Depressive disorder is sometimes a reaction to a single or severe stressor but depression in young people is more likely to arise from an accumulation and interaction over years of biological, psychological, and social factors. These factors combine:
Genetic influences (static factors): These are probably less important in uni-polar depressive conditions than in bipolar depressive conditions accounting for about 50% of the variation in depressive symptomatology in adolescents. Studies have suggested that genetic factors are often indirect, acting through temperamental traits that increase vulnerability to stress. They may also increase the risk of psychiatric disorders e.g. anxiety and conduct disorders which are in themselves risk factors for depression.
Chronic Adversity Factors (stable dynamic factors): Includes both intra-familial and extra-familial factors. The most important familial factors are parental depression and other psychiatric disorders, family discord, and parenting problems including abuse. The most important extra-familial factors are peer relationship problems (including bullying) and substance abuse.
Precipitating Stressful Events (acute dynamic factors): These most commonly arise from the chronic stressors that occur with adversity and include family problems (e.g. parental separation), breakdown in a friendship or relationship with a peer, episodes of parental mental illness. The effects of acute stressful events depends to an important extent upon their context and the degree of threat that they represent to the child and their well being e.g. the death of a grandparent has much more impact on a child if the grandparent was the primary carer or an important source of support for the child.
These factors act through psychological and biochemical processes to produce the depressive syndrome which, once established is often prolonged by maintaining factors e.g.
any of the risk factors cited above
psychological or biological "scarring" arising from the first depressive episode
the symptoms of depression itself (such as sleep disturbances which leads to poor concentration, which in turn worsens negative thinking)