1. Evaluation of Ethiology
Depression
Evaluation of Ethiology

2. Genetic Cause Strengths
Twin studies have been highly reliable in their results.
Modern research has allowed us to actually locate the genetic variations using very large sample sizes.
Modern research recognizes the interaction of environmental and biological factors.
Limitations
Correlational studies do not establish a causal relationship.
In twin studies there is a problem with population validity - samples are so small that there is difficulty in generalising results. In addition, the life of a twin may be different from the experience of the general population.
It is impossible to isolate variables and to separate out social factors.
The argument is reductionist in nature and does not account for the variations in the symptomology of depression.
It is not yet clear how the 11 genetic markers interact.

3. Biochemical Strengths
There are several longitudinal case studies and animal research which supports the theories.
Practical application of the theories have led to successful drug treatments that have improved people's lives.
Limitations
Correlational research means that causation cannot be established and bidirectional ambiguity cannot be resolved.
The Treatment Aetiology fallacy - that is, the mistaken notion that the success of a given form of treatment reveals the cause of the disorder.
Biological explanations cannot explain the range of symptoms associated with depression. There may be cultural and cognitive factors as well. The biological explanation can be seen as a reductionist approach to explain a complex human behaviour.

4. Beck’s Theory
One of the strengths of the theory is that there is significant evidence that Beck has accurately described the thinking patterns of depressed patients. These include:
Beck's own clinical observations.
Self-report questionnaires given to depressed patients.
Laboratory studies on memory bias
Studies of memory bias were carried out by Ruiz-Caballero & Gonzalez (1994) on depressed and non-depressed college students.
They gave participants a word-stem completion task to see whether in solving the task they would recall words with a positive or negative connotation.
The results indicated that depressed participants showed a memory bias for negative words.
In general, research shows that when depressed individuals are given lists of words that vary in emotional content, they tend to recall more negative words than do non-depressed individuals.

5. Beck’s Theory
Another strength of the theory is that highly successful therapy has been developed based on this theory which has led to improvement in a large number of patients.
One of the limitations of the theory is the problem of bidirectional ambiguity. Hammen argues that negative cognitions may be the result of rather than the cause of depressed moods.
Beck himself argues that the relationship is bidirectional: depression can make thinking more negative, and negative thinking can probably cause and certainly worsen depression.
The best way to resolve this issue is to carry out prospective case studies. These, however, are difficult to organize. One study by Joiner et al (1999) gave a questionnaire to university students before mid-term exams. Those who had negative thoughts before the exam who ended up doing poorly showed an increase in depressive symptoms. However, those that did well, did not. This shows that cognition must also interact with environmental stimuli in order to result in depressive behaviour.

6. Vulnerability Model
There has been extensive research on the role of stress on depression. The use of prospective longitudinal research has led researchers to believe that there is a cause and effect relationship. Often socio-cultural studies engage in method as well as researcher triangulation in order to increase the validity of the findings.
Vulnerability models have been used to explain both gender and class differences in the prevalence of depression.
One of the limitations is how we measure "stress."
Holmes & Rahe's Social Readjustment Rating Scale has ranked the stress levels of different life events to help determine how much stress an individual has experienced in the past year.
Although this assessment is frequently used, there is evidence that shows that smaller stressors - what Kanner (1981) called "hassles" - may play an even more significant role in one's overall mental health. This could be because larger life events can eventually be interpreted and seen as having significance or meaning, whereas the hassles are seen as an irritation and without purpose.
As with the other levels of analysis, it is difficult to argue social stressors are the cause of depression. It is only with the interaction of biological factors that the stressors appear to lead to the disorder. It is difficult, but not impossible, to carry out research that examines the interaction of biological, cognitive and socio-cultural factors.