Recently the serotonin “chemical imbalance” theory of depression was tested by a group of University College London scientists publishing in the journal Molecular Psychiatry. They reviewed decades of research and concluded that there was no evidence that serotonin levels or serotonin activity are responsible for depression. That was a rare challenge to a leading theory that created a paradigm of research and treatments, for selective serotonin reuptake inhibitor (SSRI) antidepressants, something most US mass shooters were on.