Scientists at University College London have conducted an umbrella review, evaluating pre-existing research, and concluded that there is little support for the idea that depression is related to abnormally low levels of serotonin.
Sahanika Ratnayake at Slate writes:
It’s true that the authors of the study are controversial figures, vocal to sometimes vituperative critics of the mental health status quo, leaving heated debates in their wake with each new publication. But the authors’ conclusion has been an open secret within mental health circles for at least a decade. The very public dispelling of this “serotonin model” has also removed a key plank in the widely believed but oversimplified myth of mental illness being caused by a “chemical imbalance.”
Also, there is this:
In the past year, 8.3 million people in the U.K. (about 12 percent of the population) took antidepressants. The most recent figures from the United States … show that approximately 23 percent of adults took prescription medication for their mental health. The vast majority of these prescriptions … are for selective serotonin reuptake inhibitors (SSRIs).