Scientists are almost certain that schizophrenia has more than one cause, although this is not yet precisely understood.
The Genetic Hypothesis:
Genetic factors appear to be important in the development of schizophrenia, but they are not sufficient to explain the entire pattern of occurrence. If an illness is entirely caused by genetic factors then identical twins share the same risk of the illness. That is, if one identical twin has the illness, the other should too. In fact, in most studies of identical twins in which one twin has schizophrenia, only about half of the other twins are affected.
A number of genes are probably involved in schizophrenia, known as “hot spots” on a cluster of genes. Researchers believe that a predisposition to develop schizophrenia is inherited, but an environmental “trigger”
must also be present to bring the illness to the surface. 
These triggers are stress factors experienced at any point of the person’s life. The role of stress is unclear; however, it is
acknowledged that stress can trigger or worsen symptoms when the illness is already present. Stress is very individual but could be
accompanied with an exam, a move,
a loss of someone, a difficult relationship, use of street drugs etc.
Scientists are almost certain that schizophrenia has more than one cause, although this is not yet precisely understood.
Viral Infection:
Another strong theory is that a viral infection is responsible for schizophrenia. This viral infection would take place during the second trimester of pregnancy. A virus, somewhat like the flu, invades the child through the mother. The infection would affect brain development. As a result, during adolescence schizophrenia would be triggered.

Neurodevelopmental Problems:
Again, during fetal development the development and placement of brain nerve cells is critical. A lack of or misplacement of these nerve cells could result in schizophrenia later in life. The brain is still developing up to and including the period of adolescence.
Birth Trauma:
Some researchers feel that schizophrenia may be the result of complications during the mother’s pregnancy or labour. 
Drug and Alcohol Abuse:
A distinction must be made between “drug/alcohol induced psychosis,” which may be temporary. Yet, it can trigger full-blown schizophrenia. So, substance
use and abuse including marihuana can trigger schizophrenia if one is
genetically or otherwise predisposed to it.
Chemical Brain Imbalance:
Research has shown that people with schizophrenia definitely have problems with certain types of brain cells and their function. There are billions of nerve cells (called neurons) in the brain. Each nerve cell has branches that send and receive messages from other nerve cells. Between each nerve cell there are gaps called a synapse. How then do brain signals cross these gaps? That’s where NEUROTRANSMITTERS come in. Scientists have discovered approximately 100 neurotransmitters in the brain. These chemicals which are released from nerve branches carry the message from the end of one nerve branch to the cell body of another. In the brain of a person with schizophrenia, something goes wrong with this communication system. 
Two neurotransmitters in particular have roles to play in schizophrenia. These neurotransmitters are called DOPAMINE and SEROTONIN. Evidence suggests that there is too much dopamine in certain areas of the brain, and this results in over-stimulation and excess sensory information which causes difficulty with concentration, thought process, reality orientation, feelings and behaviour. New evidence shows that abnormalities in serotonin activity also play an important role in the illness. The effect is that the person has a “sensitive brain” as if the nerve cells were “sandpapered.”

Nutritional Theories:
Orthomolecular medicine is the study of the effect of nutritional deficiencies in the body and how these deficiencies in vitamins, minerals and amino acids contribute to sickness. A few doctors feel that mental illnesses are partly the result of such and that diet and vitamin/mineral therapy are an important part of treating schizophrenia. Replicating nutritional studies, however, has been problematic.