Among students receiving special education services, 6% percent are identified as having an intellectual disability (National Center for Education Statistics, 2022). Genetic and environmental factors may cause intellectual disabilities, but what causes an intellectual disability is not always known. In addition, causes may be prenatal, perinatal, or postnatal.
Genetic Causes
Educators and clinicians should have a basic knowledge of chromosomal disorders to provide appropriate interventions and treatment for students whose intellectual disability is caused by a chromosomal disorder. The two most common genetic causes of intellectual disabilities are Down syndrome and fragile X syndrome.
Down syndrome occurs in approximately one in every 700 babies (Mai et al., 2019). There are different types of Down syndrome. The most common type of Down syndrome, which accounts for approximately 95% of cases, is called trisomy 21, in which a person has an extra chromosome called chromosome 21. Mosaicism, or mosaic Down syndrome, which accounts for approximately 2% of cases, is diagnosed when there is a mixture of two types of cells, some containing the usual 46 chromosomes and some containing an extra chromosome (i.e., chromosome 21). Finally, translocation, which accounts for approximately 3% of cases, is diagnosed when an additional full or partial copy of chromosome 21 attaches to another chromosome, usually chromosome 14 (National Down Syndrome Society, 2022). The extra chromosome leads to the physical features and developmental challenges that can concur among people with Down syndrome. Students with Down syndrome usually have an IQ in the mildly-to-moderately low range and are slower to speak than other children. Students with Down syndrome may also experience hearing loss, ear infections, eye diseases, and heart defects. Down syndrome is not genetically inherited. However, women who are 35 years or older are more likely to have a pregnancy affected by Down syndrome (CDCa, 2022).
Fragile X syndrome is caused by the mutation of a single gene, fragile X messenger ribonucleoprotein 1 (FMR1), and is genetically inherited. People who have fragile X syndrome do not make this protein. A female carrier has a 50% chance of passing the mutation to each of her children, whereas a male carrier will pass it to all of his daughters but none of his sons. Fragile X syndrome occurs in both sexes. However, females often have milder symptoms than males. The exact number of people with fragile X syndrome is unknown, but studies indicate that approximately 1 in 7,000 males and 1 in 11,000 females have fragile X syndrome. The severity of intellectual disability varies between individuals, but males are usually diagnosed with more severe forms of intellectual disability than females. Autism spectrum disorder also occurs more frequently in students with fragile X syndrome. Students with fragile X syndrome may also experience ear infections, strabismus (i.e., crossed eyes), seizures, sensory processing challenges, speech and language delays, and motor delays (CDCb, 2022; National Fragile X Foundation, 2022).
Less common genetic disorders may also result in intellectual disability. These include Smith–Magenis syndrome, Angelman syndrome, Smith–Lemli–Opitz syndrome, Cornelia de Lange syndrome, Rett syndrome, Cri-du-Chat syndrome, and Sotos syndrome.
- Smith–Magenis syndrome is caused by a deletion of genetic material from chromosome 17. The prevalence of Smith–Magenis syndrome is approximately one in 25,000 people. Students with Smith–Magenis syndrome have mild-to-moderate intellectual disabilities.
- Angelman syndrome is caused by the loss of a gene located on chromosome 15. Angelman syndrome affects one in every 15,000 people. Students with Angelman syndrome have profound intellectual disabilities.
- Smith–Lemli–Opitz syndrome is caused by a defective gene on chromosome 7 that is responsible for the production of cholesterol, which is essential to cells in the body. Smith–Lemli–Opitz syndrome affects an estimated one in 20,000 to 60,000 people. Symptoms vary widely, but severe cases may involve profound intellectual disability.
- Cornelia de Lange syndrome is caused by a mutation in one of a few different genes located on the X chromosome or on chromosomes 5 or 10. Cornelia de Lange syndrome occurs in about one in every 10,000 to 30,000 people. Students with Cornelia de Lange syndrome typically have mild to profound intellectual disabilities, with most falling in the mild-to-moderate range.
- Rett syndrome is caused by mutations to a gene on the X chromosome. It affects approximately one in every 10,000 to 23,000 female births. Rett syndrome in boys is extremely rare. Students with Rett syndrome typically have severe to profound intellectual disabilities.
- Cri-du-Chat syndrome is caused by missing genes on chromosome 5. It affects approximately one in every 20,000 to one in 50,000 people. Students with Cri-du-Chat syndrome have severe intellectual disabilities.
- Sotos syndrome is caused by a defect on chromosome 5 and is reported to occur in one in 10,000 or 14,000 people. Students with Sotos syndrome may or may not have an intellectual disability (Kennedy Krieger Institute, 2022).
Environmental Causes
Environmental causes of intellectual disability may occur prenatally, perinatally, or postnatally and can result in medical problems that affect a child’s development. An example of a prenatal cause is the use of alcohol by the pregnant mother, which puts babies at risk for developing fetal alcohol spectrum disorders (FASD). FASDs include fetal alcohol syndrome, alcohol-related neurodevelopmental disorders, alcohol-related birth defects, and neurobehavioral disorder associated with prenatal alcohol exposure. In addition, maternal use of drugs or tobacco can also cause birth defects and potential intellectual disabilities (CDCc, 2022).
Examples of perinatal causes of intellectual disabilities include difficulties with the birthing process such as anoxia (i.e., temporary oxygen deprivation) or other birth-related injuries. Childhood diseases such as measles that cause damage to the brain are examples of postnatal causes of intellectual disabilities. In addition, childhood head injuries and exposure to environmental toxins such as lead and mercury may cause intellectual disabilities (ARC, 2022).
Risk Factors
Several factors may contribute to the risk of developing an intellectual disability. A primary risk factor is living in poverty. Children who live in poverty are at higher risk for experiencing malnutrition, exposure to environmental toxins, and inadequate health care. In addition, experiencing child abuse and neglect is also a risk factor that may contribute to the development of an intellectual disability (Smiley et al., 2022).
National Center for Education Statistics. (2022). Students with disabilities. U.S. Department of Education, Institute of Education Sciences. Retrieved August 30, 2022, from https://nces.ed.gov/programs/coe/indicator/cgg
Mai, C.T., Isenburg, J.L., Canfield, M.A., Meyer, R.E., Correa, A., Alverson, C.J., Lupo, P.J., Riehle‐Colarusso, T., Cho, S.J., Aggarwal, D., & Kirby, R.S. (2019). National population‐based estimates for major birth defects, 2010–2014. Birth Defects Research, 111(18), 1420–1435.
National Down Syndrome Society. (2022, September 27). About Down syndrome. https://ndss.org/about#p_336
Centers for Disease Control and Prevention. (2022b, September 27). What is fragile x syndrome? https://www.cdc.gov/ncbddd/fxs/facts.html
National Fragile X Foundation. (2022, September 27). Fragile x 101. https://fragilex.org/understanding-fragile-x/fragile-x-101/
Kennedy Krieger Institute. (2022, September 27). Genetic, metabolic, and chromosomal disorders. https://www.kennedykrieger.org/patient-care/conditions/genetic-metabolic-chromosomal-disorders
Centers for Disease Control and Prevention. (2022c, S
eptember 27). Fetal alcohol spectrum disorders. https://www.cdc.gov/ncbddd/fasd/index.html
The ARC. (2011). Causes and prevention of intellectual disabilities. http://www.thearc.org/wp-content/uploads/forchapters/Causes%20and%20Prevention%20of%20ID.pdf
Smiley, L. R., Richards, S.B., & Taylor, R. (2022). Exceptional students: Preparing teachers for the 21st century (4th ed.). McGraw Hill.
