Depression is rarely the result of one thing going wrong. It is the meeting point of a vulnerability you carry and a life that presses on it, which is why two people in the same situation can be affected so differently.
Depression has no single cause. It is best explained by the biopsychosocial model, in which biological vulnerability, psychological patterns, and social circumstances combine to produce an episode. Genes and brain and stress-system differences set a baseline risk, thinking styles and early experience shape how that risk plays out, and stressful events often provide the trigger. The popular idea that it is simply a chemical imbalance is an oversimplification, not an established fact.
Why "one cause" thinking fails here
People understandably want a clean answer: a gene, a chemical, a single bad event. Depression does not offer one. Decades of research point instead to a web of contributing factors, no one of which is necessary or sufficient on its own. Someone can carry a strong family history and never become depressed. Someone else with little apparent vulnerability can be tipped into an episode by a severe enough loss.
The most useful framework is the biopsychosocial model, which treats depression as the product of three interacting domains: the biological, the psychological, and the social. A closely related idea, diathesis-stress, sharpens it: an underlying vulnerability (the diathesis) tends to produce depression only when it meets enough stress. The three sections below walk through each domain, and then a myths section clears away the most misleading shortcuts, above all the serotonin story.
One more caution before we begin. Cause and correlation are easy to confuse in mental health. Many things travel alongside depression, disturbed sleep, low activity, negative thinking, changes in brain activity, without our being able to say which came first. Some are contributing causes, some are consequences, and many are both, feeding back on each other once an episode is underway. Where the evidence genuinely supports a causal role, this page says so; where a factor is better described as a risk marker or a companion of low mood, it is flagged as such. That distinction is what keeps a model honest rather than a list of everything that ever accompanies feeling low.
The three domains of cause
These domains are not rivals competing to be the "real" cause. They are layers that stack. In most people who become depressed, more than one is at work, and they influence each other: chronic stress changes biology, biology shapes how events are experienced, and thinking patterns colour both.
Vulnerability in genes and body systems
Depression is moderately heritable, so family history raises risk without determining it. Differences in the brain's stress-response system, particularly a persistently overactive cortisol response, and in circuits that govern mood and reward, appear to contribute. Physical illness, chronic pain, and hormonal shifts can also lower the threshold. This is real biology, but it is a predisposition, not a single measurable fault.
Thinking styles and early experience
How a person habitually interprets events matters. A tendency towards harsh, global, self-blaming explanations, and towards rumination, looping over distress rather than moving through it, tends to deepen and prolong low mood. Early experiences of loss, neglect, or an environment where nothing one did seemed to help (a pattern described as learned helplessness) can lay down these patterns long before the first episode.
Life events and circumstances
Stressful events are among the strongest triggers: bereavement, relationship breakdown, job loss, financial strain, and serious illness. Chronic adversity, isolation, loneliness, discrimination, and a lack of supportive relationships all raise risk and slow recovery. Depression is not only "in the head"; the shape of a person's life is part of the explanation.
No single factor is deterministic. A gene, a thinking style, or a hard year does not, by itself, make depression inevitable. It is the accumulation and interaction across these domains, vulnerability meeting stress, that best explains why an episode begins when it does.
The interaction between the domains is worth dwelling on, because it is where the real explanatory power lies. Take a person with an inherited vulnerability and a sensitive stress-response system. On its own, that biology may never announce itself. Add a period of chronic stress, a demanding job, a strained relationship, a stretch of loneliness, and the same biology begins to matter: prolonged stress keeps the body's cortisol system switched on, which in turn affects sleep, appetite, and the brain circuits that support motivation and pleasure. Now layer on a habit of rumination, and the low mood that follows is not only felt but rehearsed, turned over and over until it settles in. None of these ingredients alone would be enough. Together they can be. This is why asking "what caused it" so often has no tidy answer even for the person living through it.
How the layers combine: a diathesis-stress view
It helps to see the domains not as a list but as a sequence that builds towards an episode. The flow below is a simplified account, not a fixed order every case follows, but it shows how the pieces fit.
An underlying vulnerability is present
Inherited risk, a sensitive stress-response system, and early experiences leave some people more susceptible than others. On its own this may never surface.
Life applies pressure
A significant loss, sustained stress, isolation, or adversity raises the load. For a vulnerable person, a level of stress that others absorb can be enough to tip the balance.
Thinking style amplifies or buffers
Rumination and harsh self-interpretation magnify the impact and keep it going, while support, perspective, and problem-solving can soften it. This is why the same event affects people so differently.
An episode takes hold and self-sustains
Low mood disturbs sleep, appetite, energy, and activity, which in turn feed the low mood, a loop that can persist even after the original trigger has passed. This self-sustaining quality is part of why depression often needs active treatment rather than simply waiting it out.
Myths worth clearing up
Some of the most common statements about the causes of depression are misleading. Correcting them matters, because a wrong model of the cause leads to wrong expectations about recovery. If you believe depression is a fixable chemical fault, you may feel betrayed when the first medication does not simply switch it off. If you believe it is a character flaw, you may punish yourself for an illness. A more accurate model is not just academically tidier; it is kinder and more practical.
Depression is simply caused by a chemical imbalance, a shortage of serotonin in the brain.
This is an oversimplification, not an established cause. A large 2022 umbrella review by Moncrieff and colleagues, pooling decades of studies, found no consistent evidence that depression is caused by low serotonin. Brain chemistry is genuinely involved in mood, but there is no single measurable imbalance that explains the condition. Importantly, this does not mean antidepressants do not work: many people are helped by them on average, even though the serotonin theory of why they help is not confirmed. Mechanism and effectiveness are separate questions.
Depression is just extreme sadness, feeling very down for a while.
Depression is more than sadness. It is a persistent condition that dampens energy, motivation, sleep, appetite, concentration, and the capacity to feel pleasure, often without any obvious sadness at all. Many people describe numbness or emptiness rather than grief. Treating it as ordinary sadness leads people to expect it to lift on its own, which it often will not.
If you really tried, you could just snap out of it.
You cannot will your way out of depression any more than you can will away a fever. The condition affects the very systems, energy, motivation, and self-regulation, that effort depends on, which is exactly why "try harder" advice tends to make people feel more defeated. Recovery usually comes through support and treatment, not sheer willpower.
Key terms
- Biopsychosocial model
- The framework used throughout this page: depression as the product of interacting biological, psychological, and social factors, not one single cause.
- Diathesis-stress
- The principle that an underlying vulnerability (the diathesis) tends to produce a disorder only when it meets enough stress. It explains why the same event affects people so differently.
- Rumination
- A repetitive, looping style of dwelling on distress and its causes. It tends to deepen and prolong low mood rather than resolve it.
Where to go next
Understanding the causes is one piece. To see how these factors show up day to day, read the symptoms. For what actually helps, and why treatment works even where the mechanism is unclear, see treatment. The research page looks harder at what the evidence does and does not settle, including the serotonin question.
Sources
- Moncrieff J, Cooper RE, Stockmann T, Amendola S, Hengartner MP, Horowitz MA. The serotonin theory of depression: a systematic umbrella review of the evidence. Molecular Psychiatry. 2023;28:3243-3256.
- Kendler KS, Gardner CO, Prescott CA. Toward a comprehensive developmental model for major depression in women. American Journal of Psychiatry. 2002;159(7):1133-1145.
- World Health Organization. Depressive disorder (depression), fact sheet. 2023.
This page is educational and is not medical advice. It does not diagnose any condition, and questioning the chemical-imbalance story does not mean depression is not real or that treatment does not help. Depression is treatable, and effective help is available. If you are struggling, please speak with a doctor or mental health professional. If you are in crisis or thinking about harming yourself, contact your local emergency services or a crisis line now: in the UK and Ireland call Samaritans on 116 123, in the US call or text 988, or find your nearest helpline at findahelpline.com.