This is still not fully understood. In both schizophrenia and bipolar affective disorder, there are abnormalities in the chemistry of the brain. This causes changes in thoughts, feelings and behaviour. There are a number of factors which make a person more likely to develop a psychotic illness. Usually, a combination of these is needed to trigger the illness.
Genetic factors play a part, probably by increasing the risk of an imbalance in brain chemistry.
Having a parent or close relative with schizophrenia or bipolar disorder means that a person will have a slightly greater than normal chance of developing the condition. A psychotic illness can be triggered in a person who is at risk by a number of factors. The first step towards getting help is to recognise that there may be a problem and to seek advice. The person with the illness may not notice the problem and deny that there is anything wrong. Often, other people notice that the ill person is not functioning as well as they normally do.