Intellectual Disability: Causes and Symptoms

Intellectual disability involves a deficiency at the cognitive, behavioral and sensory levels, which reduces the abilities for social adaptation. We'll look specifically at what intellectual disability is and why it occurs.
Intellectual Disability: Causes and Symptoms
Bernardo Peña

Written and verified by el psicólogo Bernardo Peña in 20 August, 2021.

Last update: 20 August, 2021

The definitions of intellectual disability generally follow two criteria:

  • Psychometric-statistical, based on the possibility offered by intelligence tests to reliably and predictively assess intellectual functioning.
  • Adaptive behavior, based on the ability to adapt to the environment, which doesn’t necessarily have to correlate with scores on intelligence tests.

What is intellectual disability?

The APA or American Psychiatric Association, in the DSMs, defines the essential symptoms of this disorder as:

  • An overall intellectual ability that’s well below average: IQ equal to or less than 70.
  • A significant deficit or deterioration of adaptive capacity: Depending on the effectiveness shown by the person in certain areas of their behavior, such as:
    • Social skills
    • Communication
    • Everyday problem-solving skills.
    • Personal independence.
    • Social responsibility according to their age and cultural group.
  • Onset before the age of 18: When a similar clinical picture appears for the first time after the age of 18, it constitutes dementia or an irreversible alteration of previously acquired intellectual functions, not an intellectual disability.

The most common behavioral symptoms include:

  • Passivity
  • Dependence
  • Low self-esteem, mainly in children with mild retardation
  • Low tolerance to frustration
  • Aggressiveness
  • Poor impulse control
  • Stereotyped, self-mutilating, and self-stimulating behaviors

The course of intellectual disability is a function of the underlying biological factors that have caused it and also of environmental factors. These last factors refer to educational opportunities, environmental stimulation, planning, and the execution of the treatment that’s dispensed, etc.

Therefore, with optimal environmental influence, the subject’s functioning can be improved. However, with harmful environmental influence, it can deteriorate.

Down's Syndrome

Complications of intellectual disability

In terms of complications, subjects with intellectual disability are more likely to develop pervasive developmental disorders (autism), attention deficit hyperactivity disorder, stereotypies, adaptation reactions, and above all, epilepsy, their probability of onset being higher with the greatest severity of intellectual disability.

Depending on the level of severity, four types of intellectual disability are distinguished:

  • Mild intellectual disability (IQ of 50-55 to 70), constitutes 85% of those affected. Most don’t have a known somatic etiology and are considered in regard to socio-family and cultural factors.
  • Moderate intellectual disability (IQ 35-40 to 50-55), constitutes 10% of those affected.
  • Next, serious intellectual disability (IQ 20-25 to 35-40).
  • Lastly, profound intellectual disability (IQ below 20-25).

An IQ range between 71 and 84 is considered borderline intellectual ability.

The causes of intellectual disability

Intellectual disability is a phenomenon of multifactorial etiology, with several organic and environmental factors that can be the cause of the disorder. However, we can’t forget that it can often be a consequence of the combined effects of several factors. The most frequent etiological factors (causes) are the following:

Genetic factors

Genetic factors explain approximately 50% of the cases in which it’s possible to isolate an etiology. The most frequent alterations are listed below:

  • Chromosomal aberrations. The most common is Down syndrome, caused by the presence of an accessory group of genes on chromosome 21. Others, for example, are Klinefelter syndrome or Turner syndrome.
  • Specific gene abnormalities. The most common ones involve an insufficient amount of a certain enzyme. They are, therefore, metabolism disorders, among which are galactosemia, phenylketonuria, or syndromes associated with lipid storage disorders, for example.

You may be interested in: What Is Fragile X Syndrome?

Cultural and family causes

In order to classify an individual as having a family-cultural handicap, the following requirements must be met:

  • The delay is moderate.
  • There’s no evidence of brain pathology.
  • There must be evidence of poor intellectual functioning in at least one of the parents and one or more of the siblings, if they exist.

There are two explanations for this cultural-family delay :

  • The first is genetics. Intelligence is distributed in the general population according to the normal curve. If the parent of a child is at the lower end of the curve, there’s a greater probability that the intelligence of their offspring is below the norm. If both parents are at this extreme, this probability will increase.
  • The second explanation is based on the environment. The siblings of a given family may encounter social and environmental adversities such as poverty, malnutrition, environmental deprivation, verbal deprivation, poor medical care, low expectations of success, etc.

The majority of people with moderate intellectual disability come from very low-income households with socio-environmental and educational disadvantages. According to this explanation, there could be an adequate and usable physiological anatomical potential, but the individual’s not capable of developing it for sociocultural reasons.

At present, the behavior of the child is thought to be determined by an interplay of environmental and genetic influences.

Environmental factors

These can be classified into those that occur before delivery (prenatal), during birth (perinatal), and those that occur after birth (postnatal):

fetus in utero, placenta

Prenatal factors

The first eight weeks of development are of vital importance. Exposure of the embryo to certain agents during this critical period of development can cause serious malformations and intellectual disability. Some of the most studied prenatal factors are:

  • Low birth weight: A relationship between low birth weight (less than 2.5 kg) and an increased risk of impaired mental functions has been clearly established. Some of the factors that cause low birth weight are the following:
    • Malnutrition.
    • Tobacco.
    • Alcohol.
    • Multiple births.
    • The mother’s age: Younger than 16 years old or older than 40.
    • The number of previous deliveries of the mother.
    • Weight gain by the mother.
    • Maternal stress.
  • Diabetes: Although the children of diabetic mothers don’t suffer from low birth weight, since they’re clearly overweight, they’re at high risk for neurological and intellectual deficits, as well as other problems.
  • Maternal infections: Maternal infections that occur during the prenatal period can act as a cause of intellectual disability, as they can affect the development of the central nervous system. For example:
    • Rubella
    • Syphilis
    • Cytomegalovirus
    • Toxoplasmosis
    • Toxemia
  • Rh sensitization from the second pregnancy onward

Perinatal factors

  • Anoxia or hypoxia: This is a state in which an insufficient amount of oxygen reaches the tissues, which can affect the CNS.
  • Mechanical trauma: If the child falls into an abnormal position, the instruments or techniques used for delivery can cause physical trauma that can lead to brain damage. To avoid this, cesarean sections are being performed increasingly.
  • Prematurity: Those born with less than 37 weeks of gestation or 5.5 lbs in weight are considered high risk
    because they have higher mortality rates and are more susceptible to CNS injuries.
  • Infection: For example, herpes, which is transmitted vaginally during childbirth, bacterial meningitis, etc.

Postnatal factors

The most notable are:

  • Malnutrition.
  • Infections, such as meningitis, rubella, encephalitis, etc.
  • Poisons: Lead and mercury poisoning are two of the most common causes of brain injury.
  • Injuries: The two most common causes are abuse and car accidents.

Finally, we can cite some environmental factors of a more psychological-educational nature, such as the lack of means and materials at home to promote learning. Low expectations of success, lack of stimulation of the child during its development, etc.

Conclusions regarding intellectual disability

In conclusion, intellectual disability implies a cognitive and sensory deficiency that entails difficulties with social adaptation. Its causes are multiple and, every day, investigations are revealing more interesting data.

Finally, it’s important to note that intellectual disability should be treated as soon as possible. Therefore, psychoeducational therapies can help stimulate these children.

It might interest you...
Functional Neurological Disorder: Symptoms, Causes and Treatment
Muy SaludRead it in Muy Salud
Functional Neurological Disorder: Symptoms, Causes and Treatment

The functional neurological disorder implies a series of sensory and / or motor alterations without an organic cause that justifies them.



  • Artigas-Pallarés, J. (2003). Perfiles cognitivos de la inteligencia límite. Fronteras del retraso mental. Rev Neurol36(1), 161-167.
  • Basile, H. (2008). Retraso mental y genética Síndrome de Down. Revista argentina de clínica neuropsiquiátrica15(1), 9-23.
  • Horjales, I., & Lorente, A. P. (1993). Impacto y consecuencias psicopatológicas del retraso mental en la familia. Revista complutense de educación4(2), 67-96.