Down syndrome (also known as Down’s syndrome) is the commonest congenital cause of ID in New Zealand. It is caused by chromosomal abnormalities, usually a third copy of chromosome 21 rather than just two. It is associated with ID and characteristic physical features.

Alzheimers Disease causes deterioration in memory, reasoning, emotions and behaviour . The typical brain changes seen microscopically in this condition are amyloid plaques (protein laid down in response to damage) and neurofibrillary tangles (dying nerve cells). These changes can be seen in the brains of most people with Down syndrome by the age of 40, although they may not have developed symptoms. The average age of onset of dementia for people with Down syndrome is in the 50s, but even then, only 50% have dementia by the time they are 60. People with ID are living much longer than they used to and are more likely to develop dementia than others and at an earlier age.