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Describe and Evaluate Biological Explanations for Schizophrenia. Diathesis-Stress Model. A theory that explains behaviour as both a result of biological and genetic Factors ("nature"), and life experiences ("nurture").

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diathesis stress model
Diathesis-Stress Model

A theory that explains behaviour as both a result of biological and genetic

Factors ("nature"), and life experiences ("nurture").

This model thus assumes that a disposition towards a certain disorder may

result from a combination of one's genetics and early learning.

The term "diathesis" is used to refer to a genetic predisposition toward an

abnormal or diseased condition.

According to the model, this predisposition, in combination with certain kinds

of environmental stress, results in abnormal behaviour.

This theory is often used to describe the pronunciation of mental disorders,

like schizophrenia that are produced by the interaction of a vulnerable

hereditary predisposition, with precipitating events in the environment.

  • In the diathesis–stress model, a biological or genetic vulnerability or predisposition (diathesis) interacts with the environment and life events (stressors) to trigger behaviours or psychological disorders.
  • The greater the underlying vulnerability, the less stress is needed to trigger the behaviour or disorder.
  • Conversely, where there is a smaller genetic contribution greater life stress is required to produce the particular result.
  • Even so, someone with a diathesis towards a disorder does not necessarily mean they will ever develop the disorder.
  • Both the diathesis and the stress are required for this to happen.
  • This theory was created by Holmes & Rahe.

DNA contains a set of instructions. It is the carrier of information.

  • Half of their genes come from their maternal line, and half from their paternal.
  • The DNA is composed of a series of genes, each of which ‘codes’ for a particular protein.
  • Minute differences in the DNA code create different shaped / functions in the proteins made.

As each persons DNA is different from anyone else’s, this is their specific Genotype.

  • This results in a completely unique Phenotype (the characteristic show, as a result of both the genes and the way in which it interacts with the environment).

Some genes always lead to certain characteristics; these are known as dominant genes. To produce a characteristic, dominant genes need to be on only one pair of chromosomes (one copy).

  • Some genes need more than one copy to produce a characteristic, there are known as recessive genes – if a Recessive gene is present on only one chromosome, the characteristic will not appear.
  • HOWEVER, they may be passed on and appear in a future generation.
  • Characteristics can be aspects of appearance, personality, physical health and behaviour.

Dizygotic twins (non-identical twins) share similar characteristics, much like a siblings.

  • Monozygotic twins (identical twins) are the result of an embryo viably splitting early on in development. They share almost exactly the same Chromosomal DNA.
genes and schizophrenia

Genes and Schizophrenia

Researchers have looked for a particular ‘Schizophrenia Gene’ without success.

It is now thought that combinations of certain genes might make people more vulnerable to Schizophrenia, but this does not necessarily mean that they will develop the symptoms.

The evidence shows that people who have a parent with Schizophrenia are more likely to develop it themselves.

Biochemical research has been centred on Dopamine, which is one of the chemicals that carry messages between brain cells.

The theory is that an excess of dopamine, or dopamine receptors may be involved in the development of Schizophrenia.

Complete the Gottesmann and Sheildsresearch worksheet.

pass the pen activity a genetic explanation the synapse
Pass the Pen ActivityA Genetic Explanation – The Synapse

How does this explain theDopamine Hypothesis

dopamine hypothesis

The Dopamine hypothesis states that the brain of schizophrenic patients produces more dopamine than normal brains.

  • Evidence comes from
    • studies with drugs
    • post mortems
    • pet scans
disturbance in the neurochemistry
Disturbance in the Neurochemistry
  • The first discovery in the mid 1950s was that chronic usage of large daily doses of Amphetamines could produce a psychosis that was virtually indistinguishable from schizophrenia.
  • It was found that Amphetamine could enhance neurotransmission of Dopamine, Norepinephrine and (to a lesser extent) Serotonin Synapses.
  • The second discovery was that Chlorpromazine could improve symptoms for schizophrenia.
  • It was also discovered that Chlorpromazine could prevent Dopamine from activating it’s D2 receptor subtype.
  • The knowledge that Chlorpromazine improves symptoms of schizophrenia while blocking D2 receptors for Dopamine has led to the development of drugs that have similar pharmacological properties to chlorpromazine.

Elevated Levelof Dopamine In The Brain of a Schizophrenic Patient

(specifically the D2 receptor)

Normal Level of Dopamine In The Human Brain

  • Neurons that use the transmitter ‘dopamine’ fire too often and transmit too many messages or too often.
  • Certain D2 receptors are known to play a key role in guiding attention.
  • Lowering DA activity helps remove the symptoms of schizophrenia


  • Amphetamines (agonists) lead to increase in DA levels
  • Large quantities lead to delusions and hallucinations
  • If drugs are given to schizophrenic patients their symptoms get worse
parkinson s disease
Parkinson’s disease
  • Parkinson’s sufferers have low levels of dopamine
  • L-dopa raises DA activity
  • People with Parkinson's develop schizophrenic symptoms if they take too much L-dopa
  • Chlorphromazine (given to schizophrenics) reduces the symptoms by blocking D2 receptors


Falkai et al 1988

  • Autopsies have found that people with schizophrenia have a larger than usual number of dopamine receptors.
  • Increase of DA in brain structures and receptor density (left amygdala and caudate nucleus putamen)
  • Concluded that DA production is abnormal for schizophrenia


Lindstroem et al (1999)

  • Radioactively labelled a chemical L-Dopa
  • administered to 10 patients with schizophrenia and 10 with no diagnosis
  • L-Dopa taken up quicker with schizophrenic patients
  • Suggests they were producing more DA than the control group
which came first
Which Came First?

Chickens hatch from eggs, but a mother chicken must keep an egg warm in order for it to hatch

The Chicken or the Egg?

Schizophrenia or Faulty Chemicals?

Faulty chemicals cause schizophrenia but schizophrenia may cause faulty chemicals

Drugs may influence other systems that impact on schizophrenia so cant be 100% sure about their effects