What actually helps
If you are asking this because you or someone you love is struggling, start here: depression is treatable, and most people improve. Psychotherapies such as cognitive behavioral therapy and interpersonal therapy have strong evidence, and for moderate to severe depression, antidepressant medication helps many people, often working best combined with therapy. Exercise, sleep, social connection, and treating co-occurring conditions all matter too. You do not need a settled theory of the cause to get effective help, and the cause does not have to be one broken chemical for treatment to work.
It is also worth saying plainly that depression is a real medical condition, not a personal weakness or a failure of willpower. Understanding that the biology is more complicated than a single-chemical story is not a reason to doubt your experience or to stop treatment that is working. If you are having thoughts of harming yourself, reach out to a clinician or a crisis line now.
Where the serotonin story came from, and what the evidence shows
The chemical imbalance idea took hold in the 1990s, largely alongside the marketing of a class of antidepressants called SSRIs, which act on serotonin. It offered a simple, destigmatizing message: depression is like diabetes, a chemical you are short of. The trouble is that the direct evidence for it never held up. A widely discussed 2022 umbrella review led by Joanna Moncrieff and colleagues, published in Molecular Psychiatry, pooled decades of studies and found no consistent evidence that depression is caused by low serotonin or reduced serotonin activity.
That review drew vigorous scientific rebuttals. Critics, including researchers at King's College London, argued that few experts still held the crude low-serotonin theory, that the review's method summarized older studies without new analysis, and, crucially, that whether antidepressants work is a separate question from whether serotonin causes depression. Both things can be true at once: the low-serotonin explanation is not supported, and antidepressants still relieve symptoms for many people through mechanisms that are more complex than topping up a chemical.
A richer picture: circuits, stress, and life
Most researchers today favor a biopsychosocial model. Depression emerges from an interaction of genetic vulnerability, brain and hormonal systems, chronic stress, early adversity, physical illness, and life circumstances such as loss, isolation, or poverty. There is no single lesion or lab test for it. Newer neuroscience emphasizes things like neuroplasticity, the brain's capacity to reshape its connections, as a way antidepressants and therapy may help, rather than simple neurotransmitter levels.
Another influential view treats depression as a network of interacting symptoms. Sleep loss, low mood, fatigue, and hopelessness feed one another in a loop, so the disorder is better understood as a self-reinforcing pattern than as one hidden root cause. The takeaway is not that biology is irrelevant, it is deeply involved, but that reducing depression to one molecule was always too small. A fuller understanding actually widens the menu of what can help, from medication and talk therapy to sleep, movement, and rebuilding connection.