Correlation is not causation. So what can we do to eliminate causality? We run an experiment. One with random assignment and a control group.
So in our case, how do we find out if low serotonin causes depression or the other way? We would randomly assign participants of the experiments into two groups. One would receive and SSRI, and the other would get a placebo. Then we rank the participant’s level of depression (using the Beck depression inventory or an equivalent test). If the experimental group shows a better improvement in symptoms than the control group, we can say that high serotonin reuptake can be a cause of depression. It’s as simple as that.
Sorry, try again. All you are showing with your experiment is that they are correlated. But I’m saying that the depression triggers are what cause the lack of serotinin. So it doesn’t prove anything to that. It doesn’t show which way the causation is.
It’s like saying that headaches are caused by a lack of tylenol because tylenol helps them.