Types of Intellectual Disability, Causes and Treatment

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What is Intellectual Disability (Mental Retardation)?

Intellectual Disability, formerly known as mental retardation, is characterized by deficits in general mental abilities, such as reasoning, problem-solving, planning, abstract thinking, judgment, academic learning, and learning from experience (APA, 2013). Intellectual disability falls under the category of neurodevelopmental disorders. When a person's intelligence is significantly below normal, i.e., an IQ lower than 70 and it causes impairment in daily functioning then that person might be categorized as having an intellectual disability. However, the deficits must start to exist before 18 years of age and should not be a result of brain trauma in adulthood.

What are the different types of intellectual disability?

Intellectual Disability is categorized into four different levels. They are:

1. Mild Intellectual Disability:

The IQ score of people with mild intellectual disability falls under the range of 50-55 to 70; almost 80% of people with mental retardation fall under this category. In terms of education, these people are considered educable, and as adults, their intellectual level can be compared to those of an average of 8-11 years old. In terms of social adjustment they can be compared to adolescents, however, they tend to lack imagination, inventiveness, and judgment, unlike adolescents. They generally do not show any physical abnormality or brain distortions but they require supervision due to their limited capabilities. They might be able to learn how to function independently with training and special education.

2. Moderate Intellectual Disability:

10% of people with mental retardation having an IQ score ranging from 35-40 to 50-55 falls under this category. As adults, they have intelligence levels similar to those of 4-7 years old children. They suffer from physical anomalies and have poor motor coordination. They are usually clumsy and are not capable of functioning independently. With training and supervision, they might be able to learn to take basic care of themselves and perform unskilled tasks.

3. Severe Intellectual Disability:

The IQ score of people with severe intellectual disability ranges from 20-25 to 35-40 and 3-4% of people with mental retardation fall under this category. People with severe intellectual disability have distorted motor and speech development with severe sensory defects. They are not capable of functioning independently at all and require supervision to perform simple tasks.

4. Profound Intellectual Disability:

1-2% of people with mental retardation with IQ scores below 20-25 fall under this category. People in this group are severely deficient in adaptive behavior and cannot perform any task, not even the simplest ones. If there is speech development, at all, it's rudimentary. They tend to have severe physical deformities, abnormal growth, and pathology in the central nervous system. Other physical anomalies along with convulsive seizures, mutism, and deafness are also common. They constantly need supervision the perform the most basic tasks.

What are the causes of Intellectual Disability? (ETIOLOGY)

There are various causes of intellectual disability some of which are biological and some environmental factors as well. Notable among them are:


Genetic factors play a predominant role in the cause of Intellectual Disability. Down syndrome (Trisomy 21) is a condition that is caused due to the presence of 3 chromosomes in a set instead of two. Children with down syndrome have a smaller brain structure and neurofibrillary tangles which is a facilitator of Intellectual Disability.

Phenylketonuria is another cause of intellectual disability. It is a genetic disorder that is caused due to the absence of an essential enzyme that is required to break down the amino acid phenylalanine in the body due to which various physical and mental abnormalities persist.

Fragile X Syndrome (FXS) is a heritable condition that is one of the most common genetic factors of intellectual disability. The gene FMR1 generally makes a protein FMRP the absence of which causes the FXS.

Other genetic disorders that cause intellectual disability are Tuberous Sclerosis Complex (TSC) and Lesch-Nyhan syndrome.


Although genetic factors are the leading cause of intellectual disability, environmental factors both prenatal and postnatal may be responsible. Prenatal factors include uterine environmental toxins, premature birth, hypoxia, and fetal malnutrition. Postnatal factors are bacterial and viral infections, malnutrition, and social factors like poverty, and low environmental stimulation.

According to Kaski, 2000, infections, toxic agents and trauma like physical injury also influence mental retardation.


Intellectual disability is irreversible meaning it is not completely curable however with special training programs and education children with intellectual disability can learn to function independently and perform their basic tasks. Psychological treatments focus on teaching community adjustments and adaptive behaviors for independent living. Some behavioral techniques like shaping by using rewards or chaining are also useful in helping children with intellectual disabilities to perform simple tasks.

Medicines are not the most effective form of treatment in the case of intellectual disability because it comes with their own side effects, but they might be helpful in reducing the symptoms of other psychological disorders that work as a catalyst for intellectual disability.


Sayani Banerjee
Author: Sayani Banerjee

Hey there, curious minds! I'm Sayani Banerjee, and I'm thrilled to be your companion on the fascinating journey through the realm of psychology. As a dedicated student pursuing my master's in Clinical Psychology at Calcutta University, I'm constantly driven by the desire to unravel the mysteries of the human mind and share my insights with you. My passion for teaching and my love for research come together on my blog, psychologymadeeasy.in, where we explore the world of psychology in the simplest and most engaging way possible.

Read more on: Abnormal Psychology

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