ALP 2.1 Biological etiology of MDD

The following lesson looks at biological theories with regard to Major Depressive Disorder. The lesson starts by finding out what students already know and surfacing any misconceptions about the disorder.  It also involves "meeting" some people with the disorder before delving into the biological origins of depression.

Remember that the research used here can be applied on Paper 1.  This includes research on genetics, neurotransmission and hormones.

Defining depression

First, I have students brainstorm what they know about depression. There are several ways to do this. Usually, I have them come up to the white board and do a "grafitti wall" - that is, they write what they know on the board, always aware of what has already been added. Then we read through the list without making any comments about their validity.

Another way to do this is to have them take an online knowledge test. There are several. It is important that they take a test that is about knowledge of depression - and not a self-diagnostic test. Here is one that is rather good:  Quibblo - how much do you know about depression?

After that, I give them the handout below. We first of all review the terms: affective, behavioural, cognitive and somatic. Then I have them work in threes to do two things. First, they need to figure out what the symptom is that is being investigated. Secondly, they need to classify the symptoms to give them a full picture of the disorder.

Student copy

One of the outcomes of this activity is that they realize that it is often difficult to classify a symptom in these four categories. That is part of the difficulty of trying to establish definitions in psychology - and is worth a discussion. Here is a quick overview of the symptoms that are being identified.

  1. Feelings of sadness (affective)
  2. Sluggishness/disorientation in the morning. (somatic/affective)
  3. Crying spells (behavioural)
  4. Insomnia - or hypersomnia (behavioural/somatic)
  5. A change in eating habits (behavioural)
  6. Feeling unattractive (cognitive) or social withdrawal (behavioural)
  7. Feelings of sadness (affective)
  8. A change in eating habits (behavioural/somatic)
  9. Digestion problems (somatic)
  10. Fatigue (affective/somatic)
  11. This is not a depressive symptom. This is for an anxiety disorder.
  12. Lack of cognitive clarity (cognitive)
  13. Change in habits (behavioural); feelings of apathy (affective)
  14. Often restlessness - often associated with anxiety disorders - is an outcome of depression. (Affective/behavioral)
  15. Pessimism (cognitive)
  16. Poor decision making (cognitive) or apathy (affective)
  17. Poor self-worth (cognitive)
  18. Apathy (affective) or lack of physical activity (behavioural)
  19. Poor self-worth (cognitive)
  20. Irritability - like restlessness, often associated with anxiety disorders, but also commonly seen in depressive patients. (affective)

    Building empathy

    After completing this activity, I like to show students parts of "Depression: Out of the Shadows."  This video documentary has several different cases of how people may manifest the disorder.  Here is the first section of the video.  More can easily be found on Youtube. When watching the case studies, ask students which factors they feel may be playing a role in the different symptoms shown by the different people.

      Biological etiologies of Major Depressive Disorder

      The following presentation is based on the reading: Biological approach to depression  The presentation is meant to be interactive to get students to think about the research, but also to review some key concepts from the core.

      Evaluation of biological etiologies

      I have students finalize this lesson by having them create a chart with strengths and limitations of biological arguments.  Here is a list of some potential answers.

      Evaluation of biological theories of depression


      Several studies that support the theories that have been replicated.

      Practical application of the theories has led to effective treatments.

      Modern biological research takes a more holistic, integrative approach, including the effect of environmental factors (such as ACE's) and mediating cognitive factors.


      Often dependent on animal models

      It is not possible to measure biological factors directly

      Much of the research is correlational in nature, not allowing for causation to be determined.

      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 (e.g. lower back pain in Chinese patients). There may be cultural and cognitive factors as well.

      Some biological explanations can be seen as a reductionist approach to explain a complex human behaviour.

      Summarizing the lesson

      The following worksheet is to help students test their own understanding of the lesson.

      Biological explanations of MDD worksheet

      Here are some potential answers to the questions.

      1. What is a GWAS study? Why are they important in modern research?

      This is a study that looks for gene variation in a very large sample with regard to a specific behaviour.  These are important because they are able to access large amounts of data and to then account for outliers.

      2. What is one limitation of using a twin study to investigate the etiology of depression?

      Twins are not representative of the general population. There is also an assumption made that they live in the same environment and that they experience the same stressors, but this is not necessarily the case.

      3. How do psychologists measure the level of serotonin in the brain?

      This is one of the problems with the Serotonin Hypothesis - it is not possible to directly measure levels of serotonin in the brain. We can do so by looking at indirect markers, like urine or spinal fluid. We can also look at the number of serotonin neurons in a brain post-mortem.  Finally, we can induce lower levels of serotonin through dietary changes, but this is not a precise measurement.

      4. Why do many psychologists believe that levels of serotonin is not an adequate explanation for the origins of depression?

      Because nearly half of all patients who take SSRIs do not recover from depression. In addition, even in the case where there is improvement, there is up to a month before symptoms are alleviated.

      5. What does the cortisol hypothesis explain that the Serotonin hypothesis does not?

      The cortisol hypothesis explains cognitive symptoms associated with depression.

      6. What is meant by the statement that "there is still a problem with bidirectional ambiguity because the research is still correlational in nature?"

      Research on the levels of neurotransmitters and/or cortisol is correlational in nature - that is, that no cause and effect can be established because true experimental research on humans cannot be carried out. Since it is correlational, we cannot be sure whether the level of the neurotransmitter caused the depression, whether the depression caused the lower levels of the neurotransmitter, or whether there is no true interaction between the two variables.

      7. How does the concept of Adverse Childhood Experiences link to biological theories of depression?

      These experiences most likely play a role in gene expression. As Caspi's research indicated, there is a correlation between the number of stressful experiences one has experienced and levels of depression.  In addition, there is evidence that ACE's may lead to hyperactivity of the HPA Axis, which appears to play a key role in depression.

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