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Clinical Depression » Biological Explanation of Depression

What you'll learn this session

Study time: 30 minutes

AQA spec: 3.2.4.4

The biological explanation of depression: an imbalance of neurotransmitters, such as low serotonin
How low serotonin can affect mood, sleep and appetite
The evidence for the explanation, including antidepressants
Evaluation: the chicken-and-egg problem, reductionism and ignoring life events

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Depression and the brain

Why do some people become depressed? Psychologists have more than one answer. The first answer in your specification is the biological explanation. It says that depression is caused by what is happening inside the brain. This is the influence of nature: our biology.

You already know that neurons pass messages across a synapse using chemicals called neurotransmitters (see Synaptic Transmission, Neurotransmitter Release and Neurotransmitter Reuptake). This explanation is about what happens when those chemical messengers are out of balance.

Key terms:

Biological explanation: an explanation that says mental health problems are caused by biology, such as brain chemicals.
Imbalance of neurotransmitters: too much or too little of a neurotransmitter in the brain, so messages do not pass between neurons properly.
Serotonin: a neurotransmitter that helps to control mood, sleep and appetite.
Antidepressant: a drug used to treat depression.
Reductionist: explaining something complicated using only one simple cause.

The serotonin idea

The biological explanation says that a person with depression has an imbalance of neurotransmitters in the brain. The example your specification gives is serotonin. In depression, the level of serotonin is thought to be low.

Serotonin is linked to three things that are often disturbed in depression. So low serotonin could explain the symptoms you met in Characteristics of Clinical Depression:

🙂 Mood

With low serotonin, messages that help us feel happy or calm are not passed on well. The result is persistent low mood.

😴 Sleep

Serotonin helps to control sleep. Low levels could explain sleeping far too little or far too much.

🍲 Appetite

Serotonin also affects hunger. Low levels could explain eating much less or much more than usual.

Say it in one line

Low serotonin in the brain means messages between neurons are not passed on properly, and this leads to low mood and changes in sleep and appetite.

Evidence for the explanation

The main evidence comes from antidepressants. Some of these drugs raise the level of serotonin in the brain. Many people who take them feel better. If raising serotonin helps, it suggests that low serotonin may be part of the problem. (How these drugs work is covered in a later lesson on treatments, so you only need the idea here.)

Worked example: using the evidence

Question: How does the use of antidepressants support the biological explanation?

Answer: Some antidepressants raise serotonin levels (1). Many people who take them feel less depressed (1). This suggests that low serotonin was linked to their depression (1).

Evaluating the explanation

In the exam you may be asked to say how good an explanation is. Use these points.

✅ Strengths

It is supported by evidence, because antidepressants that raise serotonin help many people.

❌ Weaknesses

The cause and effect may be the wrong way round. It does not explain why the imbalance starts. It is reductionist. It ignores life events.

The chicken-and-egg problem

Which came first, the chicken or the egg? Depression has the same problem. Does low serotonin cause depression? Or does being depressed cause serotonin to drop? Evidence that the two go together does not tell us which way round it is. This is the chicken-and-egg problem.

More evaluation points

It does not explain why the imbalance starts. Even if low serotonin is involved, why does one person's level fall and not another's?
It is reductionist. It reduces a complex problem to one chemical. Depression is unlikely to have only one simple cause.
It ignores life events. Many people become depressed after something happens, such as losing someone close or being bullied. Chemistry alone cannot explain why this happens when it does.

Nature or nurture?

This explanation puts all the weight on nature, which means biology. The next lesson looks at the psychological explanation, which focuses on nurture, the effect of experiences and the way we think. A good answer can say that both may matter, and that nature and nurture may work together.

Common mistakes

Saying that low serotonin is proven to cause depression. The evidence only suggests a link.
Forgetting to give examples of what serotonin affects: mood, sleep and appetite.
Writing "it is reductionist" without saying why. Add that it explains depression with one chemical and ignores other causes.
Mixing this up with the psychological explanation. This lesson is about nature (biology), not thoughts or experiences.

Exam-style question

(a) Outline the biological explanation of depression. [3 marks]

(b) Give one piece of evidence for the biological explanation of depression. [1 mark]

Model answer

(a) Depression is caused by an imbalance of neurotransmitters in the brain (1). For example, serotonin levels may be too low (1). This can affect mood, sleep and appetite (1).

(b) Antidepressants that raise serotonin levels help many people with depression (1).

(c) A strength is that antidepressants help many people, which supports the idea of a chemical cause (1) and shows that drug treatment can work (1). A weakness is the chicken-and-egg problem: low serotonin might be a result of being depressed, not the cause (1). It is also reductionist because it ignores life events that can trigger depression (1).

Exam tip

For an evaluation question, name a point, then add a short reason. "It is reductionist" gets less credit than "it is reductionist because it ignores life events".

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