A systematic review published in Molecular Psychiatry comes to the conclusion that depression is not caused by low serotonin levels.
"Although “the serotonin hypothesis of depression is still influential,” Moncrieff and coauthors noted, citing widely adopted textbooks published as recently as 2020 and surveys indicating that “85-90 percent of the public believes that depression is caused by low serotonin or a chemical imbalance,” the primary research indicates there is “no support for the hypothesis that depression is caused by lowered serotonin activity or concentrations.”"
