Evidence suggests that biological, psychological, cultural, and genetic factors are all involved. Biological studies have uncovered the existence of an abnormal amount of dopamine, a chemical neurotransmitter, in the brains of schizophrenics. This contributes to their auditory hallucinations, one which is widely heard of is voices in the head. Environmental conditions also contribute to the onset of schizophrenia. Unclear, disorganized communication within families is a factor, as are poor health conditions.
Psychological factors are difficult to distinguish because of the subjectivity of the area.
Heredity has a strong influence on schizophrenia. One popular theory is called the trinucleotide repeat amplification theory. Trinucleotide repeat amplification is a mutation, a genetic stutter in which a certain stretch of DNA occurs more than once when genes are copied. That explains why, in studies of identical twins with genes for schizophrenic vulnerability, only one twin could develop schizophrenia, but the children of both twins will have the same rate of the disorder. What happened was that in the first generation, only one of the twins had gone through enough trinucleotide repeats to cross the threshold to schizophrenia.