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Anorexia nervosa is a serious mental disorder, recognised in clinical classification systems, characterised by a sustained restriction of food intake, an intense fear of weight gain, and a disturbance in the way body shape or weight is experienced. It is among the most serious of psychiatric conditions and is associated with significant physical and psychological harm. This lesson examines the biological explanations for the disorder — the genetic account (drawn from family and twin studies and the search for candidate genes) and the neural account (focused on dysregulation of the neurotransmitters serotonin and dopamine and the brain regions involved in reward, control and the perception of the body). Throughout, anorexia nervosa is discussed objectively and respectfully, as a clinical condition to be understood and explained, exactly as an academic textbook would. The aim is to evaluate what biology can and cannot tell us about why the disorder develops; the companion lesson covers the psychological explanations, and a complete account is interactionist. The focus here is strictly on causes and mechanisms — never on the behaviours themselves in any detail.
Key Definition: A biological explanation of a mental disorder attributes its development to physiological factors — genetic inheritance, brain structure, and neurochemistry — rather than (or in addition to) psychological and environmental factors. For anorexia nervosa, the principal biological explanations are genetic vulnerability and neurotransmitter dysregulation.
This lesson addresses the following point from the AQA A-Level Psychology (7182) specification, Paper 3 — Eating Behaviour:
It develops the named content — genetic explanations (family studies, twin studies and concordance, heritability, candidate genes) and neural explanations (serotonin and dopamine dysregulation and the relevant brain regions) — and prepares you to describe (AO1) and evaluate (AO3) the biological account. It pairs with the lesson on psychological explanations of anorexia nervosa, and the two are frequently set against each other in extended-response questions, often with an interactionist conclusion required. Because these questions rarely include a scenario stem, the assessment objectives are typically split AO1/AO3 only, with no AO2 application required unless a stem is provided.
Before examining the explanations, a word on how this topic should be handled — in this lesson and in the exam. Anorexia nervosa should always be discussed in a measured, clinical, scientific register: as a recognised disorder with identifiable correlates and consequences, treated with the same objectivity as any other condition studied in psychology. Examiners expect, and the subject demands, that answers focus on explanation, mechanism, research and evaluation — on why the disorder arises — and never on graphic description of behaviours. This is both an ethical and an academic standard: dignity towards those affected and precision about causes go together. Keep this register throughout, and the topic poses no difficulty.
The genetic explanation proposes that vulnerability to anorexia nervosa is, in part, inherited — that some individuals carry a genetic predisposition that, given relevant environmental conditions, raises the likelihood of developing the disorder. The evidence comes from three converging sources: family studies, twin studies, and molecular genetics.
Family studies examine whether the disorder "runs in families" by asking whether the biological relatives of someone with anorexia nervosa are more likely than the general population to develop it themselves. The consistent finding is that anorexia nervosa aggregates in families: first-degree relatives (parents, siblings, children) of an affected individual show a substantially raised risk compared with relatives of unaffected people. This pattern is consistent with a genetic contribution. However, family studies alone cannot establish genetic causation, because families share not only genes but also environment — diet, attitudes to food and body, family dynamics, and cultural exposure — so a familial pattern could reflect shared experience rather than shared DNA. Family studies therefore suggest a heritable component but cannot, on their own, demonstrate it; that requires the twin design.
Twin studies provide the key evidence by comparing concordance rates — the probability that, if one twin has the disorder, the other does too — between monozygotic (MZ, "identical") twins, who share effectively all their genes, and dizygotic (DZ, "non-identical") twins, who share about half, like ordinary siblings. The logic is that if both twin types share a similar environment but MZ twins are more concordant than DZ twins, the difference is most plausibly attributable to the greater genetic similarity of MZ pairs. The consistent finding across twin studies of anorexia nervosa is that MZ concordance is substantially higher than DZ concordance, which is taken as strong evidence for a genetic contribution to the disorder. From such data, researchers estimate heritability — the proportion of the variation in liability to the disorder, within a population, that is attributable to genetic variation. Heritability estimates for anorexia nervosa derived from twin studies are generally moderate to high, indicating that genetic factors make a meaningful contribution to who develops the disorder, while also leaving substantial room for environmental influence.
Exam Tip: Describe heritability qualitatively and correctly. Heritability is a population-level estimate of the share of variation explained by genes — it is not the probability that an individual will inherit the disorder, and it does not mean the disorder is fixed or untreatable. Misstating heritability as "the chance you'll get it" is a common and penalised error.
Key Definition: Concordance rate is the probability that, if one member of a twin pair has a trait or disorder, the other does too. Heritability is the proportion of the variation in a trait within a population that is attributable to genetic differences between individuals.
Molecular genetic research seeks the specific candidate genes that might underlie this inherited vulnerability. Because the neural explanation implicates serotonin and dopamine, attention has focused on genes involved in these neurotransmitter systems — for example, genes affecting serotonin receptors and transport — as plausible candidates that could bias an individual's neurochemistry towards the disorder. More recent large-scale approaches, examining variation across the whole genome, have begun to identify genetic loci associated with anorexia nervosa, and have suggested that its genetic architecture overlaps not only with other psychiatric traits but, intriguingly, with metabolic traits — hinting that the disorder may have a partly metabolic as well as psychiatric basis. The clear message from this work is that anorexia nervosa is polygenic: it is influenced by many genes of small individual effect, rather than by any single "anorexia gene," which is the expected picture for a complex psychiatric phenotype.
It is important to be clear about how genes are thought to act, because a common misunderstanding is that an inherited predisposition means the disorder is "in the genes" in a fixed, inevitable sense. The more accurate model is one of gene–environment interaction: genes do not code for the disorder directly but for biological characteristics — perhaps a temperament marked by anxiety, perfectionism and harm-avoidance, or a particular pattern of serotonin or reward functioning — that constitute a vulnerability. Whether that vulnerability translates into the disorder depends on the environment the individual encounters, including the cultural, family and developmental factors examined in the companion lesson. This is why even the genetically most similar individuals (MZ twins) are not perfectly concordant: if anorexia nervosa were purely genetic, MZ concordance would approach certainty, yet it does not, and the substantial discordance among MZ pairs is itself powerful evidence that non-genetic factors are essential to whether the predisposition is expressed. The genetic explanation, properly understood, therefore points towards an interactionist account rather than away from one — a subtlety that strong answers exploit when reaching their conclusion.
| Genetic evidence | What it shows | Key limitation |
|---|---|---|
| Family studies | Disorder aggregates in families (raised risk in first-degree relatives) | Cannot separate shared genes from shared environment |
| Twin studies | MZ concordance > DZ concordance; moderate-to-high heritability | MZ twins may share a more similar environment than DZ twins |
| Candidate / genome-wide | Polygenic; serotonin-related and metabolic loci implicated | Individual gene effects are small; associations not deterministic |
The neural explanation attributes anorexia nervosa to dysregulation of neurotransmitter systems and to atypical functioning of the brain regions that govern reward, anxiety, impulse control and the perception of the body. Two neurotransmitters are central to the specification: serotonin and dopamine.
Serotonin is a neurotransmitter involved in the regulation of mood, anxiety and appetite, and it has long been implicated in anorexia nervosa. The general hypothesis is that dysregulation of serotonin contributes to the disorder — both to the disturbed eating and to the high levels of anxiety, perfectionism and obsessionality that frequently accompany it. The picture is complex: research has found evidence of altered serotonin activity in people with anorexia nervosa that, importantly, appears to persist after recovery, suggesting it may be a trait vulnerability rather than merely a consequence of the disorder's effects on the body. One influential interpretation is that elevated serotonin function is associated with the anxiety and rigidity characteristic of the disorder, and that food restriction may, paradoxically, reduce the availability of serotonin's precursor and so transiently dampen this distressing anxious state — offering a possible neurochemical account of how the behaviour could be negatively reinforced. The involvement of serotonin is further supported indirectly by the partial role of serotonergic medication in some patients, though, as the evaluation notes, the clinical picture here is far from straightforward.
Dopamine is centrally involved in reward, motivation and reinforcement, and dopamine dysregulation has been proposed as a contributor to anorexia nervosa, particularly to its rewarding and self-perpetuating quality. The hypothesis is that altered dopamine function in reward circuitry may distort the normal relationship between food and reward — such that, in the disorder, food restriction and control acquire a rewarding or anxiety-reducing value while eating becomes associated with anxiety rather than pleasure. Atypical dopamine activity in regions such as the striatum has been reported in people with anorexia nervosa, consistent with disturbed reward processing. This offers a neural account of one of the disorder's most puzzling features: the apparent reversal or distortion of the usual reward value of food.
It is worth dwelling on why a reward-circuit account is so theoretically attractive for this disorder. In ordinary feeding, the dopaminergic reward system makes food — especially energy-dense food — strongly rewarding, which (as the evolutionary lesson explained) was adaptive in a world of scarcity. Anorexia nervosa presents the apparent paradox of an individual overriding this powerful, evolved reward signal. A dopamine-dysregulation account proposes a resolution: if the normal coupling of food to reward is disturbed, and if restriction and the sense of control instead come to engage reward and relief, then the behaviour becomes self-reinforcing in a way that helps explain its persistence and its resistance to change. This connects to the serotonin account as well: if eating provokes anxiety (a serotonergically-mediated state) and restriction transiently reduces it, the two neurotransmitter systems may jointly sustain a cycle in which the disorder is maintained by negative reinforcement (relief from anxiety) as well as by any positive reward from control. Importantly, this remains a model of mechanism rather than a settled fact, and it is offered here to explain the logic of the neural hypothesis, not as a description of certainty.
Research by Bailer and colleagues has examined serotonin function in anorexia nervosa using brain-imaging techniques. Aim: to investigate serotonin (5-HT) receptor activity in individuals with anorexia nervosa, including those who had recovered. Method: they used neuroimaging to compare serotonin receptor binding in women with a history of anorexia nervosa against healthy controls. Findings: they reported altered serotonin receptor activity in those with anorexia nervosa relative to controls, with differences evident even in recovered individuals, and with the degree of alteration relating to levels of anxiety. Conclusion: serotonin dysregulation is associated with anorexia nervosa and appears to represent a persistent trait characteristic rather than simply a state effect of the illness, supporting a neural-vulnerability model of the disorder. This study is a key citation for the neural explanation because the persistence of the abnormality after recovery helps address the cause-versus-consequence problem discussed below.
Beyond specific neurotransmitters, neural accounts implicate several brain regions. Disturbances in body image — the misperception of body shape that is central to the disorder — have been linked to atypical functioning in regions involved in visual and bodily self-perception (including parts of the parietal and visual association cortex and the insula, which integrates internal bodily states). The prefrontal cortex, governing executive control and the capacity for rigid self-regulation, and reward-related regions such as the striatum and the amygdala (involved in anxiety), have all been implicated. The general picture is of a disorder involving distributed differences across circuits for reward, anxiety, cognitive control and the experience of the body, rather than a single localised lesion.
graph TD
A["Biological vulnerability to anorexia nervosa"] --> B["Genetic factors"]
A --> C["Neural factors"]
B --> B1["Family aggregation"]
B --> B2["Twin concordance: MZ > DZ"]
B --> B3["Candidate / polygenic; serotonin & metabolic loci"]
C --> C1["Serotonin dysregulation<br/>(anxiety, persists after recovery)"]
C --> C2["Dopamine dysregulation<br/>(reward processing)"]
C --> C3["Brain regions: insula, prefrontal cortex,<br/>striatum, amygdala"]
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