Intellectual disability is classified into four subtypes: mild, moderate, severe, and profound. The specific criteria for each subgroup are listed in the Diagnostic and Statistical Manual of Psychological Disorders.

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Intellectual Developmental Disorder (Intellectual Disability) – the specific disorder, not general category – is recognized by deficits in both intellectual functions and adaptive functions that are present in conceptual, social, and practical domains of everyday life. The onset of these deficits is required to be during the developmental period. 

Intellectual Developmental Disorder (Intellectual Disability)

Spielman, R. M., Jenkins, W. J., & Lovett, M. D. (2020, April 22). Psychology (2nd ed.). OpenStax. https://openstax.org/books/psychology-2e/pages/1-introduction

OpenStax Psychology (2nd ed.) Textbook

Citation Reference for Intellectual Developmental Disorder Node

According to the American Psychiatric Association, Intellectual Disability Disorder (IDD) has an overall general population prevalence of approximately 1%, though its prevalence rates vary by age. 

The prevalence of severe levels of IDD is approximately 6 out of 1,000 individuals. 

Prevalence of Intellectual Disability Disorder

The APA has four severity codes (degrees of IDD). 

1. mild
2. moderate
3. severe 
4. profound intellectual disability

APA's Severity Code for IDD

Risk and Prognostic factors (defined as the likely causes/factors which lead to the development of an ailment) of IDD include:

1. Prenatal aetiologies, such as genetic syndromes (ex. Variations in gene sequencing or copy number variants; chromosomal disorders)
2. Inborn metabolism errors
3. Brain malformations
4. Maternal disease (ex. Placental disease)
5. Environmental influences, such as alcohol, drugs, teratogens, or other toxins 
6. Labor and delivery-related events that may lead to neonatal encephalopathy (a disease of the newborn characterized by an inability to maintain respiration, in an altered state of consciousness, seizures, spasms, and poor tone)
7. Severe and chronic social deprivation
8. Toxic metabolic syndromes
9. Intoxications 
10. Low birthweight 
11. Premature births
12. Multiple births
13. Infections during pregnancy
14. Untreated newborn jaundice (a condition where the skin, whites of eyes, and mucus membranes turn yellow due to the inability of the liver to process red blood cells – though this is normal in newborn babies, it is supposed to be cleared after a day; untreated jaundice can cause a type of brain damage known as Kernicterus)


Risk and Prognostic Factors for IDD

Individuals with ID may have limited resources and access to health care which can result in a detrimental impact on their health.

Health in Individuals with Intellectual Disabilities (ID)

Reference for Savant Syndrome

Savant syndrome is a rare condition that exists in individuals with different developmental disorders but mostly in people with autism, this condition gives individuals remarkable mental abilities and skills such as being able to immediately calculate difficult numerical operations, calendar calculations, solve extremely difficult puzzles, artistic talents, and more.

Savant Syndrome

Intellectual Disability (ID), a term that has officially replaced 'mental retardation', is diagnosed based on criteria from the American Psychiatric Association’s DSM-5. A diagnosis requires deficits in both intellectual functions and adaptive functioning, with an onset during the developmental period. The three core criteria are: 

1. **Criteria A - Deficits in General Mental Abilities:** Confirmed through both clinical assessment and standardized intelligence testing (e.g., reasoning, problem-solving, academic learning). 

2. **Criteria B - Impairment in Everyday Adaptive Functioning:** Deficits that result in a failure to meet sociocultural standards for personal independence and social responsibility across various environments. 

3. **Criteria C - Early Onset:** The intellectual and adaptive deficits must begin during the individual’s developmental period.

Definition and Diagnostic Criteria of Intellectual Disability (ID)

An intellectual disability (ID), formerly known as mental retardation, is diagnosed when an individual meets specific criteria: earning a score approximately two standard deviations below the mean on an intelligence test (about 70 on a test with a mean of 100), having major deficits in adaptive functioning, and showing these cognitive and adaptive deficits before the age of 18.

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