There are many different factors that can trigger depression. For some people, upsetting or stressful life events, such as bereavement, divorce, illness, redundancy, and job or money worries, can be the cause.
This is often known as 'reactive depression', where depression is a reaction to the event. In other cases, depression does not have an obvious cause.
As depression can have many causes, it is sometimes divided into three broad groups - psychological, physical and social.
If you have a family history of depression, you are more likely to get depression yourself.
Studies have shown that different versions of a gene (known as 5-HTT) can be inherited, which can have an effect on a natural mood-changing chemical in your brain called serotonin. About 20% of people have got what geneticists (gene specialists) call the 'short' version of the 5-HTT gene, and it is these people who are more likely to develop depression after a stressful life event.
Research has shown that there is a link between depression and the imbalance of chemicals in the brain called neurotransmitters. People who are depressed have a lower level of certain types of neurotransmitters, such as serotonin, norepinephrine and dopamine, than people who are not. It is still not fully understood whether this imbalance is a result of depression, or a cause of it.
Children inherit the 5-HTT gene from their parents, so having a history of depression in the family can increase your risk of developing it. In other words, you have a 'genetic predisposition' for depression.
On the other hand, many people who have a family history of depression never develop the condition. Also, people with no family history of depression can become depressed.
There is rarely one single cause of depression - usually, different causes combine to trigger the condition. For example, you may feel low after an illness and then experience a stressful life event, such as bereavement, which leads to depression.
People often talk about a 'downward spiral' of events that lead to depression. For example, if a person's relationship with their partner breaks down, they are likely to feel low, they may stop seeing friends and family, and they may start drinking more - all of which can make them feel even worse and trigger depression.
Other causes of depression
Other frequent causes of depression include: