J Health Behav Med Hist 2025-31.
Medicine is missing an organ – and obesity treatment has exposed the gap.
Robert C. van de Graaf, MD, director
MEDTCC Institute for Health, Behaviour, Medicine and its History, The Netherlands
Modern medicine is organised around organs. We diagnose their diseases, train specialists to treat them, and construct entire health systems around their failures. Yet the leading causes of global morbidity and early mortality – unhealthy eating, tobacco use, alcohol misuse, drug dependence, sedentary behaviour, compulsive gambling, and harmful digital use – do not originate in any recognised organ. These conditions arise from behaviour. What is missing is the organ that produces it.
The issue is not that medicine needs a new organ. The organ already exists – medicine has simply never learned to see, understand, and treat it.
Human behaviour – what we eat, crave, inhale, inject, click, avoid, or repeat – is generated by a distributed biological system that integrates sensory signals, neural circuits, hormonal cues, metabolic states, stress physiology, social norms, and commercial and digital architectures. In modern medicine, an organ can not only be defined by its anatomical location, but also by its function. Like the immune or endocrine system, this behavioural organ consists of multiple components acting together to perform a unified role: the regulation of behaviour in a changing environment.
Despite its centrality, this system has no name, no medical specialty, and no clinical owner. Without recognition, its dysfunction remains conceptually invisible. No diagnostic framework describes its collapse; no safety standards protect it; no medical discipline prevents iatrogenic harm. To correct this omission, we need to recognise this system for what it is: the behavioural organ.
Dysfunction of this organ drives more disease than any other. It fuels obesity, type 2 diabetes, cardiovascular disease, liver disease, many cancers, depression, anxiety, burnout, criminal behaviour, relationship breakdown, and widespread disability. These conditions appear diverse, yet their origins converge in reward dysregulation, habit fixation, stress-driven behavioural collapse, sleep disruption, exhaustion, deprivation, and environments engineered for instant consumption and hyperstimulation. Seen together, they reveal a single organ under strain – one whose dysregulated outputs sustain global epidemics.
What has changed is not human biology but the environment. Over millennia, behaviour evolved as the primary interface between organisms and their surroundings: a flexible, rapid-response organ enabling survival, exploration, bonding, and energy regulation. Today, this ancient system is overwhelmed by environments designed to hijack its vulnerabilities. High-calorie palatable foods, psychoactive substances, digital reward loops, engineered attention capture, and chronic stress create conditions in which the behavioural organ is chronically dysregulated. Yet medicine continues to mainly treat downstream consequences rather than the organ whose malfunction drives them.
However, medicine is already intervening directly in the behavioural organ – often powerfully – without acknowledging it. Addiction pharmacotherapies, such as methadone, varenicline and disulfiram alter craving and reinforcement. But the most dramatic evidence comes from obesity treatment. GLP-1 agonists and bariatric procedures do not primarily target digestion or fat tissue; they target another organ. They alter appetite, satiety, reward sensitivity, motivation, impulsivity, and emotional regulation. They reshape the behavioural organ itself.
This creates a profound clinical paradox. For the first time, behaviour is being biologically altered at population scale, yet these interventions are delivered predominantly by specialists trained in gastrointestinal or endocrine organs – not in the behavioural organ whose function they are manipulating. As a result, predictable complications of behavioural-organ injury – addiction transfer, compulsive gambling or shopping, alcohol misuse after bariatric surgery, disordered eating, behavioural collapse under stress, mood instability, and disturbances in social functioning and identity – are commonly misinterpreted as psychological weakness, if recognised at all. In reality, they are often iatrogenic injuries to an organ medicine has never named.
We would not accept cardiologists inadvertently damaging the immune system, nor neurologists manipulating endocrine function without training. Yet we accept surgeons and metabolic physicians altering behaviour without any recognised background in behavioural physiology. This is no longer tenable.
By the established criteria of modern physiology – unified function, characteristic dysfunction, identifiable mechanisms, and targetable pathways – the behavioural organ qualifies unequivocally. Its mechanisms include reward processing, habit formation, attention capture, memory, stress responses, sleep–wake regulation, metabolic signalling, executive function, social norms, and environmental influence. Its dysfunction manifests in craving, compulsion, reward hijacking, habit collapse, substitution behaviour, and behavioural instability. Its treatments already exist across pharmacology, psychotherapy, behavioural interventions, coaching, and environmental redesign.
Recognising the behavioural organ is not symbolic. It is structural. Naming an organ creates coherence, responsibility, safety standards, specialist training, precision diagnostics and treatment techniques, and ethical boundaries. Most importantly, it protects patients from unintended behavioural harms of medical and commercial interventions.
The rapid rise of pharmacological behaviour-modifying obesity treatments has exposed a blind spot medicine can no longer afford. Until the behavioural organ is formally recognised, its diseases will remain misunderstood, its complications misattributed or missed, and its treatment scattered across specialties not trained to manage it.
A healthy behavioural organ produces healthier behaviour – and healthier behaviour produces fewer diseases. We do not need to invent a new organ. We need to recognise the one that already shapes the health of the world, before the consequences of ignoring it spiral further out of control.