Causes & Effects of Intellectual Disability

No one experiences intellectual disability the same way as someone else. Understanding the signs, symptoms and side effects of intellectual disability is a key component toward starting the recovery journey.

Understanding Intellectual Disability

Learn about intellectual disability

Intellectual developmental disorder, also known as intellectual disability (ID) and formerly known as mental retardation (MR), is a term used when there are limits to a person’s ability to learn at an expected level and be able to function in daily life. There are varying levels of intellectual disability, ranging from slight to very severe. Children with intellectual disability may have a hard time letting others know their specific wants and needs, as well as have a difficult time taking care of themselves. Additionally, ID may cause development to occur more slowly compared to children of the same age and lead to impairment in a child’s ability to learn, speak, walk, dress or eat without assistance.

The onset of intellectual disabilities typically occurs during the developmental periods and the features will depend upon the etiology and severity of the brain dysfunction. Delays in motor functioning, language abilities, and social milestones may be identifiable within the first two years of a child’s life if he or she has more severe intellectual disabilities. Mild intellectual disability may not be identifiable until the child reaches school-age, when challenges with academic learning become present. Early diagnosis and ongoing interventions can improve adaptive functioning throughout one’s childhood, teen years, and adulthood. It is important to mention that intellectual disability is not a disease and that there is no cure. However, with ongoing support and treatment, children with intellectual disability can learn to do many things. It may just take them more time and effort than other children.


Intellectual disability statistics

Researchers in the field believe that intellectual disabilities affect about 1% of the population in the United States. Of those affected, about 85% have mild intellectual disability. The prevalence of severe intellectual disability is estimated by the American Psychiatric Association to be approximately 6 per 1,000 people.

Causes and Risk Factors

Causes and risk factors for intellectual disability

Intellectual disability can be caused by a problem that starts any time before the age of 18. This condition can come about as a result of injury, disease, or a problem in the brain. However, for many children the cause is unknown. The most common causes and risk factors include:

Genetic: It has been established that intellectual disability can sometimes be caused by abnormal genes that have been inherited from ones parents. Some examples of genetic conditions include Down syndrome and fragile X syndrome.

Physical: There are some diseases like whooping cough, the measles, or meningitis that can cause intellectual disability. Additionally, intellectual disability can be caused by extreme malnutrition, not getting enough medical care, or by being exposed to poisons like lead or mercury.

Environmental: Problems during pregnancy that interfere with fetal brain development including drug or alcohol use, maternal malnutrition, preeclampsia, and infections during pregnancy can lead to the development of ID. Problems during childbirth, including extreme prematurity and oxygen deprivation increase the risk for intellectual disabilities as well. Additionally, traumatic brain injuries, the presence of illnesses that affect the brain, exposure to toxins such as lead, extreme malnutrition, and near-drowning have the potential to cause the onset of intellectual disabilities.

Risk Factors:

  • Prenatal alcohol or drug exposure
  • Contraction of illnesses or infections while in utero
  • Not receiving enough oxygen during birth
  • Brain injury
  • Malnutrition
  • Low parental IQ

Signs and Symptoms

Signs and symptoms of intellectual disability

The signs and symptoms of intellectual disability will vary for each individual depending upon the degree of intellectual disability. Some symptoms that may be present in those with intellectual disability include:

  • Learning and developing more slowly than other children same age
  • Difficulty communicating or socializing with others
  • Lower than average scores on IQ tests
  • Trouble learning in school
  • Inability to do everyday tasks like getting dressed or using the restroom without help
  • Difficulty hearing, seeing, walking, or talking
  • Inability to think logically

Additionally, the following categories are often used to describe each level of intellectual disability.


  • IQ 50-70
  • Slower than normal in all areas
  • No unusual physical signs
  • Can acquire practical skills
  • Reading and math skills up to grades 3-6
  • Can conform socially
  • Can acquire daily task skills
  • Integrated in society


  • IQ 35-49
  • Noticeable delays, particularly speech
  • May have unusual physical signs
  • Can learn simple communication
  • Can learn elementary health and safety skills
  • Can participate in simple activities and self-care
  • Can perform supervised tasks
  • Can travel alone to familiar places


  • IQ 20-34
  • Significant delays in some areas; may walk late
  • Little or no communication skills, but some understanding of speech with some response
  • Can be taught daily routines and repetitive activities
  • May be trained in simple self-care
  • Need direction and supervision socially


  • IQ <20
  • Significant delays in all areas
  • Cognitive abnormalities present
  • Needs close supervision
  • Requires attendant care
  • May respond to regular physical and social activity
  • Not capable of self-care


Effects of intellectual disability

Every person who has intellectual disability is a unique individual and as such they are going to be effected differently by intellectual disability.  Some common effects include:

  • Requires special teaching and training to be able to learn
  • Have difficulty solving everyday problems without support
  • Lack of interpersonal relationships
  • Difficulties finding and maintaining gainful employment
  • Difficulties performing activities of daily living
  • Suicidal thoughts and behaviors

Co-Occurring Disorders

Intellectual disability and co-occurring disorders

There are a number of disorders that co-occur in people who have intellectual disabilities. The most common co-occurring, comorbid disorders include:

  • Attention-deficit/hyperactivity disorder (ADHD)
  • Depressive disorders
  • Anxiety disorders
  • Bipolar disorder
  • Autism spectrum disorders (ASDs)
  • Stereotypic movement disorder
  • Impulse control disorders
  • Major neurocognitive disorder
  • Aggression
  • Self-injury
what past clients say

Because of the care my sister received at StoneCrest, there is peace in my family again. Thank you!

– Sister of a former client
Marks of Quality Care
  • Mental Health America Bell Seal for Workplace Mental Health 2023 - Gold Recipient
  • MPRO Governor's Award of Excellence 2017-2019
  • National Alliance on Mental Illness (NAMI)
  • The Joint Commission (JCAHO) Gold Seal of Approval