Clinical Depression: Characteristics and Theories

Depression and Everyday Sadness

Everyone feels sad or low from time to time — following a bereavement, a disappointment or a difficult period at school or work. This is a normal emotional response. Clinical depression is different. It is a persistent, debilitating condition that significantly impairs how a person thinks, feels and functions. The distinction matters because clinical depression requires professional assessment and, often, treatment.

Unipolar and Bipolar Depression

The spec identifies two main forms of clinical depression. Unipolar depression involves persistent low mood in only one direction — downward. A person with unipolar depression experiences prolonged periods of sadness, loss of interest and reduced energy, but does not experience episodes of abnormally high or elevated mood.

Bipolar depression involves cycling between episodes of very low mood and episodes of mania — abnormally elevated or irritable mood, increased energy, reduced need for sleep, and sometimes impulsive or risky behaviour. The shifts between these extremes can be rapid or may develop over weeks.

Key Fact

Unipolar depression: persistently low mood only. Bipolar depression: alternating episodes of very low mood (depression) and very high mood (mania). The key difference is that bipolar includes the manic phase; unipolar does not.

Diagnosing Unipolar Depression

The International Classification of Diseases (ICD) is the diagnostic framework used to classify and diagnose unipolar depression. Diagnosis is not based on a single symptom but on the number and severity of symptoms present over a given period. The key symptoms the spec identifies are listed below.

Low mood: a persistent feeling of sadness, emptiness or hopelessness that does not lift for most of the day, most days.

Reduced energy levels: feeling fatigued and lacking the motivation or physical energy to carry out normal tasks, even simple ones.

Changes in sleep patterns: this can mean difficulty falling asleep, waking in the night, or sleeping far more than usual. Both insomnia and hypersomnia are associated with depression.

Changes in appetite levels: this may involve eating significantly less or significantly more than usual, often leading to noticeable weight change.

Decrease in self-confidence: a persistent sense of worthlessness, self-criticism or feelings of being a burden to others.

A diagnosis of unipolar depression requires that these symptoms are present in sufficient number and severity and that they represent a change from the person’s usual functioning. The ICD criteria prevent a clinician from diagnosing depression based on a brief, understandable reaction to a life event.

Biological Theory: The Influence of Nature

The biological explanation of depression focuses on what is happening inside the body. It proposes that depression results from an imbalance of neurotransmitters in the brain. In particular, the neurotransmitter serotonin is thought to play a central role. Research has linked lower levels of serotonin activity in the brain with the symptoms of depression, including persistent low mood, disrupted sleep and reduced appetite.

This is a “nature” explanation because it locates the cause of depression in physical, biological processes. It supports a reductionist approach: it reduces the experience of depression to brain chemistry. Antidepressant medications that increase serotonin availability — particularly SSRIs (selective serotonin reuptake inhibitors), which work by blocking the reabsorption of serotonin after it crosses the synapse — are effective for many people. Their effectiveness is often cited as indirect evidence supporting the serotonin theory: if raising serotonin activity reduces symptoms, this suggests low serotonin plays a role. However, the biological approach has limitations — not everyone with depression responds to SSRIs, which suggests brain chemistry alone cannot fully account for the condition.

The psychological explanation of depression focuses on the role of thought patterns and personal history — a “nurture” account. Aaron Beck (1979) proposed the cognitive triad: three interconnected patterns of negative thinking that maintain depression. These are a negative view of the self (“I am worthless”), a negative view of the world (“things always go wrong”), and a negative view of the future (“things will never improve”). These three perspectives reinforce each other and drive the depressive state. More broadly, the psychological approach proposes that depression arises from negative schemas and attributions. A schema is a mental framework for interpreting the world; a negative schema leads a person to interpret experiences in a consistently pessimistic and self-critical way, even when evidence for a more balanced view exists. These patterns tend to operate automatically — the person does not choose to think negatively, but finds their mind defaulting to worst-case interpretations.

Attributions are explanations a person gives for events. People who attribute failures to internal causes (“it is my fault”), stable causes (“this will always be true”) and global causes (“this affects everything in my life”) are more vulnerable to depression than those who make external, unstable and specific attributions (“that particular situation went wrong this time”). These negative thinking patterns can develop through early experiences of criticism, loss or failure, and once established they become self-reinforcing: a person in a low mood interprets new events negatively, which deepens the low mood further. The psychological approach is classified as a “nurture” explanation because it emphasises the role of experience and learning in shaping thinking styles.