What, chemically speaking causes depression?

Question: What, chemically speaking causes depression?
- Keywords:
  depression,
  hormones
Asked by anon-228759 to Sophie, Jolel, Emily, Amber, Alex on 15 Nov 2019.
- Jolel Miah answered on 15 Nov 2019:
  
  Sometimes chemical inbalance could explain why people have depression, i think the causes of mental illness is more to the environment which then has an impact on the brain.

Question: What, chemically speaking causes depression?