Psychology › Mental health & disorders
Depression and mood disorders
· Condition or disorder
Well replicatedStrong, repeated evidence.
A common, treatable illness of persistent low mood or lost interest that affects sleep, energy, thinking and the body.
See it on the Psychology map →What it says
Major depression means at least two weeks of low mood or lost pleasure plus symptoms such as poor sleep, fatigue, appetite change, guilt, trouble concentrating or thoughts of death. A milder, years-long form is called dysthymia. The WHO estimates about 280 million people have depression at any time; 16 to 20 percent of Americans have an episode in their lifetime. Causes mix genes (heritability around 35 percent), stress, loss, poor sleep and illness. Talk therapy and antidepressants each help many people.
Why it matters
The WHO ranks it a leading cause of disability worldwide and a major factor in suicide, yet it is among the most treatable mental conditions.
The evidence
Large population surveys (the WHO World Mental Health Surveys, the US National Comorbidity Survey), twin studies, and hundreds of controlled trials of therapy and medication.
Open questions
Its biology is still unclear, and depression may be several illnesses sharing symptoms. How much antidepressants beat placebo in milder cases is debated.
Inside this (5)
Connections (17)
▸Sleep loss, the body clock and health raises the risk of Depression and mood disordersWell replicated
The connection
Chronic insomnia more than doubles the risk of a later depressive episode. Poor sleep disrupts emotional control and stress hormones, and depression in turn worsens sleep, creating a loop. Curiously, one night of total sleep deprivation briefly lifts mood in some depressed patients.
Who showed it and when
Population cohort studies from the 1980s onward; a 2011 meta-analysis by Chiara Baglioni and colleagues pooled them.
The evidence
Cohort studies show insomnia comes before depression; trials show CBT for insomnia reduces depressive symptoms, and a 2022 trial found it prevented new episodes in older adults.
Open questions
Why one sleepless night can temporarily relieve depression while chronic poor sleep raises its risk is not understood; the body clock is a leading suspect.
▸Learned helplessness was proposed as a model of Depression and mood disordersMixed evidence
The connection
Martin Seligman saw dogs that had endured unavoidable shocks give up even when escape became easy, and proposed that depression arises when people learn their actions do not matter. A 1978 revision added how people explain bad events: permanent, personal and pervasive explanations raise risk.
Who showed it and when
University of Pennsylvania, 1967 (Seligman and Steven Maier); revised by Lyn Abramson, Seligman and John Teasdale in 1978.
How solid is it?
A pessimistic explanatory style modestly predicts later depression in studies, but the original animal model fit depression only partly.
Open questions
In 2016 Maier and Seligman concluded passivity is the brain's default under threat and what is learned is control, reframing the link to depression.
▸Stress, cortisol and fight-or-flight can trigger Depression and mood disordersWell replicated
The connection
Major life stresses, such as losing a job, a loved one or a home, often come just before a first depressive episode. Chronic stress keeps cortisol high and can disturb sleep, mood circuits and the immune system. Later episodes may need less stress to start, a pattern called 'kindling'.
Who showed it and when
George Brown and Tirril Harris's 1978 study of women in London linked severe life events to depression; Kenneth Kendler's Virginia twin studies refined it.
The evidence
Cohort studies show severe life events raise short-term depression risk several-fold; twin studies show genes influence both stress exposure and the response to it.
Open questions
A famous 2003 gene-stress interaction (the serotonin transporter gene) failed in large repeats; why most stressed people never become depressed remains unclear.
▸Grief and the 'five stages' can be mistaken for Depression and mood disordersWell replicated
The connection
Grief after a death brings sadness, poor sleep and withdrawal that can look like depression. Grief usually comes in waves tied to the loss and leaves self-worth intact; depression is more constant and pervasive. DSM-5 (2013) dropped a rule that delayed depression diagnoses in the first months after a death.
Who showed it and when
The DSM-5 committee removed this 'bereavement exclusion' in 2013 over objections from critics such as Allen Frances; DSM-5-TR added prolonged grief disorder in 2022.
The evidence
Studies by George Bonanno and others show most bereaved people are resilient, while about 10 percent develop lasting, impairing grief or depression.
Open questions
Where normal grief ends and illness begins is contested; critics fear the 2013 and 2022 changes risk medicalizing ordinary mourning.
▸Depression and mood disorders are a leading cause of disability among Mental disorders: what they are and how commonWell replicated
The connection
Depression is one of the most common mental disorders, and because it often strikes in working years, recurs and impairs daily functioning, it causes more disability than almost any other illness. Global rankings consistently place it among the top causes of years lived with disability.
Who showed it and when
Global Burden of Disease studies, begun by Christopher Murray and Alan Lopez with Harvard and the WHO in the 1990s, now based at the University of Washington.
The evidence
Estimates built from hundreds of national surveys consistently rank depressive disorders first or second among mental disorders for disability.
Open questions
Burden estimates depend on surveys and weighting choices that vary by country; some argue they understate costs, others that they reflect overdiagnosis.
▸Anxiety disorders often co-occur with Depression and mood disordersWell replicated
The connection
About half of people with depression also have an anxiety disorder at some point, and vice versa. They share genes, the trait of neuroticism, and brain systems for threat and reward. Anxiety often comes first, in childhood or the teens, with depression following; both respond to SSRIs and CBT.
Who showed it and when
Shown in the US National Comorbidity Surveys of the 1990s–2000s and Kenneth Kendler's twin studies in Virginia.
The evidence
Large surveys show high overlap, and twin studies find the genes behind generalized anxiety and major depression are largely the same.
Open questions
Whether anxiety and depression are truly separate disorders or two faces of one 'internalizing' tendency is a central debate.
▸Bipolar disorder includes episodes of Depression and mood disordersWell replicated
The connection
Most people with bipolar disorder spend far more time depressed than manic. Their depressive episodes look like ordinary depression, so many are first diagnosed with major depression and given antidepressants alone, which can trigger mania. Mood stabilizers are usually needed instead.
Who showed it and when
Kraepelin grouped mania and depression as one illness in 1899; Jules Angst and Carlo Perris separated bipolar from 'unipolar' depression in 1966.
The evidence
Lewis Judd's long-term NIMH follow-up found bipolar I patients had symptoms in nearly half of all weeks, mostly depressive.
Open questions
Whether antidepressants should ever be used in bipolar depression, and how to spot bipolar risk at a first depressive episode, are debated.
▸The 'chemical imbalance' theory of depression oversimplified the causes of Depression and mood disordersWeak or failed to replicate
The connection
The theory reduced depression to a chemical shortfall fixable by a pill, sidelining stress, loss, sleep, inflammation and life circumstances. It helped reduce stigma, but research shows depression arises from many interacting causes, and some studies suggest the message makes people more pessimistic about recovering.
Who showed it and when
Popular from the 1990s through drug ads and doctors' explanations; challenged by researchers such as Joanna Moncrieff and Irving Kirsch, and by the 2022 umbrella review.
How solid is it?
Studies find no consistent serotonin deficit in depression; experiments show telling people their depression is chemical can lower confidence in their own ability to recover.
Open questions
Biology clearly matters in depression; the open questions are which processes, and whether they differ between subtypes of patients.
▸Burnout overlaps with Depression and mood disordersMixed evidence
The connection
Burnout's exhaustion, loss of motivation and poor concentration closely resemble depression, and the two often occur together. Burnout is tied to work and may lift with time away; depression spreads across all of life. Some researchers think severe burnout is often depression under a less stigmatized name.
Who showed it and when
Debated especially since 2015, when Renzo Bianchi and colleagues reviewed the evidence and argued that burnout and depression largely overlap.
How solid is it?
Studies find high correlations between burnout exhaustion scores and depression scales, and many people with severe burnout meet depression criteria.
Open questions
Whether burnout is a stage of depression, a distinct condition, or a useful label for work-caused distress is unresolved.
▸Suicide prevention and the 988 lifeline help people in crisis with Depression and mood disordersWell replicated
The connection
Depression is involved in a large share of suicides, so recognizing and treating it is central to prevention. Crisis lines such as 988, safety planning and follow-up after hospital discharge help people through the acute peak, while therapy and medication treat the underlying depression.
Who showed it and when
Organized prevention grew from the 1950s, with the Samaritans (London, 1953) and the Los Angeles Suicide Prevention Center (1958); the US 988 line launched in July 2022.
The evidence
Psychological autopsy studies find a mental disorder, most often depression, in most people who die by suicide; trials show treatment and follow-up contacts reduce attempts.
Open questions
Psychological autopsy methods are debated, and many people who die by suicide were never diagnosed, so reaching them is a key open problem.
▸Loneliness and social isolation raises the risk of Depression and mood disordersWell replicated
The connection
Loneliness predicts later depression even after accounting for earlier symptoms, and depression in turn makes people withdraw, so the two feed each other. Lonely people tend to read ambiguous social signals as rejection, which deepens isolation.
Who showed it and when
John Cacioppo's Chicago study of middle-aged and older adults showed in 2010 that loneliness predicted rising depressive symptoms over five years.
The evidence
Cohort studies that track both over time show the effect runs in both directions, with loneliness predicting future depression in many studies.
Open questions
Whether reducing loneliness prevents depression is not yet shown by large trials; shared causes such as personality may explain part of the link.
▸Cognitive behavioral therapy (CBT; Beck, Ellis) is a first-line treatment for Depression and mood disordersWell replicated
The connection
CBT for depression teaches patients to spot and test negative automatic thoughts and to schedule rewarding activities, countering withdrawal. In trials it works about as well as antidepressants for many patients, and its benefits tend to last longer after treatment ends.
Who showed it and when
Aaron Beck's cognitive therapy at the University of Pennsylvania; his team's 1977 trial compared it with the antidepressant imipramine.
The evidence
Hundreds of trials; a 2005 Penn–Vanderbilt study by Robert DeRubeis and Steven Hollon found it matched medication for moderate to severe depression, with fewer relapses afterward.
Open questions
Effects in newer, better-controlled trials look smaller than in early ones, and simpler behavioral activation may work about as well.
▸SSRI antidepressants (e.g., Prozac) modestly relieve Depression and mood disordersWell replicated
The connection
SSRIs relieve depression for many people, usually over two to six weeks, with benefits most visible in moderate to severe cases. On average the gain over placebo is modest, but some individuals respond strongly. Combining medication with talk therapy often works better than either alone.
Who showed it and when
Tested in hundreds of trials since the 1980s; the large US STAR*D study (2001–2006) followed real-world patients through several treatment steps.
The evidence
Andrea Cipriani's 2018 meta-analysis of 522 trials and about 116,000 patients found all 21 antidepressants studied beat placebo for acute depression.
Open questions
Whether average effects are meaningful in mild depression, why responses vary so much, and how STAR*D's reported remission rates hold up after a 2023 reanalysis.
▸Brain stimulation: ECT and TMS treat severe, treatment-resistant Depression and mood disordersWell replicated
The connection
ECT is the most effective treatment for severe or treatment-resistant depression, including depression with psychosis or high suicide risk, often working within weeks. TMS helps roughly a third to half of patients whom medications failed. Both are usually offered after other treatments.
Who showed it and when
ECT has been used for depression since the 1940s; Mark George and others tested TMS in the 1990s, and a 2007 trial led to FDA clearance in 2008.
The evidence
Controlled trials and meta-analyses, including the UK ECT Review Group's in 2003, show ECT beats sham ECT and medication for severe depression.
Open questions
Relapse after ECT is common without follow-up treatment, and a 2023 trial found ketamine about as effective for some patients without psychosis.
▸Ketamine and esketamine for depression rapidly relieve treatment-resistant Depression and mood disordersMixed evidence
The connection
For people whose depression has not improved after two or more antidepressants, ketamine or esketamine can reduce symptoms, including suicidal thoughts, within hours to a day. Effects usually fade over days to weeks, so treatment means repeated doses under medical supervision.
Who showed it and when
Yale's 2000 study, then NIMH trials led by Carlos Zarate (2006); Johnson & Johnson's Spravato nasal spray won FDA approval in March 2019.
How solid is it?
Multiple randomized trials show rapid effects, but blinding is weak; only one of esketamine's three short-term Phase 3 trials met its main goal.
Open questions
How long benefits last with ongoing use, the risk of misuse, and whether loosely regulated infusion clinics are safe are open questions.
▸Psychedelic-assisted therapy (psilocybin, MDMA) shows early promise for Depression and mood disordersMixed evidence
The connection
In trials, one or two psilocybin sessions, with preparation and follow-up therapy, have reduced depression for weeks or longer. Researchers think the drug may loosen rigid, self-critical thinking patterns, opening a window in which therapy can work.
Who showed it and when
Robin Carhart-Harris at Imperial College London (from 2016), Alan Davis and colleagues at Johns Hopkins (2020), and Compass Pathways' company trials (results from 2022).
How solid is it?
A 2021 trial found psilocybin roughly matched the SSRI escitalopram on its main measure; a 2022 trial of 233 patients found modest benefit at three weeks that faded by twelve.
Open questions
Whether effects hold up with better blinding and longer follow-up, and how they compare with existing treatments in everyday clinics.
▸Mindfulness-based therapies help prevent relapse of Depression and mood disordersWell replicated
The connection
MBCT is for people who have recovered from repeated depression. It trains them to notice early low moods and ruminative thinking without spiraling into a new episode. For people with three or more past episodes, it lowers relapse risk about as well as staying on antidepressants.
Who showed it and when
Designed by Zindel Segal (Toronto), Mark Williams (Bangor, later Oxford) and John Teasdale (Cambridge); the first trial was published in 2000.
The evidence
A 2016 meta-analysis led by Willem Kuyken, nine trials and 1,258 patients, found MBCT cut relapse risk over 60 weeks by about 31 percent versus those not given MBCT.
Open questions
Benefit is clearest for people with three or more past episodes; whether it helps those with fewer, and whether mindfulness itself is the active ingredient, is less clear.