The Chemical Imbalance Theory of Depression: What the Evidence Actually Says, and Why It Matters in the Clinic

Introduction

Few ideas in modern medicine have travelled so far on so little evidence as the "chemical imbalance theory" of depression. For a generation of patients, it was the lay-language summary of why they felt unwell and why a small white tablet was supposed to help: depression, they were told, is caused by a deficiency of serotonin in the brain, and antidepressants correct that deficiency. The story is tidy, intuitive, and reassuringly biological. It is also, on the strongest available reading of the evidence, not true.

This article is written for clinicians who actually have to sit with patients and discuss what is happening, what medication does, and what it does not do. It traces the historical origins of the monoamine hypothesis, examines the empirical evidence that was always thinner than the public messaging suggested, summarises the 2022 Moncrieff et al. umbrella review and the responses it provoked, surveys what we currently understand about the neurobiology of depression beyond serotonin, and turns to the clinical implications, including how to talk honestly with patients who arrive in the consulting room expecting a chemical-imbalance narrative.

The conclusion of this article will not be that antidepressants do not work, or that biology is irrelevant, or that prescribing is wrong. It will be that the simple chemical imbalance story is not supported by evidence, that depression is more biologically and contextually heterogeneous than that story implies, and that honest aetiological communication with patients is both ethically required and clinically beneficial.

1. Historical Origins: How a Hypothesis Became a Slogan

The 1950s pharmacological accidents

The monoamine hypothesis of depression was not built from theoretical first principles; it was reverse-engineered from clinical observations of drugs that already happened to work. In the 1950s, clinicians noticed that iproniazid, a tuberculosis drug that inhibited monoamine oxidase, produced mood elevation in some patients; that imipramine, originally trialled as an antipsychotic, had clear antidepressant effects; and that reserpine, an antihypertensive that depleted central monoamines, sometimes precipitated depressive states (Schildkraut 1965; López-Muñoz and Alamo 2009). From these observations grew the inference that depression must involve a deficiency of monoamines and that antidepressants must correct that deficiency.

In 1965, Joseph Schildkraut, working at the US National Institute of Mental Health, formalised this into the "catecholamine hypothesis of affective disorders," published in the American Journal of Psychiatry. Schildkraut proposed that "some, if not all, depressions are associated with an absolute or relative decrease in catecholamines, particularly norepinephrine, available at central adrenergic receptor sites" (Schildkraut 1965, p. 509). Importantly, Schildkraut himself was epistemically careful: he described the hypothesis as "at best a reductionistic oversimplification" and emphasised that direct biochemical confirmation was lacking (Schildkraut 1965).

Two years later, the British psychiatrist Alec Coppen published "The Biochemistry of Affective Disorders" in the British Journal of Psychiatry (Coppen 1967), proposing a parallel role for serotonin (5-hydroxytryptamine), an indoleamine. Together, the Schildkraut and Coppen papers established what became known as the monoamine theory of depression. They were widely cited, with Schildkraut's paper alone receiving more than a thousand citations in subsequent decades and being recognised as one of the most cited papers in the history of psychiatry (López-Muñoz and Alamo 2009).

From hypothesis to marketing slogan

What happened next is the part of the story that academic psychiatry has, for two decades now, been arguing about. By the early 1990s the hypothesis in its strong form was already in trouble inside academic psychiatry, but at exactly that moment the launch of fluoxetine (Prozac) in 1987 and the subsequent SSRIs through the 1990s turned the hypothesis into the marketing backbone of what would become a multibillion-dollar pharmaceutical category.

In the United States, where direct-to-consumer pharmaceutical advertising is permitted, the chemical imbalance frame became almost ubiquitous. Pfizer's television advertisements for sertraline (Zoloft) told viewers that depression "may be due to a chemical imbalance" and that "Zoloft works to correct this imbalance"; GlaxoSmithKline's advertisements for paroxetine (Paxil/Aropax) similarly stated that "with continued treatment, Paxil can help restore the balance of serotonin" (Lacasse and Leo 2005). In their landmark 2005 PLoS Medicine paper, Jeffrey Lacasse and Jonathan Leo systematically compared the claims being made in such advertisements with the contemporaneous scientific literature and concluded that "there is not a single peer-reviewed article that can be accurately cited to directly support claims of serotonin deficiency in any mental disorder, while there are many articles that present counterevidence" (Lacasse and Leo 2005). Their summary judgement was blunt: "The incongruence between the scientific literature and the claims made in FDA-regulated SSRI advertisements is remarkable, and possibly unparalleled" (Lacasse and Leo 2005).

Australia does not permit direct-to-consumer prescription drug advertising, but the same explanatory frame migrated through medical education, popular journalism, GP–patient conversations, and patient information leaflets. By the mid-2000s, surveys in the United States, the United Kingdom, Australia and elsewhere consistently showed that a large majority of the public endorsed the proposition that depression is caused by a chemical imbalance (Pescosolido et al. 2010; France et al. 2007).

2. The Evidence That Was Never There

The serotonin hypothesis has always rested on indirect inference rather than direct demonstration. The four classical lines of evidence each had problems that were known to specialists from the outset.

Tryptophan depletion studies showed that acutely lowering tryptophan availability could induce mood changes in people with a personal or family history of depression and in those in remission on serotonergic medications, but it generally did not induce depression in healthy volunteers without such histories. This is precisely the wrong pattern of results for a simple deficiency model: low serotonin should produce depression in anyone, not only in those already vulnerable (Ruhé, Mason and Schene 2007; Moncrieff et al. 2022).

5-HIAA studies of cerebrospinal fluid, the standard way of indexing central serotonin turnover, never reliably distinguished people with depression from controls. Moncrieff et al. (2022) found in their umbrella review that "two meta-analyses of a total of 19 studies of 5-HIAA in CSF (seven studies were included in both) found no evidence of an association between 5-HIAA concentrations and depression."

Platelet and post-mortem studies were similarly inconsistent, with some studies showing reduced serotonin uptake or transporter density and others not, and with confounding by prior antidepressant exposure rarely adequately controlled.

The delayed onset problem was always the most damaging conceptual difficulty. SSRIs raise synaptic serotonin within hours, but their clinical effects, when they occur, take weeks to develop. If depression were a simple serotonin deficit, recovery should track the rise in serotonin. It does not. This single fact has been recognised by serotonin researchers for decades and motivated the various "downstream" hypotheses that point to receptor adaptation, gene expression changes, neuroplasticity or BDNF rather than to serotonin levels per se (Castrén 2005; Harmer, Duman and Cowen 2017).

By the early 2000s, leading academic psychiatry textbooks were openly conceding the problem. Stahl's Essential Psychopharmacology stated plainly that "there is no clear and convincing evidence that monoamine deficiency accounts for depression; that is, there is no 'real' monoamine deficit" (Stahl 2000, cited in Lacasse and Leo 2005). The American Psychiatric Press Textbook of Clinical Psychiatry similarly acknowledged that "additional experience has not confirmed the monoamine depletion hypothesis" (Dubovsky, Davies and Dubovsky 2003, cited in Lacasse and Leo 2005). Yet the gap between this academic consensus and what patients were being told in clinics, in pharmaceutical materials, and in mental health literacy campaigns was enormous, and it was that gap that Lacasse and Leo's 2005 paper, and Moncrieff et al.'s 2022 review, have made impossible to ignore.

The Kirsch meta-analyses sharpened the empirical picture from a different angle. Drawing on the full set of trials, including unpublished trials, that had been submitted to the US Food and Drug Administration for SSRI approval, Kirsch and colleagues found that the difference between antidepressant and placebo, while statistically significant, was modest, with a standardised mean difference of d = 0.32 (95% CI 0.25–0.40), a magnitude below conventional thresholds for clinical significance for most patients with mild to moderate depression (Kirsch et al. 2008; Kirsch 2014). This work did not refute antidepressant efficacy outright, but it did dislodge the assumption that the drugs were repairing a specific neurochemical lesion.

3. The Moncrieff et al. 2022 Umbrella Review

In July 2022, Joanna Moncrieff and colleagues at University College London published "The serotonin theory of depression: a systematic umbrella review of the evidence" in Molecular Psychiatry (Moncrieff et al. 2022). The paper attracted extraordinary attention; according to its Nature/Molecular Psychiatry page it has accumulated approximately 1.86 million accesses and an Altmetric score of around 9,684, placing it in the top 5% of all scientific outputs ever tracked by Altmetric.

The review synthesised meta-analyses and high-quality systematic reviews across six domains:

1. Serotonin and 5-HIAA concentrations in plasma and cerebrospinal fluid.

2. 5-HT1A receptor binding measured by neuroimaging or post-mortem.

3. Serotonin transporter (SERT) levels, again by imaging and post-mortem.

4. Tryptophan depletion studies.

5. SERT gene (5-HTTLPR) association with depression.

6. SERT gene-by-environment interaction with stressful life events.

The conclusions, in the authors' own words, were that "the main areas of serotonin research provide no consistent evidence of there being an association between serotonin and depression, and no support for the hypothesis that depression is caused by lowered serotonin activity or concentrations" and that "some evidence was consistent with the possibility that long-term antidepressant use reduces serotonin concentration" (Moncrieff et al. 2022). The abstract also reported that "two meta-analyses of overlapping studies examining the serotonin metabolite, 5-HIAA, showed no association with depression (largest n = 1002)" and that, on tryptophan depletion, "one meta-analysis of tryptophan depletion studies found no effect in most healthy volunteers (n = 566), but weak evidence of an effect in those with a family history of depression (n = 75)."

On gene–environment interaction, the two largest and highest-quality studies, Border et al. (2019) and Culverhouse et al. (2018), found no interaction between 5-HTTLPR genotype, stressful life events and depression, contradicting the famous Caspi et al. (2003) finding that had launched a thousand candidate-gene studies. Moncrieff et al.'s overall discussion was unambiguous: "Our comprehensive review of the major strands of research on serotonin shows there is no convincing evidence that depression is associated with, or caused by, lower serotonin concentrations or activity… it is time to acknowledge that the serotonin theory of depression is not empirically substantiated."

The media response was, predictably, polarised. Headlines in the Guardian, the BBC and elsewhere ran some version of "depression is not caused by a chemical imbalance," and the paper was variously celebrated as overdue truth-telling and condemned as oversold and dangerous. The authors themselves were careful not to claim that the paper had proven antidepressants do not work; they argued, rather, that the evidence base for the specific serotonin-deficiency rationale used to justify their use to patients was not there.

4. Critical Responses: Was the Theory a Straw Man?

The Moncrieff paper provoked an immediate and substantial pushback from mainstream academic psychiatry. The most prominent rebuttal, "A leaky umbrella has little value: evidence clearly indicates the serotonin system is implicated in depression," was published in Molecular Psychiatry in 2023 by Sameer Jauhar and 35 co-authors, including many of the most prominent affective disorders researchers in the United Kingdom (Jauhar et al. 2023). Jauhar and colleagues argued that the umbrella review was methodologically flawed: that "the methodology is inconsistent with an umbrella review, with substantial bias created by the authors' chosen quality criteria, selective reporting, and interpretation of results" and that "there is an underappreciation of the complexities of neuroscience and neuropsychopharmacology" (Jauhar et al. 2023). They argued that Moncrieff et al. mistakenly assumed 5-HT1A receptors were exclusively pre-synaptic autoreceptors when most are post-synaptic heteroreceptors, that they failed to account for drug-naïve participants in some of the SERT meta-analyses (citing one in which 149 of 364 participants were unmedicated), and that they omitted relevant tryptophan depletion findings, including a Hedge's g of −1.9 (95% CI −3.02 to −0.78) for mood effects in depressed people not taking antidepressants. Their position was that "a more accurate, constructive conclusion would be that acute tryptophan depletion and decreased plasma tryptophan in depression indicate a role for 5-HT in those vulnerable to or suffering from depression, and that molecular imaging suggests the system is perturbed. The proven efficacy of SSRIs in a proportion of people with depression lends credibility to this position" (Jauhar et al. 2023).

Importantly, Jauhar and colleagues did not defend a simple chemical imbalance position. Their argument was, in effect, that Moncrieff was attacking a straw man: that no contemporary serotonin researcher believes depression is caused by a serotonin deficit in any simple sense, and that the relevant scientific question is the more nuanced one of whether the serotonergic system is perturbed in depression in ways that interact with stress, plasticity, and other systems.

This is essentially the position long taken by the American psychiatrist Ronald Pies, who in a now widely-quoted 2011 Psychiatric Times essay declared that "in truth, the 'chemical imbalance' notion was always a kind of urban legend, never a theory seriously propounded by well-informed psychiatrists" (Pies 2011), and who has reiterated the argument repeatedly since (Pies 2019). Pies acknowledges that pharmaceutical advertising promoted the chemical imbalance frame but maintains that academic psychiatry never seriously held it.

This is where the controversy turns sharp, because Lacasse and Leo, joined more recently by Moncrieff and Horowitz, have argued that this framing is historically inaccurate (Lacasse and Leo 2015; Ang, Horowitz and Moncrieff 2022). They have documented quotes from psychiatric textbooks, from official statements of national professional bodies, from patient education materials produced by mental health organisations, and from clinical practice in which the chemical imbalance framing was not merely tolerated but actively propagated by psychiatry as a profession. Ang, Horowitz and Moncrieff (2022) showed that the serotonin theory was endorsed in major English-language psychiatry textbooks well into the 2010s and was actively being defended in the academic literature, not merely passively unchallenged.

The fairest summary of this debate is probably this: the strong claim that "depression is caused by a serotonin deficiency" was not what most academic researchers wrote in the technical literature, but it was what the field was content to allow patients to believe, and in many cases what individual clinicians actively told their patients. The distinction between "what specialists believed" and "what patients were told" is the critical one, and it does not flatter the profession.

A separate, more technical critique of the umbrella review concerns the level of analysis. Umbrella reviews of biomarker research are notoriously difficult to do well, and Jauhar and colleagues are correct that some of the methodological choices made by Moncrieff et al. are arguable. But the broad conclusion, that the empirical evidence does not support a simple low-serotonin model of depression, is not seriously contested by anyone in the field. What is contested is whether that conclusion is news, whether it is overstated, and what its clinical implications are.

5. What We Actually Know About the Neurobiology of Depression

Rejecting a simple chemical imbalance story does not mean rejecting biology. It means taking biology seriously enough to acknowledge how much more complicated the picture is. Several substantive lines of contemporary research deserve mention.

Monoamines remain implicated, but not as deficits. The serotonergic, noradrenergic and dopaminergic systems are clearly involved in mood regulation, in reward processing, and in the action of antidepressants. The sophisticated current view, articulated by researchers such as Catherine Harmer, is that SSRIs may exert their early effects by altering the cognitive and emotional processing of negatively valenced stimuli, with mood improvement following from changes in how patients perceive and interact with their environment over weeks (Harmer, Duman and Cowen 2017). This is a long way from "fixing a deficit."

Neuroplasticity and BDNF. A substantial body of work, beginning with Duman and Monteggia (2006), has implicated brain-derived neurotrophic factor (BDNF) and stress-induced atrophy of hippocampal and prefrontal circuits in the pathophysiology of depression. The neurotrophic hypothesis proposes that chronic stress reduces BDNF and dendritic complexity, that depression is associated with reduced hippocampal volume, and that antidepressants of various classes (including ketamine, exercise, and electroconvulsive therapy) share an ability to enhance neuroplasticity (Duman and Monteggia 2006; Castrén 2005).

HPA axis dysregulation. Hypercortisolaemia, abnormal dexamethasone suppression, and impaired glucocorticoid receptor feedback have been documented in depression, particularly melancholic and psychotic subtypes (Holsboer 2000; Pariante and Lightman 2008). The corticosteroid receptor hypothesis frames depression as a disorder of stress-system regulation rather than monoamine concentration.

Neuroinflammation. A growing literature implicates pro-inflammatory cytokines (IL-6, TNF-α, IL-1β), microglial activation and the kynurenine pathway in a subset of patients with depression, particularly those with treatment resistance and elevated C-reactive protein (Dantzer et al. 2008; Miller and Raison 2016). This work has motivated trials of anti-inflammatory adjuncts, with mixed results.

Glutamate and the ketamine story. The dramatic and rapid antidepressant effects of subanaesthetic ketamine, an NMDA receptor antagonist, have refocused attention on glutamatergic transmission and synaptic plasticity (Berman et al. 2000; Krystal et al. 2019). Esketamine is now licensed in many jurisdictions for treatment-resistant depression. Whatever ketamine is doing, it is not raising serotonin.

Default mode network and connectivity. Neuroimaging work has identified disrupted connectivity in the default mode network (DMN), particularly increased within-DMN connectivity correlating with rumination, as a relatively replicated finding in depression (Hamilton et al. 2015; Kaiser et al. 2015). This suggests that depression involves system-level alterations in how networks engage and disengage, not just neurochemical concentrations.

Stress-diathesis and gene-environment interaction. Despite the failure of single-candidate-gene studies such as the original Caspi et al. (2003) 5-HTTLPR finding to replicate (Border et al. 2019), polygenic risk scores combined with adverse environmental exposures (childhood adversity, recent stressors) account for a meaningful portion of variance in depression liability. Depression is heritable but not in any simple Mendelian sense.

The heterogeneity problem. Major depressive disorder, as it appears in the DSM-5-TR and ICD-11, is a syndrome defined by symptom counting rather than aetiology. Two patients can both meet criteria with almost no symptom overlap. It is increasingly recognised that "depression" is not a single thing biologically and that the search for "the" pathophysiology has been chasing a category error (Fried and Nesse 2015).

Network theory of mental disorders. Denny Borsboom's network approach (Borsboom 2017) reframes mental disorders not as latent diseases producing symptoms, but as causal networks of symptoms that interact directly: insomnia produces fatigue, fatigue produces concentration problems, concentration problems produce hopelessness, and so on, with the network sometimes settling into a self-sustaining "depressed" attractor state. This perspective is consistent with depression being multiply realisable across biology, psychology and social context, and with treatment working by perturbing the network at any of multiple nodes.

The honest summary is this: depression is real, it has biological correlates, and several distinct biological systems are implicated. There is, however, no single neurotransmitter deficit, no biomarker, no biochemical test that will tell you whether a given person has the condition or how to treat them. Anyone who tells a patient otherwise is overstating what we know.

6. Why Antidepressants Still "Work" for Some People

It is critical not to confuse two separate questions: "is the chemical imbalance theory true?" and "do antidepressants help anyone?" The answer to the first is no; the answer to the second is yes, for some people, sometimes, by mechanisms that may have little to do with the rationale historically given to patients.

The Cipriani et al. (2018) network meta-analysis of 21 antidepressants in acute treatment of major depressive disorder, drawing on 522 trials and more than 116,000 participants, found that all antidepressants studied were more efficacious than placebo, with a pooled standardised mean difference of 0.30 (95% credible interval 0.26–0.34) (Cipriani et al. 2018). This is statistically robust but clinically modest, and it is consistent with the smaller meta-analyses by Kirsch and colleagues. The Cipriani analysis also showed that head-to-head differences between agents are small and that tolerability differs more meaningfully than efficacy.

The STARD trial, the largest pragmatic effectiveness trial ever conducted in depression, was originally reported as showing a cumulative remission rate of 67% across four sequential treatment steps (Rush et al. 2006). A protocol-fidelity reanalysis by Pigott et al. (2023) found that, when the original protocol's outcome measure and inclusion criteria were applied, the cumulative remission rate was approximately 35%, with sustained remission at 12 months in only a small minority of participants. The STARD investigators have contested this reanalysis (Rush et al. 2023), but the disagreement underscores how much less impressive real-world antidepressant outcomes look when scrutinised carefully.

How can drugs work without correcting an aetiological imbalance? The most coherent alternative framework is Joanna Moncrieff and David Cohen's distinction between disease-centred and drug-centred models of psychiatric drug action (Moncrieff and Cohen 2005). The disease-centred model assumes that drugs work by reversing an underlying pathological process, the way insulin reverses hyperglycaemia in diabetes. The drug-centred model suggests instead that psychoactive drugs produce characteristic mental and physical states (sedation, emotional blunting, activation, anxiolysis) that may be useful to people experiencing certain symptoms, regardless of whether those symptoms reflect any underlying biochemical lesion. SSRIs in this framing produce a state of mild emotional blunting and reduced reactivity to negative stimuli that some patients find helpful and others find intolerable; the helpful effect is not evidence of an underlying serotonin deficiency.

The active placebo critique is also relevant. Because SSRIs and other antidepressants produce noticeable physical effects (nausea, sexual dysfunction, sleep changes), patients in trials can often guess whether they have received drug or placebo, and the drug-placebo difference may be partly an enhanced placebo response in those who correctly identify their assignment (Kirsch 2014).

Finally, the question of long-term outcomes and withdrawal has moved from the fringe to the mainstream over the past five years. Mark Horowitz and David Taylor's 2019 paper in Lancet Psychiatry on hyperbolic tapering of SSRIs has reframed clinical practice, proposing that doses should be reduced in progressively smaller steps to achieve a linear reduction in serotonin transporter occupancy and minimise withdrawal (Horowitz and Taylor 2019). The Royal College of Psychiatrists' 2019 Position Statement PS04/19 on Antidepressants and Depression formally acknowledged that withdrawal can be more severe and protracted than previously recognised and called for updated NICE guidance, which followed in October 2019 (Royal College of Psychiatrists 2019). The Maudsley Deprescribing Guidelines(Horowitz and Taylor 2024) now codify hyperbolic tapering for antidepressants, benzodiazepines, gabapentinoids and Z-drugs, reflecting a substantive shift in how the field thinks about long-term antidepressant use.

7. Clinical Implications for Practitioners

This is where the rubber meets the road. What should a clinical psychologist, psychiatrist or general practitioner actually do in light of this evidence?

Honest aetiological communication

Patients deserve to be told what we know and what we do not. Telling a patient "you have a chemical imbalance and this medication will correct it" is, on the strongest available reading of the evidence, false. It is also unethical from an informed consent standpoint, because it presents a contested mechanism as an established one. A more defensible formulation might be: "Depression is real, but we don't fully understand its biological causes. There isn't a simple chemical imbalance we can measure. Antidepressants help some people, and we have evidence they reduce symptoms on average more than placebo, though the effect for any individual is hard to predict and we don't entirely understand why they work."

The deficit-framing problem

The chemical imbalance explanation is not just empirically problematic; it has documented psychological costs for patients who endorse it. Brett Deacon and Grayson Baird (2009), in a now well-known experimental study, found that participants given a chemical imbalance explanation for depression showed reduced self-blame but also significantly increased prognostic pessimism and a stronger belief that psychotherapy would be ineffective compared with a biopsychosocial frame. Kemp, Lickel and Deacon (2014) replicated and extended this in people with current or past depression, showing that bogus "chemical imbalance test feedback" produced worse prognostic pessimism, worse mood-regulation expectancies, and a perception that pharmacotherapy was more credible than psychotherapy. Subsequent work in healthy and depressed samples has broadly converged on the same finding: biogenetic explanations reduce blame but increase prognostic pessimism, decrease self-efficacy and shift treatment preferences toward medication (Lebowitz and Ahn 2018; Schroder et al. 2020).

The stigma picture is also more mixed than the public health case for biogenetic framing originally assumed. Haslam and Kvaale's (2015) meta-analysis showed that biogenetic explanations of mental illness reduce blame but increase essentialism, perceived dangerousness and desire for social distance.

In short, telling patients their depression is a chemical imbalance may make them feel less personally at fault, but it tends to make them feel more pathologically broken, less capable of recovering through their own efforts, and less interested in the psychological work that, for many, is the most durable route to lasting improvement.

How to discuss antidepressants without invoking a false aetiological model

Drawing on the drug-centred model and on what we actually know, the following framings are honest and clinically useful:

• "Antidepressants change how the brain processes emotional information. For some people this is helpful; for others it is not. We can try one and see how it goes for you."

• "The evidence is that, on average, antidepressants reduce depressive symptoms more than placebo, but the effect is modest, and we cannot reliably predict who will benefit. A trial of around four to six weeks is usually needed to know."

• "If you find the medication helps, that is real, but it does not mean you had a chemical imbalance. It means this drug, in your particular brain and circumstances, is doing something useful."

• "If you take the medication for any extended period, we will need to plan carefully for how to stop it, because withdrawal symptoms are common and can be uncomfortable."

Implications for psychological therapy

For clinical psychologists in particular, the collapse of the chemical imbalance frame is not an attack on biological psychiatry; it is a reaffirmation of the biopsychosocial model that was always supposed to be the standard of care. Psychological therapies for depression, including cognitive-behavioural therapy, behavioural activation, interpersonal therapy and mindfulness-based cognitive therapy, have effect sizes broadly comparable to antidepressants in mild-to-moderate depression and substantially better long-term durability for relapse prevention (Cuijpers et al. 2020). When patients are told their depression is a brain disease that requires a brain drug, they are less likely to engage seriously with the psychological work; when they are given an honest, multifactorial frame, they tend to engage with both medication, where appropriate, and therapy.

Australian context

The Australian context deserves specific attention. According to the Australian Institute of Health and Welfare, in 2023–24 about 18% of the Australian population (around 5 million people) were dispensed a mental health-related prescription, and about 84% of these prescriptions were written by general practitioners (AIHW 2024). Antidepressants accounted for approximately 73% of mental health-related prescription volume. Malhi et al. (2022), analysing PBS data, showed that GPs initiate around three-quarters of first antidepressant prescriptions in Australia, with psychiatrists involved in only a small fraction. Australia ranks among the highest per-capita antidepressant consumers in the OECD, with most recent OECD comparative data placing Australia at approximately 127.9 defined daily doses per 1,000 inhabitants per day, behind Iceland, Portugal, the United Kingdom and Canada (OECD 2024).

These numbers matter because they tell us where the conversations about aetiology are actually happening: in 15-minute GP consultations, often without the time, training or systemic support to engage with the nuances we have been discussing. Whatever the Royal Australian and New Zealand College of Psychiatrists position statements say, the de facto front line of psychiatric prescribing in Australia is general practice. The RANZCP's 2020 Mood Disorders Clinical Practice Guidelines (Malhi et al. 2021) provide measured, evidence-based recommendations about the use of antidepressants but say relatively little about how clinicians should explain mechanism to patients. The College's public-facing materials have, until recently, included statements that medications work by "rebalancing the chemicals in the brain," a formulation that has attracted criticism from Australian critical psychiatry voices (PsychWatch Australia 2022) and that sits awkwardly with the contemporary evidence base.

Practical clinical implications, summarised

For clinicians communicating with patients about depression and antidepressants, the following are defensible and supported by the evidence:

• Acknowledge that depression is real and has biological correlates, without claiming a specific neurochemical cause that cannot be measured.

• Avoid the phrase "chemical imbalance" as an aetiological explanation. Be willing to say "we don't know" when asked.

• Frame antidepressants as agents that can help some people reduce symptoms on average more than placebo, with mechanism not fully understood, rather than as correctives of a deficit.

• Discuss withdrawal and tapering at the time of initiation, not only at discontinuation.

• Take psychological, social, relational and contextual contributors seriously as part of formulation, not as soft additions to a "real" biological explanation.

• Document informed consent that reflects genuine clinical equipoise rather than marketing-derived simplifications.

8. Conclusion

The chemical imbalance theory of depression, in the form in which it was sold to the public and to patients from the 1990s onward, is not supported by the evidence and was never as solidly held within academic psychiatry as the public messaging implied. The 2022 Moncrieff et al. umbrella review crystallised what specialists had been quietly conceding for two decades: there is no consistent evidence that depression is caused by lowered serotonin, and the lines of research that were supposed to establish this have largely failed to do so. The vigorous responses from Jauhar and colleagues do not rescue a strong chemical imbalance position; they relocate it to a more defensible and considerably more modest claim that the serotonergic system is one of several systems implicated in a complex, heterogeneous syndrome.

This does not mean depression is "just psychological," that biology is irrelevant, or that antidepressants do not work. It means that depression is a multifactorial syndrome involving plasticity, stress regulation, immune function, network connectivity and environmental context, and that antidepressants help some people through mechanisms that may not match the rationale historically given for prescribing them.

The clinical implication is straightforward and ethically demanding: stop telling patients they have a chemical imbalance. Tell them, instead, what is true. Depression is real, our understanding is incomplete, antidepressants are one option among several, psychological therapies have comparable evidence and better durability, and the work of recovery is rarely reducible to a single pill. This is a harder conversation than the chemical imbalance script, but it is the one the evidence requires, and it is the one most likely to leave patients with a sense of agency, accurate expectations, and a treatment plan they can genuinely consent to.

If psychiatry as a field is to maintain credibility through the next decade, it will need to do what every mature science eventually does: let go of a beloved simplifying story that turned out to be wrong, sit honestly with the complexity that remains, and rebuild patient communication on a foundation of what we actually know rather than what was once convenient to say.

References

AIHW (Australian Institute of Health and Welfare) 2024. Mental health prescriptions, 2023–24. Canberra: AIHW.

Ang, B., Horowitz, M. & Moncrieff, J. 2022. 'Is the chemical imbalance an "urban legend"? An exploration of the status of the serotonin theory of depression in the scientific literature.' SSM-Mental Health, 2: 100098.

Berman, R.M., Cappiello, A., Anand, A., Oren, D.A., Heninger, G.R., Charney, D.S. & Krystal, J.H. 2000. 'Antidepressant effects of ketamine in depressed patients.' Biological Psychiatry, 47(4): 351–354.

Border, R., Johnson, E.C., Evans, L.M., Smolen, A., Berley, N., Sullivan, P.F. & Keller, M.C. 2019. 'No support for historical candidate gene or candidate gene-by-interaction hypotheses for major depression across multiple large samples.' American Journal of Psychiatry, 176(5): 376–387.

Borsboom, D. 2017. 'A network theory of mental disorders.' World Psychiatry, 16(1): 5–13.

Caspi, A., Sugden, K., Moffitt, T.E., Taylor, A., Craig, I.W., Harrington, H., McClay, J., Mill, J., Martin, J., Braithwaite, A. & Poulton, R. 2003. 'Influence of life stress on depression: moderation by a polymorphism in the 5-HTT gene.' Science, 301(5631): 386–389.

Castrén, E. 2005. 'Is mood chemistry?' Nature Reviews Neuroscience, 6(3): 241–246.

Cipriani, A., Furukawa, T.A., Salanti, G., Chaimani, A., Atkinson, L.Z., Ogawa, Y., Leucht, S., Ruhe, H.G., Turner, E.H., Higgins, J.P.T., Egger, M., Takeshima, N., Hayasaka, Y., Imai, H., Shinohara, K., Tajika, A., Ioannidis, J.P.A. & Geddes, J.R. 2018. 'Comparative efficacy and acceptability of 21 antidepressant drugs for the acute treatment of adults with major depressive disorder: a systematic review and network meta-analysis.' Lancet, 391(10128): 1357–1366.

Coppen, A. 1967. 'The biochemistry of affective disorders.' British Journal of Psychiatry, 113(504): 1237–1264.

Cuijpers, P., Karyotaki, E., Eckshtain, D., Ng, M.Y., Corteselli, K.A., Noma, H., Quero, S. & Weisz, J.R. 2020. 'Psychotherapy for depression across different age groups: a systematic review and meta-analysis.' JAMA Psychiatry, 77(7): 694–702.

Culverhouse, R.C., Saccone, N.L., Horton, A.C., Ma, Y., Anstey, K.J., Banaschewski, T., et al. 2018. 'Collaborative meta-analysis finds no evidence of a strong interaction between stress and 5-HTTLPR genotype contributing to the development of depression.' Molecular Psychiatry, 23(1): 133–142.

Dantzer, R., O'Connor, J.C., Freund, G.G., Johnson, R.W. & Kelley, K.W. 2008. 'From inflammation to sickness and depression: when the immune system subjugates the brain.' Nature Reviews Neuroscience, 9(1): 46–56.

Deacon, B.J. & Baird, G.L. 2009. 'The chemical imbalance explanation of depression: reducing blame at what cost?' Journal of Social and Clinical Psychology, 28(4): 415–435.

Dubovsky, S.L., Davies, R. & Dubovsky, A.N. 2003. 'Mood disorders.' In: Hales, R.E. & Yudofsky, S.C. (eds), American Psychiatric Press Textbook of Clinical Psychiatry, 4th edn. Washington, DC: American Psychiatric Press.

Duman, R.S. & Monteggia, L.M. 2006. 'A neurotrophic model for stress-related mood disorders.' Biological Psychiatry, 59(12): 1116–1127.

France, C.M., Lysaker, P.H. & Robinson, R.P. 2007. 'The "chemical imbalance" explanation for depression: origins, lay endorsement, and clinical implications.' Professional Psychology: Research and Practice, 38(4): 411–420.

Fried, E.I. & Nesse, R.M. 2015. 'Depression sum-scores don't add up: why analyzing specific depression symptoms is essential.' BMC Medicine, 13: 72.

Hamilton, J.P., Farmer, M., Fogelman, P. & Gotlib, I.H. 2015. 'Depressive rumination, the default-mode network, and the dark matter of clinical neuroscience.' Biological Psychiatry, 78(4): 224–230.

Harmer, C.J., Duman, R.S. & Cowen, P.J. 2017. 'How do antidepressants work? New perspectives for refining future treatment approaches.' Lancet Psychiatry, 4(5): 409–418.

Haslam, N. & Kvaale, E.P. 2015. 'Biogenetic explanations of mental disorder: the mixed-blessings model.' Current Directions in Psychological Science, 24(5): 399–404.

Holsboer, F. 2000. 'The corticosteroid receptor hypothesis of depression.' Neuropsychopharmacology, 23(5): 477–501.

Horowitz, M.A. & Taylor, D. 2019. 'Tapering of SSRI treatment to mitigate withdrawal symptoms.' Lancet Psychiatry, 6(6): 538–546.

Horowitz, M.A. & Taylor, D. 2024. The Maudsley Deprescribing Guidelines: Antidepressants, Benzodiazepines, Gabapentinoids and Z-drugs. Oxford: Wiley-Blackwell.

Jauhar, S., Arnone, D., Baldwin, D.S., Bloomfield, M., Browning, M., Cleare, A.J., et al. 2023. 'A leaky umbrella has little value: evidence clearly indicates the serotonin system is implicated in depression.' Molecular Psychiatry, 28(8): 3149–3152.

Kaiser, R.H., Andrews-Hanna, J.R., Wager, T.D. & Pizzagalli, D.A. 2015. 'Large-scale network dysfunction in major depressive disorder: a meta-analysis of resting-state functional connectivity.' JAMA Psychiatry, 72(6): 603–611.

Kemp, J.J., Lickel, J.J. & Deacon, B.J. 2014. 'Effects of a chemical imbalance causal explanation on individuals' perceptions of their depressive symptoms.' Behaviour Research and Therapy, 56: 47–52.

Kirsch, I. 2014. 'Antidepressants and the placebo effect.' Zeitschrift für Psychologie, 222(3): 128–134.

Kirsch, I., Deacon, B.J., Huedo-Medina, T.B., Scoboria, A., Moore, T.J. & Johnson, B.T. 2008. 'Initial severity and antidepressant benefits: a meta-analysis of data submitted to the Food and Drug Administration.' PLoS Medicine, 5(2): e45.

Krystal, J.H., Abdallah, C.G., Sanacora, G., Charney, D.S. & Duman, R.S. 2019. 'Ketamine: a paradigm shift for depression research and treatment.' Neuron, 101(5): 774–778.

Lacasse, J.R. & Leo, J. 2005. 'Serotonin and depression: a disconnect between the advertisements and the scientific literature.' PLoS Medicine, 2(12): e392.

Lacasse, J.R. & Leo, J. 2015. 'Antidepressants and the chemical imbalance theory of depression: a reflection and update on the discourse.' The Behavior Therapist, 38(7): 206–213.

Lebowitz, M.S. & Ahn, W. 2018. 'Blue genes? Understanding and mitigating negative consequences of personalized information about genetic risk for depression.' Journal of Genetic Counseling, 27(1): 204–216.

López-Muñoz, F. & Alamo, C. 2009. 'Monoaminergic neurotransmission: the history of the discovery of antidepressants from 1950s until today.' Current Pharmaceutical Design, 15(14): 1563–1586.

Malhi, G.S., Bell, E., Bassett, D., Boyce, P., Bryant, R., Hazell, P., Hopwood, M., Lyndon, B., Mulder, R., Porter, R., Singh, A.B. & Murray, G. 2021. 'The 2020 Royal Australian and New Zealand College of Psychiatrists clinical practice guidelines for mood disorders.' Australian and New Zealand Journal of Psychiatry, 55(1): 7–117.

Malhi, G.S., Acar, M., Kouhkamari, M.H., Chien, T.H., Juneja, P., Siva, S. & Baune, B.T. 2022. 'Antidepressant prescribing patterns in Australia.' BJPsych Open, 8(4): e120.

Miller, A.H. & Raison, C.L. 2016. 'The role of inflammation in depression: from evolutionary imperative to modern treatment target.' Nature Reviews Immunology, 16(1): 22–34.

Moncrieff, J. & Cohen, D. 2005. 'Rethinking models of psychotropic drug action.' Psychotherapy and Psychosomatics, 74(3): 145–153.

Moncrieff, J., Cooper, R.E., Stockmann, T., Amendola, S., Hengartner, M.P. & Horowitz, M.A. 2022. 'The serotonin theory of depression: a systematic umbrella review of the evidence.' Molecular Psychiatry, 28(8): 3243–3256.

OECD 2024. Health at a Glance 2023: Pharmaceutical consumption. Paris: OECD Publishing.

Pariante, C.M. & Lightman, S.L. 2008. 'The HPA axis in major depression: classical theories and new developments.' Trends in Neurosciences, 31(9): 464–468.

Pescosolido, B.A., Martin, J.K., Long, J.S., Medina, T.R., Phelan, J.C. & Link, B.G. 2010. '"A disease like any other"? A decade of change in public reactions to schizophrenia, depression, and alcohol dependence.' American Journal of Psychiatry, 167(11): 1321–1330.

Pies, R.W. 2011. 'Psychiatry's new brain-mind and the legend of the "chemical imbalance".' Psychiatric Times, 11 July.

Pies, R.W. 2019. 'Debunking the two chemical imbalance myths, again.' Psychiatric Times, 2 August.

Pigott, H.E., Kim, T., Xu, C., Kirsch, I. & Amsterdam, J. 2023. 'What are the treatment remission, response and extent of improvement rates after up to four trials of antidepressant therapies in real-world depressed patients? A reanalysis of the STAR*D study's patient-level data with fidelity to the original research protocol.' BMJ Open, 13(7): e063095.

PsychWatch Australia 2022. 'Myth persists at RANZCP: College of Psychiatrists still claims that meds rebalance brain chemicals.' Available at: psychwatchaustralia.com.

Royal College of Psychiatrists 2019. Position Statement PS04/19: Antidepressants and depression. London: RCPsych.

Ruhé, H.G., Mason, N.S. & Schene, A.H. 2007. 'Mood is indirectly related to serotonin, norepinephrine and dopamine levels in humans: a meta-analysis of monoamine depletion studies.' Molecular Psychiatry, 12(4): 331–359.

Rush, A.J., Trivedi, M.H., Wisniewski, S.R., Nierenberg, A.A., Stewart, J.W., Warden, D., et al. 2006. 'Acute and longer-term outcomes in depressed outpatients requiring one or several treatment steps: a STAR*D report.' American Journal of Psychiatry, 163(11): 1905–1917.

Rush, A.J., Trivedi, M.H., Wisniewski, S.R., Nierenberg, A.A., Stewart, J.W. & Fava, M. 2023. 'The STAR*D data remain strong: reply to Pigott et al.' American Journal of Psychiatry, 180(12): 919–920.

Schildkraut, J.J. 1965. 'The catecholamine hypothesis of affective disorders: a review of supporting evidence.' American Journal of Psychiatry, 122(5): 509–522.

Schroder, H.S., Duda, J.M., Christensen, K., Beard, C. & Björgvinsson, T. 2020. 'Stressors and chemical imbalances: beliefs about the causes of depression in an acute psychiatric treatment sample.' Journal of Affective Disorders, 276: 537–545.

Stahl, S.M. 2000. Essential Psychopharmacology: Neuroscientific Basis and Practical Applications, 2nd edn. Cambridge: Cambridge University Press.