The Biological Theory of Depression (AQA GCSE Psychology): Revision Note

Influence of nature: imbalance of neurotransmitters

• Biological explanations of depression take the ‘nature’ side of the nature/nurture debate

  • One biological explanation assumes that human behaviour (such as depression) is the result of inherited traits 

  • Another biological explanation  is the role of neurotransmitters in depression

Neurotransmitters

• Neurotransmitters are chemicals that carry the message chemically across the synaptic cleft, after the signal has travelled electrically down the axon to the terminal buttons

• An action potential begins the process of neurotransmission in the form of an electrochemical impulse:

  • This impulse travels down the axon to the terminal knob of the neuron 

  • The terminal knob contains the neurotransmitter which is then released into the synaptic cleft 

• Molecules of the neurotransmitter which are not passed across the synaptic cleft are taken back up into the axon of the presynaptic neuron (known as reuptake)

• The key neurotransmitter in the study of depression is serotonin

  • Serotonin is implicated in mood disorders such as depression i.e. low levels of serotonin have been linked to depressive symptoms

  • The idea that low serotonin levels are implicated in depression is known as the serotonin hypothesis

  • Serotonin also affects memory, sleep and appetite

Evaluation of the biological explanation of depression

Strengths

• There is research support: McNeal and Cimbolic (1986) found lower levels of serotonin in the brains of people with depression compared to a non-depressed control group, suggesting a link between low serotonin and depression

• The development of antidepressant medications that target serotonin (SSRIs) has been shown to be effective in treating depression, which supports the idea that serotonin plays a role in the disorder

Weaknesses

• Low serotonin could be an effect of depression rather than a cause: negative thinking associated with depression can itself change neurons and neurotransmitter production

• Moncrieff et al. (2022) conducted a large-scale systematic review and found no consistent evidence of an association between serotonin and depression

• The theory is too simple, as some people with low serotonin don't have depression

  • The diathesis-stress model offers a better explanation: depression results from a biological vulnerability (e.g. low serotonin) combined with a stressor (e.g. divorce, bereavement), rather than low serotonin alone