Depression is not simply a chemical imbalance. The idea that low serotonin causes depression dominated public understanding for decades, but a landmark 2022 review of prior research found no convincing evidence that serotonin levels or serotonin activity are responsible for the condition. The reality is more complex: depression involves changes in brain structure, stress hormones, inflammation, genetics, and life circumstances, all interacting in ways that a single-chemical explanation can’t capture.
Where the Chemical Imbalance Idea Came From
The story begins in the 1960s. Researchers noticed that certain drugs happened to relieve depressive symptoms, and those drugs worked by increasing the concentration of chemical messengers (called monoamines, which include serotonin and norepinephrine) in the gaps between brain cells. Working backward from that observation, scientists proposed what became known as the monoamine hypothesis: depression is caused by a deficiency of these chemical messengers, and correcting that deficiency with medication restores normal function.
The hypothesis was appealing because it offered a clean, biological explanation and a straightforward treatment rationale. Pharmaceutical marketing amplified it further, and by the 1990s “chemical imbalance” had become the default way doctors and patients talked about depression. But the hypothesis was always an inference. No one had directly measured a serotonin shortage in a living person’s brain and linked it to their depressive symptoms.
What the 2022 Serotonin Review Found
In 2022, a research team led by Joanna Moncrieff at University College London published an umbrella review in Molecular Psychiatry, pulling together decades of existing meta-analyses and systematic reviews on serotonin and depression. Their conclusions were striking. Studies comparing serotonin levels and its breakdown products in the blood or brain fluid found no difference between people with depression and healthy controls. Research on serotonin receptors and the serotonin transporter (the protein most antidepressants target) showed weak, inconsistent evidence that actually pointed toward higher serotonin activity in people with depression, the opposite of what the chemical imbalance theory predicted.
The authors concluded there is “no support for the hypothesis that depression is caused by lowered serotonin activity or concentrations.” This didn’t surprise most researchers, who had already moved on from the simple chemical imbalance model. But it was a significant moment for public understanding, because so many people had been told their depression was fundamentally about low serotonin.
What’s Actually Happening in the Brain
Rather than a single chemical being too low, depression involves structural and functional changes across several brain systems. One of the most well-supported findings involves a protein called brain-derived neurotrophic factor, or BDNF, which acts like fertilizer for brain cells. BDNF helps neurons grow new connections, maintain existing ones, and adapt to changing demands. In depression, BDNF levels drop, particularly in the prefrontal cortex and hippocampus, two regions critical for mood regulation and memory. When BDNF decreases, neurons lose the tiny connection points (called dendritic spines) where they communicate with each other. The result is a brain that’s less flexible and less able to process emotional information normally.
Stress appears to be a major driver of these changes. Chronic stress activates the body’s hormonal stress response, leading to persistently elevated cortisol. Multiple meta-analyses show that people with major depression have higher daytime cortisol levels than people without it. Over time, excess cortisol damages the hippocampus, causing measurable volume loss, and triggers atrophic changes in the prefrontal cortex while enlarging structures in the amygdala, the brain’s threat-detection center. Most researchers now believe these stress-related hormonal disturbances are a cause of depression rather than a consequence of it.
The Role of Inflammation
Depression also has an inflammatory dimension that the chemical imbalance story completely overlooked. People experiencing depressive episodes show elevated levels of several inflammatory markers, including interleukin-6 (IL-6), tumor necrosis factor (TNF), and interleukin-1β (IL-1β), the same molecules your immune system ramps up when you’re fighting an infection. During depressive episodes, people also show reduced levels of natural killer cells, lower zinc and albumin levels, and changes in red blood cell counts consistent with systemic inflammation.
This inflammatory response isn’t just a side effect of feeling bad. In elderly patients, IL-1β levels are directly proportional to depression severity. In postpartum women, those with depressive symptoms have higher IL-1β than those without. Animal research shows that blocking inflammatory signals can reverse depressive-like behavior, and that knocking out anti-inflammatory pathways makes animals more prone to it. Overactive inflammatory markers have even been associated with suicidal ideation. The relationship between inflammation and depression runs deep enough that some researchers view certain forms of depression as partly an inflammatory condition.
Genetics Set the Stage, Not the Script
Large twin studies from population-based samples estimate that depression is roughly 36% to 44% heritable. That means genetics account for a meaningful chunk of risk, but they’re far from the whole picture. The remaining variance comes from environmental factors unique to the individual: childhood experiences, relationships, trauma, financial stress, social isolation, and so on.
This lines up with what’s called the diathesis-stress model. A person may carry a genetic predisposition that makes their stress response more reactive, their inflammatory system more sensitive, or their BDNF regulation less efficient. But that predisposition typically needs to meet an acute stressor, like a job loss, a breakup, or sustained adversity, before depression develops. Early life experiences matter especially: childhood deprivation and insecure attachment have been linked to lasting changes in stress hormone regulation, creating a biological vulnerability that persists into adulthood.
If It’s Not a Chemical Imbalance, Why Do Antidepressants Work?
This is the question that understandably trips people up. If low serotonin doesn’t cause depression, why do drugs that increase serotonin help some people feel better? The answer is that these medications likely work through downstream effects that have little to do with correcting a chemical deficit.
One well-documented effect is that SSRIs promote the growth of new neurons in the hippocampus, the brain region that shrinks during prolonged depression. In animal studies, chronic SSRI use increases the number of neural progenitor cells, essentially jump-starting the brain’s ability to build new connections. This process takes weeks to develop, which lines up with the well-known clinical observation that antidepressants don’t start working for two to four weeks, far too long if they were simply topping off a low chemical tank. The therapeutic benefit likely comes from gradually restoring the brain’s structural flexibility rather than fixing a chemical shortage.
This reframing doesn’t mean antidepressants are useless. For many people, they provide real, measurable relief. It means the reason they help is more nuanced than the explanation most patients receive.
A More Honest Picture of Depression
The current scientific understanding treats depression as a condition with biological, psychological, and social roots that interact constantly. Chronic stress reshapes your brain’s structure and hormone regulation. Inflammation alters how your immune and nervous systems communicate. Genetic predispositions make some people more vulnerable to these cascading effects. And life circumstances, from poverty to loneliness to trauma, provide the triggers that set everything in motion.
None of this reduces to “it’s all in your head” or “just think positive.” The brain changes in depression are measurable and real: shrinking hippocampal volume, elevated cortisol, disrupted neural connections, elevated inflammatory markers. But calling it a chemical imbalance was always an oversimplification that made depression look like a problem with one faulty ingredient when it’s actually a system-wide disruption with many entry points and many potential paths to recovery.