Still, despite the influential scope of this theory, "no comprehensive review has yet synthesised the relevant evidence," the researchers share. That is, until now.
Based on data from 17 different studies on serotonin, the researchers found "there is no convincing evidence that depression is associated with, or caused by, lower serotonin concentrations or activity."
The authors went on to state: “Most studies found no evidence of reduced serotonin activity in people with depression compared to people without, and methods to reduce serotonin availability using tryptophan depletion do not consistently lower mood in volunteers.”
It's worth noting that in some studies, there was minor evidence that pointed toward a tie between increased serotonin activity and depression, but the researchers believe these results were likely "influenced by prior use of antidepressants and its effects on the serotonin system."
All this to say, while this study doesn't definitively disprove a link, based on the data points from thousands of people, the researchers don't believe there is strong evidence depression and serotonin are connected.