Major depressive disorder doesn’t have a single cause. It develops from a combination of genetic vulnerability, brain chemistry changes, life experiences, and sometimes underlying medical conditions. Around 332 million people worldwide live with depression, affecting roughly 5.7% of adults, and the mix of contributing factors varies from person to person.
Genetics Set the Stage
Depression runs in families, but inheritance isn’t straightforward. Twin studies published in JAMA Psychiatry estimate that genetics account for 36% to 44% of the risk in women. In men, heritability estimates are lower, ranging from 18% to 24%, though the confidence intervals are wide enough that the true figure could be higher. The remaining risk comes from environmental factors unique to each individual, meaning shared family environment (growing up in the same household) contributes surprisingly little on its own.
No single “depression gene” has been identified. Instead, hundreds of small genetic variations each nudge risk up slightly. What you inherit isn’t depression itself but a nervous system that may be more reactive to stress, less efficient at regulating mood, or more prone to the neurochemical imbalances described below.
Neurotransmitter Imbalances
The oldest and most widely known explanation for depression centers on chemical messengers in the brain. Serotonin, norepinephrine, and dopamine all help regulate mood, motivation, sleep, appetite, and attention. The “monoamine hypothesis” proposes that depression arises when levels of these chemicals drop too low or their signaling becomes disrupted. Nearly every effective antidepressant works by increasing the availability of one or more of these messengers in the gaps between nerve cells, which lends strong indirect support to this idea.
Serotonin is the most studied of the three. When researchers experimentally lower serotonin production in the brain (by depleting its building block, tryptophan), people who have a history of depression or a family predisposition tend to develop depressive symptoms. Reduced serotonin has also been linked to negative memory bias and disrupted emotional processing. Norepinephrine, which helps govern alertness and energy, shows signs of abnormal metabolism in depressed patients. Dopamine, the brain’s core reward signal, appears to drive anhedonia specifically: when dopamine activity drops in the brain’s reward center, people lose the ability to feel pleasure and perform poorly on tasks involving rewards.
Two other chemical systems also play a role. GABA, the brain’s main calming signal, is consistently found at lower concentrations in the prefrontal cortex of people in acute depressive episodes. And glutamate, the brain’s primary excitatory signal, is implicated by the rapid antidepressant effects seen when its receptors are blocked, a mechanism behind ketamine’s use in treatment-resistant depression.
Structural Changes in the Brain
Depression isn’t just a chemical problem. It’s visible on brain scans. The hippocampus (critical for memory and emotional regulation) and the medial prefrontal cortex (involved in decision-making and self-referential thought) are consistently smaller in people with major depressive disorder compared to healthy controls. Multiple meta-analyses report moderate reductions in hippocampal volume, and these reductions affect several subregions of the structure.
These changes aren’t static. They worsen with illness progression: more depressive episodes, longer illness duration, and failure to achieve remission are all tied to greater volume loss. On the other hand, antidepressant treatment has been shown to partially reverse these reductions over time. People who have both depression and an anxiety disorder tend to show even more pronounced shrinkage in the prefrontal cortex than those with depression alone.
High levels of cortisol, the body’s primary stress hormone, appear to drive much of this damage. Cortisol causes nerve cells to shrink and retract their connections, particularly in the hippocampus and prefrontal cortex. Higher baseline cortisol levels are directly correlated with smaller volumes in both regions.
The Stress Hormone Connection
Your body’s stress response system, called the HPA axis, is a feedback loop linking the brain to the adrenal glands. Under normal conditions, a stressful event triggers a burst of cortisol, which helps you respond, and then the system shuts itself off. In many people with depression, this feedback loop gets stuck in the “on” position: cortisol levels stay elevated throughout the day, and the system fails to suppress properly when tested.
Evidence for this pattern has been accumulating since the 1960s. People with depression often show higher daytime cortisol, elevated levels of the hormones that trigger cortisol release, and abnormal results on suppression tests designed to check whether the system can turn itself off. The link is reinforced by Cushing’s disease, a condition of extreme cortisol excess that frequently causes depression as a direct symptom. Chronic psychological stress produces a milder but sustained version of the same cortisol elevation, which over months and years can reshape brain structure and neurotransmitter function.
Adverse Childhood Experiences
Difficult experiences in early life are among the strongest predictors of adult depression. A large longitudinal study published in Molecular Psychiatry found that adverse childhood experiences from the prenatal period through adolescence were robustly associated with moderate and severe depressive symptoms in young adulthood. Threat-related adversities, such as abuse and violence, carried larger associations than other types of hardship.
The timing of these experiences matters. Researchers have proposed several models to explain how early adversity gets “biologically embedded”: through cumulative damage over years, through the recency of stressful events, or through sensitive developmental windows when the brain is especially vulnerable. Imaging studies have confirmed that a history of early-life stress is associated with reduced hippocampal and prefrontal cortex volumes, the same structural changes seen in depression itself. In essence, childhood adversity may prime the brain’s stress system and reshape its architecture in ways that lower the threshold for developing depression later.
Chronic Low-Grade Inflammation
A growing body of evidence connects depression to the immune system. People with major depressive disorder consistently show elevated blood levels of inflammatory markers, particularly IL-6, TNF-alpha, and C-reactive protein (CRP), compared to people without depression. These aren’t the dramatic spikes you’d see with an infection but rather a sustained, low-level inflammatory state.
The relationship runs in both directions. A systematic review covering more than 20,000 participants found that depression predicts future increases in IL-6, and elevated CRP and IL-6 predict the future onset of depression. Women with depression appear particularly likely to show elevated inflammatory markers across multiple categories. In adults, this inflammatory pathway seems especially relevant, though in younger people the connection between inflammation and depressive symptoms is weaker, suggesting inflammation may become a more important driver as people age.
Reduced Neuroplasticity
Healthy brains constantly form new connections between nerve cells, strengthen useful pathways, and prune unused ones. This flexibility, called neuroplasticity, depends heavily on growth-promoting proteins. The most important of these in the context of depression is brain-derived neurotrophic factor (BDNF), which supports neuron survival, the formation of new synapses, and the overall adaptability of brain circuits.
Postmortem studies of people who had depression reveal decreased levels of BDNF and its receptor in the brain. Animal studies tell a consistent story: stress reduces BDNF in the cortex and hippocampus, and mice engineered to produce less BDNF develop depression-like behaviors. When BDNF drops, structural neuroplasticity suffers. Nerve cells lose connections, circuits become rigid, and the brain becomes less able to adapt to challenges. Effective antidepressant treatment reverses this pattern, restoring BDNF levels and, over time, rebuilding lost neural connections. Some researchers view impaired neuroplasticity as the initial domino that tips into a full depressive episode.
Why Women Are Affected More Often
Depression is diagnosed in women roughly twice as often as in men, with lifetime prevalence estimated at about 21% for women versus 12% for men. The reasons are both biological and social. Hormonal fluctuations across the menstrual cycle, pregnancy, the postpartum period, and menopause all create windows of vulnerability. More than 10% of pregnant women and new mothers experience depression. Women also show higher heritability estimates for depression in twin studies, suggesting their genetic architecture may confer greater susceptibility.
Part of the gap, however, reflects differences in how depression manifests and gets reported. Women tend to experience the “classic” symptoms that align with diagnostic criteria: persistent sadness, fatigue, slowed movement, pessimism, and unexplained crying. Men with depression are more likely to present with anger, irritability, risky behavior, early-morning waking, and increased alcohol use. These symptoms are less likely to be recognized as depression by either the person experiencing them or their doctor. Men are also less likely to seek help, sometimes viewing depression as a source of shame, which means many male cases go undiagnosed.
Medical Conditions That Trigger Depression
Some chronic diseases can directly cause depression through their biological effects on the brain, not simply because being sick is stressful. Neurological conditions are the clearest examples: Parkinson’s disease damages dopamine-producing neurons, cerebrovascular disease disrupts blood flow to mood-regulating brain regions, and multiple sclerosis destroys the insulating material around nerve fibers. Each of these can produce depression as a direct symptom of the disease process itself.
Endocrine disorders are another common culprit. Hypothyroidism slows metabolism throughout the body, including in the brain, and frequently presents with fatigue, low mood, and cognitive sluggishness that are indistinguishable from depression. Cushing’s syndrome floods the body with cortisol, producing depressive symptoms through the same HPA axis mechanism described above. Sleep apnea, which fragments sleep and reduces oxygen delivery to the brain, can also mimic or trigger depression. In all of these cases, treating the underlying medical condition often improves depressive symptoms significantly.