Types of intellectual disabilities

An intellectual disability may mean difficulty communicating, learning, and retaining information.

What is an intellectual disability?

An intellectual disability is characterised by an IQ below 70 (the median IQ is 100), and significant difficulty with daily living such as self-care, safety, communication, and socialisation.

People with an intellectual disability may process information more slowly than others, have difficulty communicating and managing daily living skills, and also have difficulty with abstract concepts such as money and time.

About 3% of Australians have an intellectual disability, making it the most common primary disability (Australian Institute of Health and Welfare).

An intellectual disability may be caused by a genetic condition, problems during pregnancy and birth, health problems or illness, and environmental factors.

Types of intellectual disabilities

Fragile X syndrome (FXS)

FXS is the most common known cause of an inherited intellectual disability worldwide. FXS is a genetic condition caused by a mutation (a change in the DNA structure) in the X chromosome.

People born with Fragile X syndrome may experience a wide range of physical, developmental, behavioural, and emotional difficulties, however, the level of severity can be very varied.

Some common signs include a developmental delay, intellectual disability, communication difficulties, anxiety, ADHD, and behaviours similar to autism such as hand flapping, difficulty with social interactions, difficulty processing sensory information, and poor eye contact (Better Health).

Boys are usually more affected by the syndrome than girls – it affects around 1 in 3,600 boys and between 1 in 4,000 – 6,000 girls (Better Health).


Down syndrome

Down syndrome is not a disease or illness, it is a genetic disorder which occurs when someone is born with a full, or partial, extra copy of chromosome 21 in their DNA.

Down syndrome is the most common genetic chromosomal disorder and cause of learning disabilities in children (Mayo Clinic). In Australia, approximately 270 children, or one in 1,100, are born with Down syndrome each year.

People with Down syndrome can have a range of common physical and developmental characteristics as well as a higher than normal incidence of respiratory and heart conditions.

Physical characteristics associated with Down syndrome can include a slight upward slant of the eyes, a rounded face, and a short stature. They will also have some level of intellectual and learning disabilities, but this can vary widely from one person to to another.

Find out more:

Myths about Down syndrome
Blog: Growing up with Down syndrome


Developmental delay

It’s important to know that children develop and mature at different rates. However, when a child develops at a slower rate compared to other children of the same age, they may have a developmental delay. One or more areas of development may be affected including their ability to move, communicate, learn, understand, or interact with other children.

Sometimes children with a developmental delay may not talk, move or behave in a way that’s appropriate for their age but can progress more quickly as they grow. For others, their developmental delay may become more significant over time and can affect their learning and education.


Prader-Willi Syndrome (PWS)

Prader-Willi syndrome (PWS) is a rare and complex genetic disorder which affects around 1 in 10,000 – 20,000 people (Better Health Channel). This disability is quite complex and it’s caused by an abnormality in the genes of chromosome 15.

One of the most common symptoms of PWS is a constant and insatiable hunger which typically begins at two years of age. People with PWS have an urge to eat because their brain (specifically their hypothalamus) won’t tell them that they are full, so they are forever feeling hungry.

The symptoms of PWS can be quite varied, but poor muscle tone and a short stature are common. A level of intellectual disability is also common, and children can find language, problem solving, and maths difficult.

Someone with PWS may also be born with distinct facial features including almond-shaped eyes, a narrowing of the head, a thin upper-lip, light skin and hair, and a turned-down mouth.

Find out more:

Myths about Prader-Willi syndrome
Blog: Kate’s story about living with Prader-Willi syndrome
PWS Association of Australia


Fetal alcohol spectrum disorder (FASD)

FASD refers to a number of conditions that are caused when an unborn foetus is exposed to alcohol. When a mother is pregnant, alcohol crosses the placenta from the mother’s bloodstream into the baby’s, exposing the baby to similar concentrations as the mother (Better Health Channel).

The symptoms can vary, however, include distinctive facial features, deformities of joints, damage to organs such as the heart and kidneys, slow physical growth, learning difficulties, poor memory and judgement, behavioural problems, and poor social skills.

Many cases are also often misdiagnosed as autism or ADHD as they can have similarities such as learning and behavioural difficulties, poor memory, and hyperactivity.

It is not well understood what quantity of alcohol can affect an unborn child, but it is important to note that there is no proven ‘safe amount’ of alcohol that women can drink during pregnancy. The World Health Organisation recommends that mothers-to-be, or those planning on conceiving, should completely abstain from alcohol.


Environmental and other causes

Sometimes an intellectual disability is caused by an environmental factor or other causes. These causes can be quite varied but can include:

  • Problems during pregnancy such as viral or bacterial infections
  • Complications during birth
  • Exposure to toxins such as lead or mercury
  • Complications from illnesses such as meningitis, measles or whooping cough
  • Malnutrition
  • Exposure to alcohol and other drugs
  • Trauma
  • And even unknown causes