1. The Etiology of Depression (BLOA)

2. Neurotransmitters

3. Catecholamine and Serotonin Hypothesis
Joseph Schildkraut came up with the Catecholamine Hyphothesis in 1965 suggesting that depression is associated with low levels of noradrenaline (or norepinephrine).
This lead to the development of the Serotonin Hypothesis, which suggests depression is associated with serotonin.

4. Janowsky et al. (1972)
Physostigmine, a drug that increases level of noradrenaline produced was given to participants.
Within minutes, participants became depressed with self-hate and even suicidal wishes.
These results show how depression may be caused by disturbances in neurotransmission.

5. Delgado and Moreno (2000)
Found abnormal levels of noradrenaline and serotonin in patients suffering from major depression.
Correlation does not mean causation- results do not indicate neurotransmitters directly cause depression.

6. Burns (2003) Criticizes the serotonin hypothesis.
Claims he have never seen depression caused by low levels of serotonin throughout his many years of researching brain serotonin metabolism.
The serotonin hypothesis cannot be tested as it is not possible to measure levels of brain serotonin in a human with our technology.

7. Lacasse and Leo (2005)
Argues that although many researches and studies are done in this field, none provides real evidence- all they are showing is how the brain is poorly understood.
Criticizes how drugs affecting serotonin levels are among the most prescribed drugs to treat depression and other psychological disorders due to the serotonin theory.

8. Hormones

9. Types of hormones affecting depression
Estrogen
Testosterone

10. Estrogen
Low estrogen is probably a risk for depression, though estrogen out of balance is probably a risk for anxiety. 
The most straightforward connection is with serotonin. Estrogen boosts serotonin by both making more of it and keeping it around after it's made. So that's likely to be the reason that estrogen helps fight depression and helps with sleep.

11. Estrogen has a mixed effect on norepinephrine (more in women) and allows them to feel better and alert. But for some women, high norepinephrine might be the cause of the anxiety and panic attacks that estrogen seems to produce in some women.
The risk for depression is higher in women. Estrogen levels are larger during puberty, which explains moodswings for women and depression in teenagers, and estrogen levels fluctuate in a declining manner during perimenopause.

12. Testosterone
Testosterone is the principal androgen, or male sex hormone, although it appears in both genders.
Older men tend to have a decrease in testosterone.
Low testosterone levels trigger symptoms such as anorexia, fatigue, which may have an effect on mood.
Some 30% of men over age 55 have it.

13. Older men would be particularly vulnerable to these effects, because serotonin receptors are already decreased from normal aging
Depression is a major risk factor for suicide, and older men have the highest suicide rate of any age group in the United States.
People with low testosterone increase the risk of depressive illness and must be identified early so they can have the opportunity for early treatment

14. Genes

15. If you have a parent or sibling with depression, you may be 1
If you have a parent or sibling with depression, you may be 1.5 to 3 times more likely to develop the condition compared to those who aren’t closely related to someone who’s depressed
The genes that we receive of our parents are able to determine what illnesses we are more likely to get

16. Supporting Study Nurnberger and Gershon 1982
Reviwed the results for 7 twin studies
Findings: - Concordance rate for major depressive disorder is higher in MZ twins than DZ twins - MZ twins – 65%, DZ twins – 14% - Suggests genetic material may predispose people to depression

17. Contradicted by Caspi et al. 2003
- Below 100% - Correlation does not mean causation
Could be environmental factors, stress, traumatic events
Warned that speculation about clinical implications of these findings is premature

18. Other biological factors that could contribute to depression
Poor diet
Lack of proper exercise
Onset of another mental disorder/illness eg. cancer
Techno-brain burnout (overuse of technological devices)
Under-stimulation of effort driven rewards system (not getting a sense of satisfaction from activities that should be rewarding)