Genetic Factors:
Family Studies:

• Family studies have established that schizophrenia is more common among biological relatives of a person with schizophrenia, and that the closer the degree of genetic relatedness, the greater the risk.
• E.g., children with two schizophrenic parents have a concordance rate of 46%, children of one schizophrenic patient have 13% and siblings 9%
• Many researchers now accept that the fact that schizophrenia appears to run in families may be more to do with common rearing patterns or other factors that have nothing to do with heredity

Twin Studies:

• If monozygotic (identical) twins, who share 100% of genes, are more concordant in terms of a trait like schizophrenia than dizygotic (fraternal) twins, then this suggest the greater similarity is due to genetics
• Joseph (2004) calculated that the data for schizophrenic twin studies prior to 2001 shows a concordance rate for Mz twins of 40.4% and for Dz twins of 7.4%
• More recent, methodologically sound (e.g. 'blind' diagnoses, where the researchers don't know whether the twins are Mz or Dz) have reported lower rates for Mz twins
• This still shows support for the genetic position, as the Mz concordance rate is many times higher than the Dz rate

Adoption Studies:

• Studies of genetically related individuals who have been reared apart are used to separate environmental and genetic factors
• Tienari et al (2000) carried out a study on 164 adoptees whose biological mothers had been diagnosed with schizophrenia
• 11 (6.7%) also received a diagnosis of schizophrenia, compared to just four (2%) of the 197 control adoptees
• Investigators concluded that these findings showed that the genetic liability to schizophrenia had been 'decisively confirmed'

The Dopamine Hypothesis:

• This states that messages from neurons that transmit dopamine fire too easily or too often, leading to the characteristic symptoms of schizophrenia
• Schizophrenics are thought to have abnormally high numbers of D2 receptors on receiving neurons, resulting in more dopamine binding and therefore more neurons firing
• Dopamine neurons play a key role in guiding attention, so disturbances in this process may lead to the problem relating to attention, perception and thought found in schizophrenics (Comer, 2003)

Evidence for Dopamine Hypothesis:

Amphetamines:

• This is a dopmanie agonist, stimulating nerve cells containing dopamine causing the synapse to be flooded with this neurotransmitter
• Large doses of the drug can cause the characteristic hallucinations and delusions of a schizophrenic episode

Antipsychotic Drugs:

• These block the activity of dopamine in the brain, eliminating symptoms such as hallucinations and delusions
• The fact that these drugs alleviated many of the symptoms of schizophrenia, strengthened the case for being a contributing factor in this disorder

Parkinson's Disease:

• Low levels of dopamine activity are found in people suffering from Parkinson's
• It was found that some people take the drug L-dopa to raise their levels of dopamine were developing schizophrenic symptoms (Grilly, 2002)