The Medical Model
In the Medical model all behaviour is viewed as being due to biological processes in the body. Therefore any abnormality is due to a malfunction of the body or due to genetic/inherited problems. It assumes that abnormalities are caused by either physical factors, are inherited or due to abnormal neuroanatomy.
It assumes that all mental disorders are related to some change in the body and are like physical disorders eg, illness. These can be caused by biochemical substances, genes, neuroanatomy or micro organisms.
It also states that illness is inherited and is passed down the generations from parents to children. Investigations have been carried out on twins and have shown a high concordance rate for mental disorders such as schizophrenia and a low concordance rate for other mental disorders such as phobias.
Certain genes can also lead to illness as they tell the body how to function. They determine the production of neurotransmitters and hormones. Genes also determine the structure of the brain and it has been found that people who have schizophrenia have larger ventricles. Neurotransmitters transmit signals across a from one neuron (nerve cell) to another ‘target’ neuron. Serotonin is an example of a neurotransmitter and is regarded by some researchers as a chemical that is responsible for maintaining mood balance, and that a deficit of serotonin leads to depression.
Image of neurotransmitter being transmitted:
However it is not clear whether abnormal biochemistry is a cause of abnormal behaviour. This is because concordance rates are never 100% and it is likely that the individuals could inherit the problem but that it only develops due to certain stressors. The medical model can also be viewed as being more humane as it lead to more humane treatment for patients as many mental illnesses were regarded as demonic possessions. On the other hand mental disorders can be viewed as being different to physical diseases. Szasz(2000) believes that mental disorders are not diseases at all.
ncpsych 
Excellent summary – really good information about that approach.
“people who have schizophrenia have larger ventricles.” – that’s right, but note that this implies brain damage of some type. The ventricles i.e. fluid-filled regions would be larger if the brain itself was reduced in size.
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