What Is Intellectual Developmental Disorder?

When a parent finds out their child might have intellectual developmental disorder — also known as intellectual disability, intellectual and developmental disability, or IDD — they might have more questions than answers. IDD is a catch-all term for a range of different conditions (including what years ago was called mental retardation, a designation that is no longer used), and families may need to wait to see how children change as they grow. But they can still help their child get the support they need to learn and develop new skills.

Intellectual developmental disorder involves limitations to both cognitive functioning and adaptive skills that start in childhood. Some people are born with it — for example, kids with Down syndrome or fetal alcohol syndrome, or some kids with autism. Others develop IDD when they are older, from a brain injury or an illness such as epilepsy or meningitis.

Cognitive functioning is what people usually mean when they talk about intelligence — the capacity to think and learn. It’s what’s measured by an IQ test. Adaptive skills, which measure a child’s ability to function in daily life, are broken down into three categories:

• Conceptual: These include things like language, time and number concepts, and executive functions.
• Social: These involve interactions with other people, including reading social cues and being able to act appropriately.
• Practical: These involve the practical tasks of daily living, like getting dressed, bathing, or using money.

While all children with IDD have below-average cognitive functioning, when it comes to adaptive skills, some kids have trouble in one or two of these areas, while others have delays across all three.

Children with IDD may have mild, moderate, severe, or profound disability. As they grow up, those with mild disability can live independently with minimal support. Those with moderate disability can live independently with moderate levels of support — for example, they may be able to go to familiar locations in their community but need help to go to a new destination. Those with severe IDD need assistance with daily life, and those with profound IDD require 24-hour care.

According to the CDC, about 2 percent of U.S. children ages 3–17 have been diagnosed with intellectual disability, and approximately 85 percent of those diagnosed with IDD have mild impairments.

A psychiatrist or clinical psychologist can give a child a battery of tests to assess them in the two key areas: cognitive abilities and adaptive skills.

To test cognitive ability, clinicians administer an intelligence test appropriate for the child’s age to measure their ability to problem-solve both with and without language. That results in an IQ score, which clinicians use as one part of completing an evaluation for possible intellectual disability. In the past, a score of 70 or below indicated an IDD diagnosis, but currently there is no specific cutoff score. Instead, diagnosis is at the discretion of the clinician.

“If a child is 10 years old but has the cognitive functioning of a 6-year-old, we would still be able to get an accurate measure of their IQ,” says Jenna Lebersfeld, PhD, a pediatric neuropsychologist who works with the Special Olympics.

It may be more challenging to accurately assess intelligence when people are at the severe or profound level of intellectual disability, Dr. Lebersfeld says. A clinician will initially administer an IQ test for the child’s biological age range, and if that is too challenging, they may use other tests intended for kids outside their age range. This would give the clinician and family an overall estimate of the child’s developmental age equivalent; however, an IQ score can’t be generated in this situation.

To assess adaptive skills, clinicians typically ask questions to a caregiver about the child’s functioning at school, at home, and in the community.

“We’ll ask questions that are developmentally appropriate for a child’s age range,” Dr. Lebersfeld says. For example, is the child able to put on a jacket? Are they able to initiate a conversation with someone else? Can they catch a ball?

Sometimes, instead of using a questionnaire, occupational therapists will meet with a child and use their own observations to assess their adaptive skill levels.

When a child under the age of 5 is not meeting developmental milestones they may be diagnosed with what’s called global developmental delay. Children with global developmental delay should get reassessed when they are over the age of 5, which should lead to a more specific diagnosis. Some children who had delays when they were very young do not have intellectual disability when they get older.    

Kids with autism sometimes have IDD as well, but IDD and autism spectrum disorder are separate diagnoses. People with intellectual and developmental disability have challenges in learning and problem-solving, while those with autism have differences in social communication and restricted and repetitive patterns of behavior and interests.

It can be hard to properly diagnose autistic children with intellectual disability, especially at younger ages, because behavioral challenges or social and language delays may interfere with testing procedures used to make an assessment. Initial testing might find a higher level of impairment than actually exists.

“A lot of kids with autism don’t have an intellectual disability, it’s just that their verbal skills are delayed,” Dr. Lebersfeld says, while their nonverbal skills are age appropriate. “They may be able to do math and science and understand things conceptually, they just aren’t able to communicate it.” Teachers, family members, and friends might think a child with autism doesn’t have the cognitive ability to do things in their everyday life, she says, and therefore won’t give them the opportunities they deserve.

With early intervention and therapies such as speech therapy, occupational therapy, physical therapy, behavioral therapy, and other supports, autistic kids can improve their abilities, which can help lead to a more accurate diagnosis.

“When they’re younger, there may not be a large difference between somebody who’s typically developing and someone with intellectual disability,” Dr. Lebersfeld says. All children — with or without IDD — will often learn and develop at a quicker rate when younger, and as they get older, skills tend to plateau, she explains. Those with IDD will plateau earlier. As kids with IDD get older, the difference in cognitive ability between those with and without IDD gets bigger and bigger, Dr. Lebersfeld says. Cognitive skills usually don’t improve much after age 25, whereas life skills can continue to progress — the same as for individuals without intellectual disability. 

When it comes to adaptive skills, kids with IDD will “still make progress and keep learning, but it may be slower and take more repetition,” she says.

Often, kids with IDD stabilize at a developmental level that’s much younger than their age. Others, especially those whose disability was mild and resulted from a later accident or illness, can improve and no longer qualify for the diagnosis.

An initial diagnosis of IDD can be difficult for parents who don’t know what the future will hold for their child. Support groups of families who have children with IDD can help parents navigate what they can expect at different ages.

From birth to age 3, all children may be able to get services from each state’s Early Intervention Program, which is required by federal law under the Individuals With Disabilities Education Act (IDEA). Depending on the child’s needs, families may get help with nursing, nutrition services, and different kinds of therapy. Occupational therapy, for example, can help kids learn to dress and feed themselves. Cognitive therapy can help them improve their working memory and reasoning skills. Speech therapy can help with communication skills.

When children reach school age, parents can meet with the local school district to formulate an Individualized Education Program (IEP). Many kids with IDD can attend the local public schools with the right support. Parents often find they need to be strong advocates for their child to make sure the IEP each year has developmentally appropriate goals and the teachers and therapists that can help them achieve them.

Outside of school, families of children with IDD can get involved in activities such as Unified Sports, one of the programs Special Olympics runs for kids with intellectual and developmental disability. Unified Sports integrates kids with and without IDD in team sports, which can break down stereotypes about kids with IDD and help promote inclusivity in the community. To get involved or learn more, families can find a local Special Olympics program.