The biological model
The biological or medical model sees psychological problems as illnesses with physical causes. This viewpoint would suggest that people with a particular genetic makeup are seen to have a predisposition.
Depression often runs across generations in families, which suggests a possible genetic component. The closer the genetic relationship, the more likely people are to share a diagnosis of depression. First degree relatives - brothers, sisters, sons, daughters, fathers and mothers - share 50% of their genes. First degree relatives of people diagnosed with depression are two or three times more likely to receive a similar diagnosis than first degree relatives of those who have not received a diagnosis of depression (Oruc et al, 1998).
Monozygotic (MZ) twins - identical twins - share 100% of their genes, whereas Dizygotic (DZ) - non-identical - twins share around 50% of their genes. Studies support the idea that a higher proportion of identical twins share the disorder. In a study of nearly 200 pairs of twins it was found that when an MZ twin was diagnosed with unipolar disorder, there was a 46% chance that the other twin would receive a similar diagnosis, whereas the figure for DZ twins is 20% (McGuffin et al, 1996).
Research from families and twins are limited by the fact that they usually share the same environment. However, this problem has been partly overcome by adoption studies. These studies compare people who have been adopted at an early age with their biological and adoptive relatives. Since they were raised apart from their biological relatives, similarities with those relatives would indicate genetic influences. Adopted children who later develop a mood disorder appear to be much more likely to have a biological parent who has a mood disorder, becomes alcoholic or commits suicide, even though the adopted children are raised in very different environments (Wender et al, 1986).
Most studies of adoptees diagnosed with depression show that their biological relatives are more likely than their adoptive relatives to have a similar diagnosis: 20% of their biological relatives compared with 5-10% of their adoptive relatives, which is the rate for the general population (Harrington et al, '93).
DNA markers have been used to identify the gene or genes involved in mood disorders. This viewpoint investigates the inheritance of mood disorders within high-risk families and then searches for a DNA segment that is inherited along with a predisposition to develop the disorder. In a study within the Amish community in Pennsylvania, four extended families were found to have a high likelihood of developing depression (Egeland et al, 1987). From the 81 people studied, fourteen were diagnosed as having bipolar disorder. This disorder appeared to be linked with a specific genetic marker on chromosome 11. However, later research, both within the Amish Community and in other populations, failed to replicate this finding (Kelsoe et al, 1989). This suggests two possibilities: the gene for bipolar disorder may not actually be on chromosome 11, or several genes play a role, only one of which is on chromosome 11. The second possibility is supported by the observation that a gene on the X chromosome has also been implicated in bipolar disorder (Nemeroff, 1998).
Neurotransmitters are the focus of research into the biochemistry of depression. Genes act by directing biochemical events. Biochemical processes may play a causal role in affective disorders that involve imbalances in the neurotransmitters serotonin and noradrenaline.
Antidepressant drugs are often effective in reducing the symptoms of unipolar disorder. One group of antidepressant drugs increases the levels of the neurotransmitters seretonin and noradrenaline. This finding formed the basis of one of the main biochemical theories of depression. Schildkraut (1965) proposed that too much noradrenaline at certain sites causes mania, whereas too little causes depression. Later research suggests that serotonin plays a similar role.
It is not exactly known, however, how these neurotransmitters affect depression. One theory suggests that low levels of both neurotransmitters can lead to depression, another theory states that low levels of either neurotransmitter can lead to depression. Yet another theory suggests that it is the balance between these neurotransmitters and their relationship to other neurotransmitters that contributes to depression (Barlow et al, 1999).
It may be that hormones such as cortisol are also implicated in depression. These hormones are regulated by neurotransmitters. Recent research indicates that the relationship between hormones and neurotransmitters needs to be examined in order to understand the contribution of both to depression (Ladd et al, '96).
These theories usually start from the idea that depression is related to some loss experienced in early childhood. Freud thought there was a similarity between those suffering from depression and the response to the death of a loved one. Both involve extreme sadness, loss of appetite, disturbed sleep and withdrawal from social life.
Freud thought that people were either under or over-gratified during their infant years, either receiving too little or too much love from their parents. According to Freud, both experiences involve loss. Those that received too little love feel unworthy and consequently have low self esteem. Those who received too much love want the experience to last all their lives, yet the love they received as a child is not forthcoming as they grow older. Both seek to compensate for their loss and become overly dependent on others for love, affection, self worth and self- esteem. It may be that a loss in adulthood brings back the loss people experienced in childhood, which enhances their feelings of rejection and unworthiness. Blatt suggests that depression can be seen as an appeal for love and security.
Freud suggests that depression also involves guilt and self-criticism, which can be related to loss during childhood. Anger or hostility may be directed at parents if they do not give the child the love and warmth that is needed for healthy development. However, these destructive feelings are turned inwards and the child blames itself for the loss, which leads to guilt and self-criticism. The loss of a loved one in adult life may bring back these childhood experiences. The adult may also blame the loved one for dying or deserting them and feel anger as a consequence. This anger may then be internalised, leading to guilt that may usher in depression.
Obviously some people become depressed without losing a loved one. Freud explained this by suggesting the loss can be imagined or symbolic. They may for example feel that unless they meet certain perceived expectations they will be rejected and unloved.