“If I get angry, don’t you think that releases different neurotransmitters in my brain? So am I angry because of the neurotransmitters, or because I stubbed my toe?”
Popa raises a good question, which is one of causality. How can we know which variable causes which. Or maybe there is a third element at work influencing both.
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.