Although previously the monoamine systems were considered to be responsible for the development of major depressive disorder (MDD), the available evidence to date does not support a direct causal relationship with MDD. There is no simple direct correlation of serotonin or norepinephrine levels in the brain and mood. In other words, after a half-century of research, the chemical-imbalance hypothesis as promulgated by the drug companies that manufacture SSRIs and other antidepressants is not only without clear and consistent support, but has been disproved by experimental evidence.

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