Just want to add that low serotonin level does not mean the depression is biological in origin. That's a correlation. If I fall down and my knee bleeds, it does not mean that a messed up knee caused my fall. But once I fell, my knee started to bleed and if I go to the doctor he might not know which caused which. Of course he is not going to make that mistake but when it comes to depression, doctors do make that mistake because it's pretty confusing. So many things are linked together it's scary. So environmental factors that affect serotonin levels, which in turn change the physiology of the brain (e.g. number of serotonin receptors), which in turn makes one change his/her behavior and expose himself or herself to different environments and even perceive things differently (seeing life as more depressing than one used to), and that changes the biology again....