About two to three percent of people meet ID criteria
Intellectual disability—also disorder of intellectual development, and in UK usage sometimes “learning difficulty”—is a neurodevelopmental condition with major limits in intellectual function and adaptive behaviour beginning early in life. DSM-5 lists reasoning, planning, judgment, and learning among the skills affected. Roughly two to three percent of people qualify; most cases are mild.
DSM-5 counts reasoning, problem solving, planning, abstract thought, judgment, school learning and learning from experience as the intellectual functions at stake, and requires deficits to be confirmed both clinically and by standardised testing. Syndromic forms arrive with other medical or behavioural signs; non-syndromic or idiopathic cases make up thirty to fifty percent. Mild forms often show no physical signs, though a genetic cause such as Down syndrome can bring characteristic features.
Severity shapes the picture. Mild disability, which covers 75 to 90 percent of those affected, may go unnoticed until school, and experts may be needed to tell it apart from a specific learning disorder. Moderate disability usually surfaces in the first years, often through delayed speech, and needs substantial support. Severe and profound forms, about 3.5 and 1.5 percent of cases, call for lifelong supervision. Early warning signs include late sitting, crawling or walking and trouble with dressing or feeding oneself. Roughly 22 percent of people with the condition have epilepsy, rising to about half at the profound level, and overlap with autism can muddy diagnosis.
Causes vary. In a third to a half of affected children none is found. About a quarter trace to genetic disorders, including Down, Fragile X, Williams and Prader–Willi syndromes and untreated phenylketonuria, though only around 5 percent are inherited. Others follow alcohol exposure in pregnancy, oxygen shortage at birth, meningitis, measles or whooping cough without prompt care, and lead or mercury poisoning.
Iodine deficiency, which touches about 2 billion people, is the leading preventable cause where it is endemic, and famine-driven malnutrition lowers intelligence in places such as Ethiopia. The category is clinical, not a measure of worth; support scaled to need widens independence.
Source: Intellectual disability
