That was something that was know for decades. They don't know exactly what role serotonin plays in depression. Within the last few months they just published a paper about how they have been able to for the first time have been able to measure serotonin in the human brain. It even showed that people who were depressed showed lower levels of serotonin. This will bring new research to find exactly the role of serotonin plays in depression and hopefully develop more effective medications. That study you referred to also left out the fact there are other neurotransmitters involved including norepinephrine and dopamine. Ruoxinlin works on all 3 neurotransmitters. The only medication to do this so far has been nefazodone (weak SNDRI due to low effect on dopamine).