The World Health Organization (WHO) has defined learning disabilities as a state of arrested or incomplete development of mind. Somebody with a general learning disability is said to have a significant impairment of intellectual, adaptive and social functioning. A learning disability is not acquired in adulthood and is evident from childhood.

The term general learning disability has now been recommended in the UK to replace terms such as mental handicap or mental retardation. A child with a general learning disability or intellectual impairment finds it more difficult to learn, understand and do things compared with other children of the same age. The degree of disability can vary greatly, being classified as mild, moderate, severe or profound.

General learning disability is different from from specific learning difficulty such as dyslexia which means that the person has one difficulty such as in reading, writing but has no problem with learning in other areas.  It is characterised by generally poor performance in tasks such as learning, short-term memory and problem solving.

People with intellectual disabilities have an increased prevalence of health problems and their health needs are often unrecognised and unmet. For example, anxiety disorders are the most prevalent of the major mental illnesses and are present in 15–20% of the population but for people with intellectual disability, the prevalence of anxiety disorders has been shown to be much higher, particularly for hose with a mild to moderate intellectual disability. This is likely to be because they are aware that they are functioning less well than other people and because they find it significantly harder than others to cope with the demands of everyday life Anxiety disorders in people with intellectual disability can often be overlooked. Communication difficulties may make it harder for the person to describe their feelings and fears. Their difficulties may result in aggressive or destructive behaviour when they find situations difficult to cope with or are trying to avoid something which they find stressful

Although life expectancy is increasing, with people with mild learning disabilities approaching that of the general population, the mortality rates among people with moderate to severe learning disabilities are three times higher than in the general population, possibly due either to the impact of the cause of the disability, complex additional physical difficulties or because of less effective health care than the general population. The median age at death for people with learning disabilities is about 25 years younger than for those who do not have learning disabilities.

A classification of mild, moderate, severe and profound has been used to describe the degree of learning disability. However, this classification is in many ways inappropriate because each person with general learning disability must be assessed and treated as an individual. A person’s degree of intellectual impairment provides very little information about a person’s social, educational and personal needs, and the kind of help and support

About 2% of the population is estimated to have an intellectual impairment. It has been estimated that 1,191,000 people had learning disabilities in England in 2011, including: 286,000 children (180,000 boys, 106,000 girls) aged 0-17. As all children are assessed and monitored in infancy and all children attend school, it is likely that all or almost all of these children are receiving services to support them and their families

However of the 905,000 adults aged 18+ (530,000 men and 375,000 women), estimated to have an intellectual impairment only 189,000 (21%) are known to learning disabilities services. This means that the vast majority of adults with an intellectual impairment are not receiving services which would provide support for them and their families

The population for people with learning disabilities shows a sharp reduction in prevalence rates after age 49 due to reduced life expectancy. A sharp increase in prevalence for those under 20 may reflect increased survival rates among more severely disabled children.

The number of people with intellectual disabilities increased by 53% over the 35-year period from 1960 to 1995 as a result of improvements in socio-economic conditions, medical  care  and access to services for children

The causes of intellectual impairment include:

Genetic disorders such as Down’s syndrome, fragile X syndrome, Klinefelter’s syndrome and Turner syndrome

Metabolic disorders such as Tay-Sachs disease, Gaucher’s disease, Niemann-Pick disease) and Hurler’s syndrome.

Nutritional deficiency such as iodine deficiency.

Damage caused before the child is born by infections such as rubella; or by drugs or alcohol.

Damage caused during labour by prolonged labour, trauma, and deprivation of oxygen

Accidental or non-accidental injury, severe infections, poisoning or malnutrition   in infancy or childhood

For many people with an intellectual impairment there is no known cause