What Causes Depression? The Science Behind It

“It’s a chemical imbalance.” For decades, that phrase was how doctors and patients made sense of depression. It was simple. It was biological. It suggested a fix.

It was also an incomplete picture that has been substantially revised by neuroscience.

Depression does not result from a single chemical being too low. It involves disrupted neural circuits, impaired neuroplasticity, altered stress-response systems, genetic vulnerability, and the long shadow of environmental experience. Understanding what actually causes depression is not just an academic question. It changes how treatment decisions get made.

What This Page Covers

• The neurobiology of depression beyond the serotonin model
• What genetics research shows and what it does not
• The role of stress, trauma, and epigenetics
• Neuroinflammation as an emerging mechanism
• The causal role of lifestyle factors
• What this means for treatment

The Neuroscience: Beyond the Chemical Imbalance

The monoamine hypothesis, the idea that depression results from reduced levels of serotonin, norepinephrine, or dopamine, emerged in the 1960s and drove the development of antidepressants. That research was meaningful, and SSRIs work for many patients.

But depression does not spring from simply having too much or too little of certain brain chemicals. Many chemicals are involved, working both inside and outside nerve cells. There are millions of chemical reactions that make up the dynamic system responsible for your mood.

Current neuroscience identifies several overlapping systems:

Disrupted neural circuits. The amygdala, hippocampus, prefrontal cortex, and thalamus are all involved in mood regulation. In depression, these regions show altered activity patterns. The amygdala becomes hyperactive in response to negative stimuli. The prefrontal cortex, which normally regulates the amygdala’s threat response, loses some of that inhibitory control. The result is a brain that perceives and weights negative information differently.

Impaired neuroplasticity. The hippocampus is smaller in some people with depression, with studies finding a 9 to 13% smaller hippocampus in women with a history of depression compared to those without. The more depressive episodes a person has experienced, the smaller the hippocampus tends to be. Stress suppresses neurogenesis in the hippocampus, which impairs the brain’s ability to adapt and recover.

Glutamate and GABA dysregulation. Beyond monoamines, the balance between glutamate (the brain’s primary excitatory neurotransmitter) and GABA (the primary inhibitory one) is disrupted in depression. This has become a primary focus of rapid-acting antidepressant research, including ketamine’s mechanism of action.

BDNF and synaptic strength. Brain-derived neurotrophic factor (BDNF) supports neuron survival and plasticity. Reduced BDNF in depression contributes to the synaptic weakening that underlies many depressive symptoms. Most antidepressants, including SSRIs, increase BDNF over time as a downstream effect.

In October 2025, researchers at McGill University published findings in Nature Genetics identifying two specific types of brain cells, neurons and microglia, that show measurable changes in gene activity in people with depression. The researchers described this as the first time scientists had identified what specific brain cell types are affected by mapping gene activity together with DNA regulation mechanisms.

The Genetics of Depression

Twin studies show that approximately 40% of depression risk is genetically determined. That is a meaningful genetic contribution, but also a reminder that the majority of depression risk is shaped by factors other than genetics.

A genome-wide association study of nearly 1 million participants catalogued 354 depression-risk genetic loci, heavily concentrated in synaptic and neurodevelopmental genes. These findings suggest that depression vulnerability is written, in part, into the genes that govern how brain circuits develop and how neurons communicate.

But having a genetic predisposition does not make depression inevitable. The same UK Biobank research that identified these genetic patterns found that lifestyle factors significantly modified risk, even in people with high genetic vulnerability. A healthy lifestyle decreased the risk of depression across populations with varied genetic risk.

Stress, Trauma, and the HPA Axis

The hypothalamic-pituitary-adrenal (HPA) axis is the brain and body’s stress-response system. When a threat is perceived, the HPA axis releases cortisol. This is adaptive in the short term. In chronic stress, it becomes damaging.

Prolonged cortisol elevation suppresses neurogenesis in the hippocampus and promotes inflammatory processes that alter mood regulation. It also sensitizes the stress-response system itself, making future stressors more likely to trigger depression.

Early life stress has a particularly lasting effect. Epigenetic changes such as DNA methylation and altered microRNA expression are key mediators of the relationship between early environmental adversity and brain development. Specifically, hypermethylation of the NR3C1 gene, which encodes the glucocorticoid receptor, alters HPA axis feedback and cortisol regulation. These molecular changes can persist for decades, creating a biological predisposition to depression that emerges under subsequent stress.

This mechanism explains why a history of childhood adversity is one of the strongest clinical risk factors for adult depression, and why some people who appear to be managing well under ordinary circumstances become severely depressed when significant stressors arise.

Neuroinflammation: The Emerging Driver

Inflammation is not just a peripheral immune response. The brain has its own immune cells, called microglia, and they participate in neuroinflammatory processes that affect mood.

People with depression show elevated inflammatory markers compared to controls, and neuroinflammatory processes are increasingly understood as a contributing mechanism. Inflammatory cytokines disrupt serotonin synthesis, activate the HPA axis, and promote neurodegeneration in hippocampal tissue.

This has clinical implications. Some patients whose depression does not respond well to standard antidepressants show elevated inflammatory biomarkers, and researchers are actively exploring anti-inflammatory approaches as supplementary or alternative treatments.

Lifestyle Factors: Causally Related, Not Just Correlated

One of the most important developments in depression research is the growing evidence that lifestyle factors are not merely correlated with depression, they are causally related to it.

A study of 287,282 participants from the UK Biobank followed over nine years found that seven lifestyle factors, including physical activity, moderate alcohol intake, healthy diet, adequate sleep, social connection, reduced sedentary behavior, and non-smoking, each independently reduced depression risk. Using Mendelian randomization, the researchers confirmed the causal direction: lifestyle shaped depression risk, not only the reverse.

This matters clinically because it means lifestyle is a legitimate treatment lever, not just an adjunct. Exercise increases BDNF and serotonin availability. Sleep restoration improves cortisol regulation. Social connection reduces neuroinflammation. These are not soft lifestyle suggestions. They are neurobiological interventions.

What This Means for Treatment

The multifactorial nature of depression explains why treatment often requires more than one approach, and why the same medication does not work the same way for everyone.

Antidepressants address one or more aspects of the biological picture: monoamine availability (SSRIs, SNRIs), glutamate signaling (ketamine), or HPA axis function. Psychotherapy, particularly CBT, changes the cognitive and behavioral patterns that sustain depressive cycles. Lifestyle changes alter neuroplasticity and inflammatory burden over time.

The combination of medication, therapy, and lifestyle change consistently outperforms any single approach because it targets multiple causal pathways simultaneously.

Understanding what causes your depression, whether the primary drivers are biological, stress-related, trauma-related, or some combination, shapes which components of treatment are most likely to produce meaningful change.

About SiggyMD

SiggyMD’s clinical model is designed for the complexity that the biology of depression demands. Daily check-ins track symptom trajectory, sleep changes, and medication response in real time. A licensed prescriber reviews every clinical decision, adjusting the treatment plan as the data changes rather than waiting for a quarterly appointment to discover that the current approach is not working.

“Depression is not one condition. It has subtypes, drivers, and complicating factors that require different approaches,” says Daniel Montville, MD, Psychiatrist, of the SiggyMD clinical team. “The intake is designed to capture the full clinical picture, so the treatment plan addresses the actual drivers rather than applying a one-size approach.”

For a deeper look at how depression is classified and treated, read about major depressive disorder: symptoms, types, and what treatment involves.

Start your anonymous intake at SiggyMD. No account, no email, no name required.

What Members Are Saying

BK

B.K., 39

Major Depression, Treatment History

“I had been told it was a serotonin problem my whole life. When my prescriber explained the HPA axis piece and how my early stress history had sensitized my stress-response system, the depression I had been experiencing for years finally made sense. The treatment approach shifted based on that understanding.”

LM

L.M., 52

Depression, Post-Stressor Onset

“I thought I had handled everything fine. Then a major life stressor hit and I crashed harder than I had ever expected. Understanding that chronic stress changes the brain’s stress-response system made sense of why this happened when it did.”

Member stories reflect real experiences. Names and identifying details have been changed to protect privacy. Results vary. You can begin anonymous intake without an account, name, email, or payment.

The Bottom Line

Depression is not a simple chemical imbalance. It is a disorder of neural circuits, neuroplasticity, stress systems, genetics, and environmental experience. Understanding this does not make depression any less real or any more the patient’s fault. It makes it possible to match treatment to the actual drivers, which is what produces better outcomes.

Treatment works. The science of why it works, and for whom, is improving rapidly.

Sources

1. Harvard Health Publishing. What causes depression? Updated 2023.

2. Scioli AG, et al. Molecular Mechanisms of Depression: The Interplay Between Genes and Receptors. International Journal of Molecular Sciences. 2025;26(23):11325.

3. Zhao Y, et al. The brain structure, immunometabolic and genetic mechanisms underlying the association between lifestyle and depression. Nature Mental Health. 2023;1(10):736-750.

4. McGill University. For the first time, scientists pinpoint brain cells linked to depression. ScienceDaily. 2025.

5. Marques AH, et al. Menopause-Associated Depression: Impact of Oxidative Stress and Neuroinflammation. PMC Review. 2024.

6. Castrillo S, et al. From trauma to depression: structural, synaptic, epigenetic, and molecular pathways. Frontiers in Psychiatry. 2025.

7. National Library of Medicine. Biology of depression. Wikipedia synthesis of primary literature. Accessed June 2026.

8. Bhatt S, et al. Molecular pathways of major depressive disorder converge on the synapse. Molecular Psychiatry. 2023.