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Major Depressive Disorder involves depressed mood or anhedonia plus neurovegetative changes (sleep, appetite, energy), cognitive symptoms (guilt, worthlessness, concentration), and psychomotor changes lasting at least two weeks. MDD is the most common mental disorder with significant heritability; episodes are triggered or maintained by psychosocial stressors and neurobiological dysregulation. Severity ranges from mild to severe with psychotic features.
MDD is not sadness amplified — it is a distinct syndrome with a characteristic cluster of symptoms that cohere because they share underlying neurobiological mechanisms. The diagnostic anchor is straightforward: a person must experience either depressed mood or anhedonia (loss of interest or pleasure in previously enjoyable activities) most of the day, nearly every day, for at least two weeks. These are the gatekeeper criteria — at least one must be present. To that anchor, you add any four from a list of seven remaining symptoms: changes in sleep, appetite or weight, psychomotor agitation or retardation, fatigue, feelings of worthlessness or excessive guilt, difficulty concentrating, and recurrent thoughts of death or suicidal ideation. Five or more symptoms, two-week minimum, significant functional impairment — that is MDD.
The neurobiological substrate connects directly to what you know about the limbic system. The amygdala is hyperreactive in MDD — it fires more vigorously to negatively valenced stimuli and shows impaired habituation, which explains the negativity bias and emotional reactivity that pervades the disorder. The hippocampus shows structural volume reduction in chronic MDD, a consequence of sustained elevation of cortisol via the HPA axis. Glucocorticoids at high prolonged concentrations are toxic to hippocampal neurons and suppress neurogenesis — this hippocampal atrophy likely contributes to the cognitive impairments (memory, concentration) and the impaired contextual regulation of negative emotion. The HPA axis dysregulation also directly drives many neurovegetative symptoms: disrupted cortisol rhythm perturbs sleep architecture, appetite regulation, and energy metabolism.
The symptoms of MDD cluster into three broad domains. Mood and hedonic symptoms: depressed mood, anhedonia, feelings of worthlessness, guilt, death ideation. These reflect disrupted reward and emotional regulation circuitry, particularly in the prefrontal cortex–amygdala axis. Neurovegetative symptoms: sleep disruption (most commonly insomnia, though hypersomnia also occurs), appetite and weight changes, and fatigue. These reflect the downstream effects of HPA dysregulation and disrupted monoamine signaling. Cognitive and psychomotor symptoms: concentration difficulties, psychomotor slowing or agitation. These reflect reduced prefrontal engagement and disrupted dopaminergic function in frontostriatal circuits.
MDD is episodic, and understanding its course is clinically essential. Most people will experience multiple episodes over a lifetime — recurrence rates exceed 80% after a third episode. The kindling hypothesis offers one explanation: each episode lowers the threshold for the next. Early episodes typically follow identifiable psychosocial stressors; later episodes can emerge with minimal provocation, as if the system has been sensitized by prior activations. This explains why ongoing maintenance treatment (medication or psychotherapy) is recommended after multiple episodes even when the person feels well — the goal is preventing the next kindling event.
Heritability estimates for MDD are approximately 40%, meaning genetic factors explain a meaningful portion of risk but environmental factors remain dominant. The diathesis-stress model provides the most empirically supported account: genetic and developmental vulnerabilities (the diathesis) lower the threshold at which environmental stressors (the stress) tip the system into a depressive episode. Vulnerabilities include genetic polymorphisms affecting serotonin and HPA function, childhood adversity that calibrates the stress response system toward hyperreactivity, and cognitive styles (especially rumination) that perpetuate negative mood. This model directly informs treatment: you can target either the diathesis (via medication that modifies HPA or monoamine function) or the stress response (via psychotherapy that modifies cognitive and behavioral patterns).