Comprehensive review finds little evidence that depression is caused by an imbalance of serotonin in the brain.

The serotonin theory of depression: a systematic umbrella review of the evidence – Molecular Psychiatry

News Release: No evidence that depression is caused by low serotonin levels, finds comprehensive review – University College London

Commentary from the authors: Depression is probably not caused by a chemical imbalance in the brain – new study – The Conversation

Other commentaries:

Expert reaction to a review paper on the ‘serotonin theory of depression’ – Science Media Centre

Little evidence that chemical imbalance causes depression, UCL scientists find – The Guardian