I've had an attempt at doing a 24 mark essay condensing and integrating all the explanations of depression together in the unlikely event that a question like this might come up in the exam. I've basically repeated everything I've already posted about the explanations but shortened and combined it together, as well as adding a paragraph on the cognitive explanations of depression at the end. I'm assuming this is enough Ao1 and Ao2 for 24 marks:

Explanations of Depression using Empirical Evidence (8+16)

▲ Biological   ▲ Psychological   ▲ Cognitive

 Biological explanations of depression explain how children can inherit different types of the 5H77 gene, which is responsible for the transmission of serotonin. Therefore, having a history of depression in the family can increase ones chance of getting it. This is known as Genetic Predisposition.
 Much research has been conducted to support the biological explanation that depression can develop due to genetic inheritance. Wender et al studied adopted children and found that their biological relatives were 7 times more likely to develop depression than the adoptive relatives. These findings suggest that there is a genetic factor in depression, and although other adoptive studies have found a relatively weak correlation studying this, they have found a similar result.
 Additionally, Harrington et al has presented supporting research, finding that people who share a 50% genetic relationship with someone who has depression have a 20% chance of developing the disorder. This shows strong evidence to confirm the proposals of the genetic explanation of depression, as like Wender it accepts the basic prediction of genetic theory. Nevertheless, Harrington et al's  study was compared to only 5% of the general population, therefore making it hard to generalise as a whole. 
 Using twins has also proven to be very effective in investigating how genetic factors link to depression, where one twin has the disorder and to investigate the likelihood (concordance rate) of the other twin having it. Supporting evidence comes from McGuffin et al, who found a 46% concordance rate in Mz twins, compared to 20% among Dz. Both these rates are dramatically higher than the general life time risk of developing depression, and highlight strong indications that depression can be genetically inherited. However the fact that these concordance rates are not 100% shows that genetics cannot be the sole explanation of depression.

 Psychological explanations of depression centre around Freud's Theory of The Unconscious Mind and The Structures of Personality. His original view linked depression to the oral stage of development (0-18 months); children whose needs are not met by their parents at this stage can become over-dependant on people, which makes them more vulnerable to experiencing depression late in life. Freud further argued that depression is like grief as it occurs as a reaction to the loss of an important relationship. He distinguished between 'actual' and 'symbolic' loss, which can both lead to depression by causing an individual to re-experience childhood episodes where they experienced the loss of affection from a significant individual.
 This theory can be supported by studies from Waller et al, who found that men who had lost their fathers during childhood scored higher on the depression scale than those whose father did not pass away. Similar to this Bifulco found  that children whose mothers died during childbirth were more likely to experience depression in later life.  Although this study cannot support that depression occurs due to the loss of an important relationship (it is not possible for a child to develop an emotional bond with their mother during childbirth), it does however support Freud's theory that unmet needs from the parent during the oral stages of development can root to depression.  
 Furthermore, Kendler et al can be used to support Freud's assumptions, finding that female twins who had experienced parental loss through separation had an above average tendency to experience depression in later life, confirming the basic predictions of Freud's theory that symbolic loss can lead to depression. 

 The cognitive explanation of focuses on Beck's Cognitive Triad, which suggests that depression is caused by faulty cognitions; negative thoughts about ones self, the world and the future. The individual can become trapped by these negative thoughts, which can eventually lead to depression. It is also thought that we can develop cognitive distortions, such as overgeneralising things and 'selective abstraction' where an individual will focus only on one single aspect of a situation and ignore the others.
 Beck developed his theory into two schemas to characterise depression: sociotropy,  which involves basing self-esteem on the approval of others, and autonomy, which involves basing self-esteem on success and achievements.
 In support of this explanation, Evans et al conducted a prospective study and found that women with the highest scores for negative beliefs were more likely to become depressed than those with lower scores. This of course accepts the assumption from Beck that faulty cognitions are a factor of depression.
  Practical applications from Butler and Beck can also be used to support this approach. Through reviewing 14 meta-analyses on Beck's cognitive behavioural therapy they concluded that 80% of adults benefitted from it. It was also found to be more effective than drug therapy, and had a low relapse rate. This supports the predisposition that depression has a cognitive bias, and therefore suggests that knowledge of the cognitive explanation of depression can improve the quality of people's lives.