behavior and ability to learn. In addition, Fragile X can affect communication skills, physical appearance, and sensitivity to noise, light, or similar information.

    Fragile X syndrome is the most common form of inherited intellectual and developmental disability.

    People with Fragile X syndrome may not have noticeable symptoms, or they can have serious symptoms that range from learning disabilities to cognitive and behavior problems.

    What Causes Fragile X Syndrome?

    Fragile X results from a change or mutation in the Fragile X Mental Retardation 1 gene, which is found on the X chromosome. The gene normally makes a protein called Fragile X Mental Retardation Protein, or FMRP. This protein is important for creating and maintaining connections between cells in the brain and nervous system. The mutation causes the body to make only a little bit or none of the protein, which often causes the symptoms of Fragile X.

    Not everyone with the mutated FMR1 gene has symptoms of Fragile X syndrome, because the body may still be able to make FMRP. Severity of symptoms will depend on how much of the protein the body can make and the number of cells affected.

    Please note that females—due to having two X chromosomes—can usually make at least some FMRP. Therefore symptoms of Fragile X are often milder in females than in males.

    About 1 in 4,000 males and 1 in 8,000 females have Fragile X syndrome.

    What Are the Symptoms of Fragile X?

    People with Fragile X don’t all have the same signs and symptoms, but they do have some things in common:

    Cognitive functioning. Many people with Fragile X have problems with intellectual functioning. These problems can range from the mild, such as learning disorders or problems with mathematics, to the severe, such as an intellectual or developmental disability. The syndrome may affect the ability to think, reason, and learn.

    NOTE: Because many people with Fragile X also have attention disorders, hyperactivity, anxiety, and language-processing problems, a person with Fragile X may have more capabilities than his or her IQ score suggests.

    Physical features. Most infants and young children with Fragile X don’t have specific features connected to the syndrome. But when they start to go through puberty, many will begin to develop physical features that are typical of those with Fragile X. These include: a narrow face, large head, large ears, flexible joints, flat feet, and a prominent forehead.

    Behavioral, social, and emotional. Most children with Fragile X have some behavioral challenges. They may be afraid or anxious in new situations.

    They may have trouble making eye contact with other people. Boys, especially, may have trouble paying attention or be aggressive. Girls may be shy around new people. They may also have attention disorders and problems with hyperactivity.

    Speech and language. Most boys with Fragile X have some problems with speech and language. They may have trouble speaking clearly, may stutter, or may leave out parts of words. They may also have problems understanding other people’s social cues, such as tone of voice or specific types of body language. Girls usually do not have severe problems with speech or language.

    Some children with Fragile X begin talking later than typically developing children. Most will talk eventually, but a few might stay nonverbal throughout their lives.

    Sensory. Many children with Fragile X are bothered by certain sensations, such as bright light, loud noises, or the way certain clothing feels on their bodies. These sensory issues might cause them to act out or display behavior problems.

    NOTE: Between 6 percent and 20 percent of children with Fragile X syndrome have seizures. Seizures associated with the syndrome are more common in boys than in girls.

    Sources: The Eunice Kennedy Shriver National Institute of Child Health and Human Development/National Institutes of Health, and the Centers for Disease Control and Prevention, and Special Olympics.
  • Autism

    What Is Autism?

    Autism Spectrum Disorder (ASD), or autism, is a complex neurological and developmental disorder that affects how a person acts, communicates, learns, and interacts with others.

    ASD affects the structure and function of the brain and nervous system. Because it affects a child’s development, ASD is called a developmental disorder. ASD can last throughout a person’s life. People with this disorder have problems with communication, interactions with other people (social skills), restricted interests and repetitive behaviors.

    Different people with autism can have different symptoms. For this reason, autism is known as a spectrum disorder—a group of disorders with a range of similar features. ASD includes:

    • Autistic disorder (“classic” autism). Autistic disorder is often what people think of when they think of autism.
    • Asperger syndrome. Asperger syndrome is sometimes said to be a milder version of classic autism, mostly affecting social behaviors. Unlike people with autism, many people with Asperger syndrome have normal or above-average intelligence and language skills.
    • Pervasive developmental disorder not otherwise specified (PDD-NOS, or “atypical” autism). PDD-NOS includes some, but not all, of the features of classic autism and/or Asperger syndrome. This category also includes childhood disintegrative disorder and Rett syndrome, two conditions in which a child develops normally for several months or years, then loses skills related to language, movement and coordination, and other cognitive functions.

    What Are the Symptoms?

    The symptoms of one person with autism can be very different from another’s. One person with autism may have mild symptoms, while another may have more serious symptoms, but they both have ASD.

    Despite the range of possible symptoms, there are certain actions and behaviors that are common. In general, the main signs and symptoms of ASD relate to: communication; social behaviors; and routines or repetitive behaviors, sometimes called stereotyped behaviors.


    • Does not respond to his/her name by 12 months of age
    • Cannot explain what he/she wants
    • Language skills are slow to develop or speech is delayed
    • Doesn’t follow directions
    • Seems to hear sometimes, but not other times
    • Doesn’t point or wave “bye-bye”
    • Used to say a few words or babble, but now does not

    Social Behavior:

    • Doesn’t smile when smiled at
    • Has poor eye contact
    • Seems to prefer to play alone
    • Is very independent for his/her age
    • Seems to tune people out

    Steroeotyped Behavior:

    • Gets “stuck” doing the same things over and over and can’t move on to other things
    • Shows deep attachment to toys, objects, or routines
    • Spends a lot of time lining things up or putting things in a certain order
    • Repeats words or phrases

    Other Behaviors:

    • Has unusual movement patterns
    • Doesn’t know how to play with toys
    • Does things “early” compared to other children
    • Walks on his/her toes
    • Throws intense or violent tantrums
    • Is overly active, uncooperative, or resistant
    • Seems overly sensitive to noise

    How Common Is Autism?

    Researchers don’t know exactly how many people in the United States have ASD.

    The latest estimates suggest that about one out of every 68 children—or 1.5% of children—in the United States has autism. About 36,500 of every 4 million children born each year in the U.S. will have autism.

    ASD occurs in all racial, ethnic and socioeconomic groups. Studies in Europe, North America and Asia show an average prevalence of people with autism between 1% and 2% of the population.

    ASD is about 4.5 times more common among boys than among girls.

    Communicating Without Words

    For athletes with autism, the social experience at Special Olympics events can be both rewarding and empowering. Not only do our athletes learn new skills (and get to show them off), these athletes compete in a “safe” environment where they can interact with other people in a non-pressured, relaxing way.

    Special Olympics volunteers and coaches are experienced working with young people and adults who may have trouble communicating or who have sensory issues. We will find a way—and a sport—that is best suited to each athlete.

    Sources: The Eunice Kennedy Shriver National Institute of Child Health and Human Development/National Institutes of Health, and the Centers for Disease Control and Prevention, and Special Olympics.
  • Other Intellectual Disabilities

    Apert Syndrome

    Apert syndrome is a genetic disease in which the seams between the skull bones close earlier than normal. This affects the shape of the head and face.

    Apert syndrome can be passed down through families (inherited). The syndrome is inherited as an autosomal dominant trait, which means that only one parent needs to pass on the faulty gene for a child to have the condition.

    Some cases may occur without a known family history.

    Apert syndrome is caused by mutations in a gene called fibroblast growth factor receptor 2. This gene defect causes some of the bony sutures of the skull to close too early, a condition called craniosynostosis.

    Symptoms include fusion or severe webbing of the 2nd, 3rd, and 4th fingers; webbing or fusion of toes; hearing loss; slow intellectual development; prominent eyes; under-development of the mid-face; skeletal (limb) abnormalities; and small stature.

    Williams Syndrome

    Williams syndrome is a developmental disorder that affects many parts of the body. This condition is characterized by mild to moderate intellectual disability or learning problems, certain personality characteristics, distinctive facial features, and heart and blood vessel (cardiovascular) problems.

    Williams syndrome is caused by the deletion of genetic material from a specific region of chromosome 7. The deleted region includes more than 25 genes.

    People with Williams syndrome typically have difficulty with visual-spatial tasks such as drawing and assembling puzzles, but they tend to do well on tasks that involve spoken language, music, and learning by repetition (rote memorization). Affected individuals have outgoing, engaging personalities and tend to take an exceptional interest in other people. Attention deficit disorder (ADD), problems with anxiety, and phobias are common among people with this disorder.

    Young children with Williams syndrome have distinctive facial features including a broad forehead, a short nose with a broad tip, full cheeks, and a wide mouth with full lips. Many affected people have dental problems such as small, widely spaced teeth and teeth that are crooked or missing. In older children and adults, the face appears longer and more gaunt.

    A form of cardiovascular disease called supravalvular aortic stenosis (SVAS) occurs frequently in people with Williams syndrome. SVAS is a narrowing of the large blood vessel (the aorta) that carries blood from the heart to the rest of the body. If this condition is not treated, the aortic narrowing can lead to shortness of breath, chest pain, and heart failure. Other problems with the heart and blood vessels, including high blood pressure (hypertension), have also been reported in people with Williams syndrome.

    Additional signs and symptoms of Williams syndrome include abnormalities of connective tissue (tissue that supports the body’s joints and organs) such as joint problems and soft, loose skin. Affected people may also have increased calcium levels in the blood (hypercalcemia) in infancy, developmental delays, problems with coordination, and short stature. Medical problems involving the eyes and vision, the digestive tract, and the urinary system are also possible.

    Fetal Alcohol Syndrome

    Fetal alcohol spectrum disorders (FASDs) are a group of conditions that can occur in a person whose mother drank alcohol during pregnancy. These effects can include physical problems; difficulty with behavior and learning; or a mixture of all three.

    Signs of FASDs can be physical or intellectual. That means they can affect the mind or the body, or both. Because FASDs make up a group of disorders, people with FASDs can show a wide range and mix of signs.

    Physical signs of FASDs can include abnormal facial features such as narrow eye openings and a smooth philtrum (the ridge between the upper lip and nose), small head size, short stature, and low body weight. In rare cases, there are problems with the heart, kidneys, bones, or hearing.

    Intellectual and behavioral signs of FASDs might include problems with memory, judgment or impulse control, motor skills, academics (especially in math), paying attention, and low IQ. Specific learning disabilities are also possible.


    Prader-Willi syndrome is the most common of the genetic disorders that cause life-threatening obesity in children. The syndrome affects many aspects of a person’s life including eating, behavior and mood, physical growth, and intellectual development.

    The syndrome usually results from deletions or partial deletions on chromosome 15 that affect the regulation of gene expression, or how genes turn on and off. Andrea Prader and Heinrich Willi first described the syndrome in the 1950s.

    One of the main symptoms of Prader-Willi syndrome is the inability to control eating. Other symptoms include low muscle tone and poor feeding as an infant, delays in intellectual development, and difficulty controlling emotions.

    Individuals with Prader-Willi syndrome have varying levels of intellectual disabilities. Learning disabilities are common, as are delays in starting to talk and in the development of language.

    Phenylketonuria (PKU)

    Phenylketonuria (pronounced fen-l-kee-toh-NOOR-ee-uh), or PKU, is an inherited disorder that that can cause intellectual and developmental disabilities (IDDs) if untreated. In PKU, the body can’t process part of a protein called phenylalanine, which exists in all foods containing protein. If the phenylalanine level gets too high, the brain can become damaged.

    All children born in U.S. hospitals are tested routinely for PKU soon after birth, making it easier to diagnose and treat early. Children and adults who are treated early and consistently develop normally. Depending on the level of phenylalanine and tolerance for phenylalanine in the diet, PKU is classified into two different types: classic (which is the severe form) and moderate.

    Children with untreated PKU appear normal at birth. But by age 3 to 6 months, they begin to lose interest in their surroundings. By age 1 year, children are developmentally delayed and their skin has less pigmentation than someone without the condition. If people with PKU do not restrict the phenylalanine in their diet, they develop severe intellectual and developmental disabilities.

    Other symptoms include: behavioral or social problems; seizures, shaking, or jerking movements in the arms and legs; stunted or slow growth; skin rashes, such as eczema; small head size (microcephaly); musty odor in urine, breath, or skin that is a result of the extra phenylalanine in the body; and fair skin and blue eyes, due to the body’s failure to transform phenylalanine into melanin.

    Cerebral Palsy

    The term cerebral palsy refers to any one of a number of neurological disorders that appear in infancy or early childhood which permanently affect body movement and muscle coordination, but don’t worsen over time. Even though cerebral palsy affects muscle movement, it isn’t caused by problems in the muscles or nerves. Rather, it is caused by abnormalities in parts of the brain that control muscle movements.

    The majority of children with cerebral palsy are born with it, although it may not be detected until months or years later. The early signs of cerebral palsy usually appear before a child reaches 3 years of age. The most common are a lack of muscle coordination when performing voluntary movements (ataxia); stiff or tight muscles and exaggerated reflexes (spasticity); walking with one foot or leg dragging; walking on the toes, a crouched gait, or a “scissored” gait; and muscle tone that is either too stiff or too floppy.

    A small number of children have cerebral palsy as the result of brain damage in the first few months or years of life; brain infections such as bacterial meningitis or viral encephalitis; or head injury from a motor vehicle accident, a fall, or child abuse.

    People with CP have problems with movement and posture. Many also have related conditions such as intellectual disability; seizures; problems with vision , hearing, or speech; changes in the spine (such as scoliosis); or joint problems (such as contractures).

    Sources: The Eunice Kennedy Shriver National Institute of Child Health and Human Development and the National Institutes of Health, the American Association of Intellectual and Developmental Disabilities, and the Centers for Disease Control and Prevention, and Special Olympics.