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General definition: affective (mood) disorder characterised by feelings of sadness and general withdrawal (oxford psychology dictionary)

Two main diagnostic manuals used:

Diagnostic and Statistical Manual of Me ntal Disorders (DSM)
Only available in English Language.
Diagnosis is based on a number of axes which include information about the biological, psychological and social aspects of a person’s condition.

DSM can classify three types of depression:
1. Major Depressive Depression (MDD) or Unipolar Disorder – most common and most severe
Reactive – triggered by external events e.g. death of lovedone, unemployment.
Endogenous – triggered by internet events e.g. family history, negative thinking

2. Dysthymic disorder – not as intense but lasts longer
3. Manic-depressive disorder (bipolar disorder) – moods alternate between manic (extreme excitement) & depression.

International Classification of Disease (ICD)
Mostly used in UK.
Available in most world languages
Not just for psychological disorders

Interesting facts!
DSM & ICD lay out specific symptoms/criteria with indication of severity etc for each psychological disorder
Both overlap quite but may be slight differences in symptom lists & disease names e.g. either classification system has a disorder called ‘psychopathy’. DSM has ‘antisocial personality disorder (APD)’, ICD-10 uses the term ‘sociopathy’ or ‘dissocial personality disorder’.
Prevalence – about 17% of the population suffer from depression at some point in their lives.
Women are twice as likely to suffer as men.


For major depression, the patient must show five (or more) of the symptoms for longer than two weeks (must include either symptom 1 or 2):
1. depressed mood
2. loss of interest
3. weight/appetite change
4. sleep disturbance
5. slowness/restlessness
6. fatigue and loss of energy
7. guilt/feelings of worthlessness
8. lack of concentration
9. thoughts of death

Severity of depression is measure using The Beck Depression Inventory (BDI). This is a 21-item self-report diagnostic tool designed to measure the severity of symptoms in individuals with depression. Each question assesses a specific symptom. Sum of all BDI item scores indicates the severity of depression thus BDI is able to distinguish between different types of depression.


Biological explanations have focused on several areas such as genes and biochemistry.
Genetic explanations assume depression has a genetic basis which might not be due to one gene only but a number of genes that predispose a person to depression.
The genes may begin their expression later or in an individual’s development which makes them more sensitive to negative life experiences throughout their life
If this explanation is valid then we would expect a higher rate of depression amongst genetically similar relatives of sufferer because of genetic predisposition.

Family studies select people who already have depression and examine whether other members of their family have been, or might be, diagnosed with depression. If there is a genetic link, relatives should show higher rates of depression than rest of population.
Harrington et al. (2003) found that 20% of the relatives of a sufferer have depression, compared to a figure of around 10% for the population at large. In order words, first degree relatives are twice more likely to develop depression than the general population.
This suggests that genetics do play a role in depression because as the study shows if there is genetic predisposition then depression is very likely to occur.
However, it is not clear whether these findings are due to genetic influences or to a shared environment because family studies often show inconclusive results.

The best way of disentangling genetic factors from environment is to look at adoption studies. They examine