Encyclopedia section
Causes & Mechanisms
the biology and mechanisms.
Autoimmune and Infectious Contributors to Depression
Autoimmune processes and infections can contribute to depression, ranging from rare but treatable autoimmune encephalitis that mimics psychiatric illness to broad epidemiological links between immune activation and mood disorders.
Read →Cerebrovascular Disease and Vascular Depression
Damage to the brain's blood vessels — small-vessel disease, white-matter lesions, and stroke — can disrupt the frontal-subcortical mood circuits, producing a distinctive form of depression especially in later life. Many of its risk factors are modifiable.
Read →Chronic Stress and Allostatic Load in Depression
Chronic stress, formalized through McEwen's framework of allostasis and allostatic load, describes how the cumulative wear-and-tear of sustained stress dysregulates the HPA, immune, metabolic, autonomic, and neuroplastic systems all at once. It is the master upstream driver that integrates the other mechanisms of depression.
Read →Default Mode Network and Circuit Dysfunction
The circuit hypothesis reframes depression as a network disorder — abnormal activity and connectivity in the large-scale brain networks that govern mood, self-reflection, and emotion, most prominently the default mode network and its link to rumination.
Read →Early-Life Adversity and Developmental Programming in Depression
Childhood adversity is the single most powerful environmental risk factor for adult depression, related to it in a graded, dose-dependent way. It works by biologically embedding early experience — durably reprogramming the stress, immune, and neural systems toward a vulnerable phenotype that can persist for decades.
Read →Genetics, Epigenetics, and Gene-Environment Interaction in Depression
Depression is genuinely heritable (about 35–40% for unipolar) but in a highly polygenic, no-single-gene way. The early "candidate-gene" findings largely failed to replicate; what survives is a picture of inherited vulnerability inseparably entangled with environment and regulated by epigenetics.
Read →Glutamatergic Dysfunction and Excitotoxicity in Depression
Dysfunction of glutamate — the brain's principal excitatory neurotransmitter and the substrate of synaptic plasticity — contributes to depression through disrupted signaling, impaired clearance, and excitotoxic damage. The discovery that ketamine works rapidly reoriented the field toward this system.
Read →Hormonal Disturbances as a Driver of Depressive Symptoms
Thyroid, reproductive hormones, and neurosteroids exert powerful effects on mood, and their disturbance can produce, precipitate, or shape depression. These are among the most clinically actionable causes — and the key to depression's sex differences.
Read →HPA-Axis Dysregulation in Depression
Dysregulation of the hypothalamic-pituitary-adrenal axis — chronic cortisol elevation with broken feedback (glucocorticoid resistance) — is the most replicated neuroendocrine finding in depression. It is the biological bridge through which life stress becomes brain pathology, yet it has stubbornly resisted becoming a treatment target.
Read →Inflammation as a Driver of Depression
In a substantial subset of patients — perhaps a quarter to a third — depression is driven or sustained by chronic low-grade immune activation, with depressive symptoms arising partly as the brain's response to inflammatory signaling. The evidence is genuinely causal, and it points toward a biomarker-stratified psychiatry.
Read →Medication- and Substance-Induced Depression
Certain prescribed drugs, substances of use, and withdrawal states can directly cause depressive symptoms — a potentially reversible cause that recognizing is fundamental to the depression differential. Several historically-blamed drugs have been exonerated.
Read →Metabolic Dysfunction as a Driver of Depressive Symptoms
Depression and metabolic disease — obesity, type 2 diabetes, insulin resistance, cerebrovascular disease — are robustly and bidirectionally linked, with metabolic dysfunction driving depression in an identifiable immunometabolic subgroup. This reframes some depression as a whole-body disorder of energy and metabolism.
Read →Mitochondrial Dysfunction as a Driver of Depressive Symptoms
Depression, or a fatigue-and-anhedonia-dominant subset of it, may reflect a failure of cellular energy production in the brain's energy-hungry mood circuits. Mitochondria are the cellular convergence point where inflammation, metabolic dysfunction, and chronic stress meet.
Read →Monoaminergic Dysfunction in Depression
The classic theory that depression stems from a deficiency of serotonin, norepinephrine, and dopamine — the basis of the "chemical imbalance" narrative — has collapsed as a simple deficiency model. The monoamine systems remain genuinely involved in mood, but as downstream, plasticity-mediated levers rather than the broken part.
Read →Neuroplasticity, BDNF, and Neurotrophic Deficits in Depression
Impaired neuroplasticity — a deficit in the brain's capacity to form and remodel synaptic connections, driven by reduced BDNF signaling — is the leading candidate for depression's final common pathway. It is the endpoint on which the diverse upstream causes converge and on which every effective antidepressant acts.
Read →Nutritional Factors and Deficiencies
Dietary patterns and specific nutrients influence the risk and course of depression, and deficiencies and poor diet contribute causally in some people. Diet is a modifiable, low-risk lever that acts upstream of much of depression's biology.
Read →Oxidative and Nitrosative Stress in Depression
Depression involves an imbalance between damaging reactive oxygen and nitrogen species and the body's antioxidant defenses, producing oxidative damage to an exquisitely vulnerable brain. It is largely the chemical link connecting mitochondrial dysfunction, inflammation, and excitotoxicity.
Read →Reward-Circuit Dysfunction and Anhedonia
Anhedonia — the loss of pleasure, interest, and motivation — reflects a specific dysfunction of the brain's dopaminergic reward circuitry. It is among depression's most disabling and treatment-resistant features, and serotonergic drugs address it poorly.
Read →Sleep and Circadian Dysregulation
Disrupted sleep and circadian rhythms are not just symptoms of depression but causal contributors that trigger, maintain, and predict it. Sleep is also one of depression's most modifiable and treatable levers.
Read →Social and Environmental Determinants
Loneliness, adversity, poverty, discrimination, and meaninglessness are among the most powerful contributors to depression — causal in their own right, and biologically embedded through the stress and inflammatory pathways. This is where biography becomes biology.
Read →The Gut Microbiome and the Gut-Brain Axis in Depression
The trillions of microorganisms in the gut communicate bidirectionally with the brain and influence mood, and disruption of this ecosystem (dysbiosis) may contribute to depression. The animal evidence is striking, but the human evidence is early and correlational.
Read →The Pathophysiology of Depression (Overview)
An integrative synthesis of depression's many biological, psychological, and social contributors — showing they are not competing theories but interconnected facets of a single heterogeneous web of dysregulation. It maps how the causes converge, relate, and combine differently across individuals.
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