The biologists believe that all psychological abnormalities have an underlying chemical or physical cause. The four main causes are: infection, genetics, neurochemicals and neuroanatomy.


This is the idea that a bacteria or virus is caught from others and it damages the brain in some way.

  • Research has shown a link between glandular fever and anorexia nervosa.
  • Weakness- The research was done on an extremely small sample size of only 4 people, therefore there is not enough evidence to support this theory.


This is the theory that you inherit a mental disorder from your parents because it is passed to you in your genes.

  • This theory has been tested by studying identical twins, otherwise known as monzygotic or MZ twins.
  • Biologists have studied twins with mental disorders to find out if one twin has a disorder then how likely is it that the other twin will also develop the disorder?
  • They found that if one twin had annorexia then in 48% of cases so did the other, the percentage for depression was 46% and for schizophrenia was 45%. These percentages are known as concordance rates.
  • Weakness- For this theory to be viable the concordance rates would need to be around 90% rather than around 50%.
  • Weakness- Just because a disorder runs in a family it does not mean it has been passed down in genes, it could have been learnt from the family environment.
  • Weakness- Anorexia nervosa is a modern disorder so where would the gene for it suddenly have come from?


This is the idea that a change in chemicals in the brain causes mental abnormality.

  • People with depression are found to have low levels of serotonin in their brains and people with schizophrenia are found to have high levels of dopamine in their brains.
  • Weakness- (Chicken & Egg) We cannot establish whether it is the change in chemical levels which causes the abnormality or whether it is the abnormality which changes the chemical levels.


This is the idea that in some way the structure of the brain has been damaged causing an abnormality to occur.

  • For example, Alzheimers and Dementia are both thought to be caused by the malfunction and loss of brain cells.
  • Weakness- There is a lack of evidence for the neuroanatomy theory.

Weakness- Overall the theory is not scientific as it cannot demonstrate a cause and effect between two variables; only a link.  Also, it is reductionist and deterministic.

Reductionist- all complex behaviour is reduced to its simplest form.

Deterministic- you have no choice over your behaviour or the development of a mental disorder.