Depression is classified as a mood disorder. To meet the criteria for having depression the sufferer will of have to have felt low in mood for two weeks or more and display at least five of the symptoms that are laid out by the DMS-IV. These symptoms typically include: continuous low moods and sadness, changes in appetite and weight, morbid thinking and thoughts of suicide, insomnia or excessive sleeping (hypersomnia), a considerable loss of energy or fatigue, being unable to concentrate and make even menial decisions, apathy and excessive feelings of guilt and hopelessness. (N.H.S, 2012) These symptoms can vary in severity from person to person and many other symptoms that are also linked to depression may be evident. Depression can affect anyone at any age (though it does seem to be more prominent in females.) (Stewart, 2010) The psychological causes of depression are very vast, however three theories on how depression comes about will be explored in this essay. These theories include the cognitive -behavioural model and the behavioural.
The cognitive- behavioural model has a strong focus on reinforcement (whether positive or negative) as an explanation for depression. Studies done by Seligman (1975) also state that depression can also be learned though classical conditioning. Seligman's experiment involved placing a dog in a cage that is partitioned; one side of the cage's floor has an electric current running through it that would shock the dog, the other half has no electric current. Seligman restrained the dog on the side with the electric current; over time, the dog learned that it could not escape the electric shock. Eventually he let the dog go unrestrained so it was able to escape to the side that was safe from the electric shock. However, the dogs made no attempt to try and escape and continued to endure the shocks passively. Seligman thought that this was because; the dogs had learned that they had no control over the outcome of their situation, no matter their reaction so they gave up trying.
It is plain to see how this experiment could be used in explaining why some people become depressed as it does have good face validity (it makes perfect sense.) For example, if a person has many traumatic or unpleasant throughout their life they may come to believe that no matter what they try to do, they cannot change or improve their situation. Seligman called this 'learned helplessness' which has many symptoms that are portrayed in depression. However, a critique of Seligman's work would be that he used dogs and not humans so it is difficult to say whether the experiment can be applied to humans at all. It also did not take into account whether biological causes could contribute to the symptoms the dogs showed or depression in general. Indeed, Pratt (1980) also stated that the dogs acted more like trauma victims and those suffering from Post-Traumatic Stress Disorder (PTSD) than depression. He also criticised the experiment by saying that the electric shocks could have also traumatised the dogs to the point of debilitation and being 'paralysed by terror.' Pratt also made the distinction that even though the dogs showed passivity, lower aggression and helplessness they did not show the typical signs of depression such as sadness or low mood, and in any case deciphering these moods from a dog would be particularly difficult. It could be said that in fact, Seligman's experiment is not particularly relevant to depression at all, but more to PTSD. (Kathryn Hahner, 2012) The other issue if is that of ethics, repeating the experiment now would certainly raise issues to do with cruelty to animals, as it caused the dogs suffering and that could be said even if humans were used. Another criticism that could be made of Seligman's work is that it does not explain why people blame themselves or external factors on their bad experience; neither does it explain why some people who are depressed, blame their successes on luck. Abramson (1978) expanded on this criticism, and concluded that people attribute their failures either internally (self-blame), globally (thinking their failure will affect everything they do) and in a stable manner (where they think their bad situation will last forever). Moreover, those who base their success on luck are more likely to become depressed; however, he did believe that it was these perceptions on their experiences that caused the learned helplessness. This in turn shows that Seligman's theory may be reductionist as his work on focuses on recurrent bad experiences alone that cause people to feel helpless and depressed. 
An additional cognitive- behavioural theory on the cause of depression is that of Aaron Beck (1974). He hypothesised that people with negative thoughts towards themselves, or those that have low self-esteem are far more susceptible to suffer from depression, and that the negative perceptions that they held towards themselves were built up through negative experiences. Beck assumed that experiences in childhood could lead to a cognitive triad that could lead to depression. This triad is built up of three areas in which people hold negative thoughts: the self, the world and the future. In addition, those people who have depression or are susceptible to depression magnify their bad experiences, but minimise the good experiences. He also stated that those who are likely to become depressed suffer from bias towards focussing on only negative situations, and think in a 'black and white', all or nothing fashion. Weissman and Beck (1978) started using self-schemas to try to find out how people perceived themselves and the world around them. They found that those people with negative self-schemas were far more likely to get depression. White (1985) agreed that there was enough evidence to suggest that Beck's theory was correct, yet it does not show the true causality of the depression and that he did not recognise that logical errors might be caused by biological factors, such as a chemical imbalance in the brain. (Richard Gross, 2000) Beck's theory is based on questionnaires, which leads to the question: 'can you tell whether someone is depressed just through doing a questionnaire?' His research is probably not entirely reliable as the participators of the questionnaire are affected by social desirability- people may want the assessor to like them and not to stigmatise them; so they form their answers to try to influence the assessor's view of them.
Behavioural theories of depression like that of Ferster (1960) and Lewinsohn (1974) suggest it is lack of positive reinforcement, which causes depression. For example, the loss of a loved one may cause depression due to loss of the positive reinforcement, this can also happen with the loss of a job. Lewinsohn also suggested that when others give the depressed person attention this reinforces their depressive symptoms and behaviour. Yet, in turn when there is lack of attention by family or friends and lack of reinforcement this can equally exacerbate depressive symptoms. He also suggested that those with a lack of social skills were at a greater risk of becoming depressed because they lacked social reinforcement. Lewinsohn and his colleague MacPhillamy also found that people who are experiencing depression believe that they have far fewer pleasant experiences than those who are not depressed. However, they also suggested that it was the depression, which was making people less inclined to take part in pleasant activities. This leads to question whether depression causes negative thinking and perceptions or that the negative perceptions were the cause of the depression. (McIlveen, 1996) Lewinsohn's theory is a clear example of operant conditioning as the person who has depressive symptoms then gets reinforcement for them by attention from others; this encourages the behaviour to be repeated. Conversely, there are some definite criticisms that can be made towards Lewinsohn's model. Blaney (1977) suggested that one of the main problems with the model is that it cannot be tested or proved affectively, and that Lewinsohn's theory was based on observation and that he used 'analogue subjects.' (Clark, 1999) In other words, he based his work on similar studies that have been done before on the cause of depression. Therefore, in this way Lewinsohn's model lacks objective. His theory focuses more on description of what depressive symptoms occur when someone lacks or does not engage in pleasant activities than actually pin- pointing the cause of the depression to begin with.
In conclusion, it is fair to say that there is no one cause of depression. Both the behavioural and cognitive- behavioural theories seem to have very good face validity to such that you could apply them to 'real- life' situations and case studies of those with depression. Yet, both theories do also have their flaws, the main one being that both do not focus on biological or genetic causes for depression. The NIMH (National Institute of Mental health) agrees that it is most definitely a mixture of cognitive, genetic and environment factors, that contribute to the cause of depression. (NIMH, 2009) Subsequently, all mental illnesses are complex and depression is no exception, therefore it cannot have a singular, simple explanation.