Intellectual disability is also called mental retardation, intellectual developmental disability, or general learning disability. It is an illness of the brain. Intellectual disability becomes visible in early childhood.

What is an intellectual disability? 

To have an intellectual disability, a person must: 
 Have an intelligence quotient (IQ) score of less than 70 (this is just over two people in a hundred) and 
 Have trouble with parts of daily life.

Types of intellectual disabilities 
There are two major types of intellectual disability.

The first is syndromic intellectual disability. This means that the person has a syndrome that causes intellectual disability, as well as medical issues and other problems.  Down syndrome and fetal alcohol syndrome are two examples of syndromic intellectual disabilities.

The second is non-syndromic intellectual disability. This means that the intellectual disability is not a part of a syndrome.

Levels of intellectual disability 
There are three levels of intellectual disability, based on how severe a person's disability is:
 The first and most common is mild intellectual disability. A person with mild intellectual disability can usually act without help from other people, but may need help with things like paying taxes. 
 The second level is moderate intellectual disability. Someone with moderate intellectual disability has an IQ between 40 and 55. They cannot live by themselves, but can learn to perform basic tasks. 
 The third level is severe intellectual disability. Someone with severe intellectual disability needs a lot of help and can only do simple things.
Mild intellectual disability may not be noticed until a child starts school. Moderate and severe intellectual disability can be seen before a child starts school.

What does intellectual disability look like? 

People with syndromic intellectual disabilities may have a "typical look."  For example, the picture on the right shows facial features that many people with fetal alcohol syndrome have.  People with non-syndromic intellectual disability do not look any different than a person without a disability.  The symptoms of intellectual disability are all behavioral.

People with intellectual disability may have some or all of these:
 They may learn to crawl, walk, or talk later than other children.
 They may have trouble remembering things.
 They may have trouble learning how to act in public.
 They may have trouble figuring out problems.
 They may learn how to take care of themselves slower.
 They may not learn when to not say or do something.
Most children with intellectual disabilities can learn.   However, they usually will not learn things as fast as other children. They may need to be taught things in a certain way in order to learn them and remember them.

Intellectual disability is different from dementia. In dementia, people forget things, and they lose skills they once had. People with an intellectual disability never learn those skills.

Causes 
Often, no-one knows what caused a child's intellectual disability.  This happens in between one-third and one-half of all cases of intellectual disability.

The top three most common causes of intellectual disability are Down syndrome, DiGeorge syndrome, and Fetal alcohol syndrome.

Genetic disorders 
Genetic disorders are caused by problems with a child's genes.  Children are born with these disorders.
 For example, Down syndrome happens when a child has an extra copy of chromosome 21.  Another example is  DiGeorge syndrome. This is caused by the deletion of a small segment of chromosome 22.

Problems during pregnancy 
Substances called teratogens can prevent a fetus from developing normally.  If a teratogen enters a woman's body while she is pregnant, it can cause developmental problems, like intellectual disabilities.

Examples of teratogens that can cause intellectual disabilities include:
 Alcohol
 Alcohol is the most common cause of intellectual disability which can be prevented.  Alcohol is poisonous to a fetus and can cause fetal alcohol syndrome if a mother drinks while pregnant
 Illegal drugs
 Certain medications, like warfarin (a blood-thinning medicine) and thalidomide
 Certain toxic chemicals, like lead and mercury, if a woman is exposed to enough of them during her pregnancy
 Certain diseases, like rubella and syphilis, if the mother has them during pregnancy

Problems during birth 
For example, if a child does not get enough oxygen during birth, it can hurt the brain and lead to intellectual disability later.

Diseases and trauma 
Some childhood illnesses, like measles and whooping cough, can cause intellectual disability if not treated properly.  So can infections that affect the brain, like meningitis and viral encephalitis.

Brain injuries can cause intellectual disability at any age.

Malnutrition 
Lack of proper nutrition can lead to intellectual disability over time.

Not having enough iodine in the body 
Iodine deficiency (not having enough iodine in the body) can lead to several medical issues, including intellectual disability.  The most common way to prevent this is by adding iodine to salt.  This is a much more common problem in developing countries.

Diagnosis 

In the DSM, there are three requirements a person has to meet in order to be diagnosed with an intellectual disability.

Low intelligence quotient (IQ) 
First, the person must have a low intelligence quotient (IQ). IQ is measured with an IQ test.

If the person has an IQ of below 70, they may have an intellectual disability. However, they must still meet the other two requirements to be diagnosed with an intellectual disability.

Trouble with daily activities 
Second, to qualify for an intellectual disability, a person must have trouble in more than one area of normal daily activities. These activities are often called "adaptive behaviors" or "activities of daily living (ADLs)."   Some examples of adaptive behavior are:
 Getting dressed
 Using the bathroom
 Eating and drinking
 Being able to have a conversation
 Acting properly in different situations
To see if a child is having trouble with these, a doctor will talk to people who know the child, and will watch the child's behavior.

Beginning in childhood 
The final requirement is that the symptoms of intellectual disability have to begin in childhood or adolescence. If the issues do not start at a young age, they are probably caused by a different illness of the brain.

Management 
There is currently no cure for intellectual disability. Those affected can learn to cope and do many things, if they get enough support and are taught well. There are many places around the world for someone with intellectual disability to get help. These places can take care of people with intellectual disabilities, as well as help them find jobs, find a house of their own, or help them take care of their children.

There are some different ways for people with intellectual disability and those around them to learn how to help the person with the disability. One kind is psychosocial treatment. This is meant for very young children. Psychosocial treatment helps them learn basic skills and increase learning over their lifetime. Another kind is behavioral treatment. This is meant to help young people, but can be used for adults as well. Behavior treatment helps teach language skills as well as social skills like sharing or following instructions. A third kind of help is cognitive-behavioral treatment. This is a combination of the previous two treatments. Cognitive-behavioral therapy helps children with intellectual disability both learn skills and learn how to plan ahead. Another type of help a person with intellectual disability can get is family-oriented help. Family-oriented help focuses on teaching family members how to help the person in their family with intellectual disability.

Many people with an intellectual disability have other health problems, for which they will be given specific drugs. As an example, autistic children with developmental delay may use anti-psychotics or mood stabilizers to help with behavior. Giving drugs to intellectually disabled people needs to be monitored; side-effects often occur, and are wrongly diagnosed as problems with behavior or as psychiatric problems.

History 

People have had intellectual disability throughout history. People with intellectual disability have had a lot of trouble in the past. The oldest idea of where intellectual disability came from was in ancient Greece. Hippocrates thought that intellectual disability was caused by an issue with the four humors. For several hundred years in Europe, churches took care of people with intellectual disabilities. In the 17th century, Thomas Willis suggested that intellectual disability was a disease caused by issues with the structure of the brain. In the 18th and 19th centuries, people with intellectual disabilities were put in asylums. The asylums would give them basics like food and shelter, but were not always good to the people in them. In the early 20th century, people with intellectual disability were made to not be able to have children and could not marry. It was thought that this would reduce the amount of intellectual disabilities in the future. This is not done anymore because it does not follow the idea of human rights.

In the 1950s, a group called the Civitans started to help people with intellectual disability. In the '70s, many people wanted to remove the stigma around people with intellectual disability. Now, people with intellectual disability are treated as people with something to be fixed instead of less than “normal” people. There are also fewer people with intellectual disability being sent to asylums.

Words that were used to describe people with an intellectual disability have changed a lot.  The most common words used today are “special”, “challenged”, "learning disabled" and “developmentally delayed”. Some previous words to describe people with intellectual disability are “cretin”, “idiot”, “imbecile”, “moron”, and “retarded”, all of which are now regarded as insulting.

Society 
People with intellectual disability are often treated badly by people without disabilities. They are often not allowed to make choices about their own lives and are not considered a full part of society. Their abusers are often people who are supposed to care for them. 39-83% of women with intellectual disability will be sexually abused before they are 18 years old.

The dignity and human rights of people with intellectual disability are protected by the Convention on the Rights of Persons with Disabilities as well as other disabilities and equally like other persons without disabilities.

References

Psychiatry
Psychology
Symptoms