What are the causes of depression?
Families (genetic predisposition)
As with many other diseases, a person's vulnerability to depression is clearly influenced by familial factors. Research is attempting to identify the role that genes play in causing depression. The exact way in which genes affect mood is not yet clear, but twin studies show that in a pair of identical twins (i.e. the same genes), mood disorders (including depression) occur in both twins in 33-75% of cases and in 9-23% for non-identical twins (i.e. share only some genes). This clearly shows that there is an inherited vulnerability to development of mood disorders, but also demonstrates that other factors must also operate in order for this vulnerability to be realized. The situation is similar to the complex causal issues in the emergence of various cancers and different forms of cardio-vascular disease.
Family studies of people with mood disorders show that the first-degree relatives of people with major depression (children, brothers, sisters, parents) have a 6%-17% risk of developing major depression themselves.
Alcohol and other drugs
Various drugs, including alcohol, interfere with the normal balance of brain chemicals. Twin studies have suggested that genetic factors influence the risks for developing both alcoholism and depression. The theory is that while different genetic factors influence the vulnerability to alcoholism and to major depression, they frequently act together to increase the likelihood of developing both of these conditions.
People who misuse various other substances are also more likely to become depressed because these drugs may also affect mood negatively. In addition drugs when abused can be, and are, associated with behaviors (getting into fights, being arrested, running up large debts or losing a driver's license), which increase life stress.
Alcohol is probably the most commonly abused drug in the world and much research has been devoted to studying its relationship with depression. It appears that someone with a significant alcohol problem has a risk of developing depression that is as much as five times higher than the risk faced by the general population. It also seems that women with problems with their alcohol use are at greater risk of depression than men.
Research has also shown that small amounts of alcohol may temporarily relieve depressive symptoms, but that larger amounts, consumed over time, will seriously worsen them. People with mood problems frequently try to use substances to improve their mood. This may be partially effective in the short term but makes things worse eventually. Chronic intoxication is associated with substantial increase in the frequency and severity of depression and also increases the risk of suicide.
These facts have clear practical implications: people with depressive symptoms, occurring in the context of high alcohol consumption should stay off alcohol, while those who receive treatment for depression, but continue to drink, are much less likely to recover from their depression. Depressed people are also more likely to act impulsively, e.g. self-harm or commit suicide when drunk.
Brain chemicals (Neurotransmitters)
Neurotransmitters are brain chemicals which are involved in sending messages from one part of the brain to the other. There is good evidence that disturbances in the levels or the availability of some of these substances contribute to the development of depression.
There are two neurotransmitters that are thought to be of particular importance. These are called norepinephrine (or noradrenaline) and serotonin (or 5-hydroxytryptamine or 5-HT). Antidepressant medications act by increasing the availability of either or both of these neurotransmitters at certain sites in the brain. For example, SSRIs raise levels of serotonin and venlafaxine raises levels of both serotonin and norepinephrine.
There are many other classes of neurotransmitter that have been investigated in mental disorders (including dopamine, acetylcholine, and gamma-aminobutyric acid) but there is less evidence that these have a role to play in depressive illness.
Physical illness
Sometimes the symptoms of a recognizable physical illness will resemble or include the symptoms of depression. These conditions are often neurological (brain and nerve related) but also include several endocrine (hormonal) and metabolic conditions (such as diabetes mellitus).
Also, some medications used to treat certain various conditions may cause depression as a side effect, e.g. the effect of certain drugs used to treat raised blood pressure. In other cases, treatment of a specific and identifiable disease may resolve the depression, as may occur, for example with successful treatment for thyroid dysfunction (hypothyroidism).
Depressive illness therefore, in some cases, can be seen to be a component of specific physical illnesses with now well-understood causes. In other cases, as noted above, the causal circumstances for depressive illness are, however, complex and by no means as well understood.
Depression may also develop, as a secondary consequence, in people who are physically unwell, particularly with long-standing (chronic) conditions where pain or disability is prominent. It can be difficult to decide which symptoms (such as weight loss or tiredness or sleep disturbance) are related to the illness and which have developed as a consequence of depression, but if the person has low mood and loss of interest in things around them, depression is likely and should be appropriately trusted.
Hormones
Hormones are substances produced and released by glands in the body, for example, thyroid hormone -thyroxin- from the thyroid gland in the neck. Such substances enter the blood-stream and are carried to all parts of the body. They act as chemical messengers, producing specific physical effects in various organs including the brain.
A number of brain-active hormones are known to influence mood and to be involved in causing depression. Cortisol, which is released from the adrenal glands situated just above the kidneys, is probably the most studied of these mood-influencing substances and its blood-levels are sometimes found to be raised in people with depression. Other hormones potentially involved in depressive illness include thyroxin (from the thyroid), growth hormone (from the pituitary - a gland that is attached to the base of the brain), and the male and female sex hormones (such as testosterone and estrogen), which come from glands called the gonads - the testes in men and the ovaries in women. Generally, reduced levels of these hormones may be linked to depression.
Stress and grief
Stress is a part of life but most stressful life experiences do not lead to the development of depression. Depression is not the same as being sad, worried or upset about a difficult episode in your life and the distinction needs to be kept very clear.
However, major life events that are associated with various forms of loss, and/or with high levels of stress, such as the breakdown of a significant emotional relationship (divorce or separation), loss of a job, or being diagnosed with a serious illness (which is associated with loss of health) may result in the development of depression, especially in those who are predisposed to or are otherwise vulnerable to the disorder, e.g. because of other physical illness or have a genetic vulnerability. Grief and depression may both involve similar symptoms, e.g. sadness, crying, loss of weight and appetite, withdrawal from social interactions, but they are not the same.
Grief is a normal reaction to the loss of a loved one or some other important part of one's life. It generally follows a course that, put simply, involves phases of denial and shock, preoccupation and resolution. It resolves with the passing of time (usually over weeks).
Depression on the other hand is a mental illness that requires specific treatment and will not necessarily just get better with time. Usually certain symptoms are more severe than in a normal grief reaction, and continues for longer. In some instances, grief may evolve into depression. This may occur when the loss is sudden, violent or unexpected, e.g. following the accidental death of a child, or when the grieving person has a previous history of depression.
Personality
Personality is a rather difficult concept to define. Personality describes the habitual and characteristic emotional and behavioral patterns that a person uses in his/her social and personal interactions, and is relatively consistent over time.
Research into the links between personality and depression has identified a set of characteristics called 'neuroticism'. These characteristics include the tendency to be anxious, emotionally unstable, self-conscious and easily upset. 'Neuroticism' appears to be a marker of vulnerability to depression.
Depression, when present, also tends to cause certain aspects of a person's personality to become exaggerated. Anxiety or fussiness or irritability may become more prominent, making the person more difficult, or quite changed in his/her behavior and relationships. Such 'personality changes' are usually reversed with treatment. Depression is a treatable disorder, and does not cause long-term 'damage' to a person's personality.
Child abuse
People who have experienced traumatic events early in life, such as childhood abuse (physical, emotional or sexual abuse), may be vulnerable to depression as a consequence. The outcome of such abuse depends on a number of factors including the nature, severity and duration of the abuse, as well as the child's other vulnerabilities and strengths, and the availability of effective supports.
It is thought that survivors of childhood abuse are at increased risk of depression as well as a number of other mental health problems, including substance abuse, relationship difficulties and personality problems. However, this varies a great deal and many people - perhaps the majority - with traumatic childhoods are well-adjusted and emotionally healthy as adults.
