HookOne meal, one lifetime of disgust
In the 1950s the psychologist John Garcia noticed something that should not have been possible. Rats given sweetened water and then, hours later, made nauseous by radiation refused that water ever again — after a single pairing, with a delay of hours between the taste and the sickness. Standard conditioning said learning needed the stimulus and response to arrive almost together, and to be repeated. Garcia's rats broke both rules, and only for taste: pair the sickness with a light or a noise instead and nothing was learned. The finding was rejected by journals for years before it became one of the most reliable in psychology. Martin Seligman later gave it a human face when he developed a lifelong revulsion to sauce béarnaise after a single bout of flu that had nothing to do with the sauce — his own preparedness in action.
Taste aversion is the doorway into this whole topic because it shows eating is never just chemistry — it is chemistry shaped by evolution, learning and culture all at once. Topic 4.13 asks you to hold four things together: why we prefer the foods we do (an evolutionary story plus a learning story), the neural and hormonal switches that tell the body to start and stop eating (the hypothalamus, ghrelin and leptin), and what happens at the two extremes where those systems and our psychology come apart — anorexia nervosa and obesity, each with a biological explanation and a psychological one. The examiner's favourite move is to make you weigh the biology against the psychology rather than pick a side.
ModelFood preferences — the evolutionary story and the learned one
The evolutionary explanation says our preferences are ancient survival tools. A sweet tooth pointed our ancestors towards ripe, calorie-dense fruit; a taste for fat and salt tracked scarce, valuable nutrients; and a dislike of bitterness helped them avoid the alkaloid poisons that plants use to defend themselves. Two mechanisms matter most for the exam. Taste aversion (the Garcia effect) is one-trial, long-delay learning that ties nausea to a novel flavour — adaptive because an animal that eats a toxin once and lives cannot afford to test it again slowly. Neophobia, the reluctance to eat unfamiliar foods, is its cautious cousin: a new food might be nutritious or might be poison, so hesitation is safe. Both are examples of Seligman's biological preparedness — we are primed to learn some associations far faster than others.
But evolution only sets the defaults. Learning, social and cultural influences do the fine-tuning. Through classical conditioning a neutral food paired with a pleasant context becomes liked; through operant conditioning a food that earns praise (or is used as a reward) is eaten more. Children copy what they see — Birch's work showed they eat more of foods they watch peers and parents enjoy, which is why neophobia softens with repeated, pressure-free exposure. And culture writes the final menu: what counts as food at all (insects, offal, blue cheese), when it is eaten, and with whom. The strong evaluation point is that neither account alone is enough — evolution cannot explain why one culture prizes chilli heat and another finds it painful, and learning cannot explain why sweetness is preferred from birth. This is a nature-and-nurture topic, not a nature-versus-nurture one.
MechanismThe switches — hypothalamus, ghrelin and leptin
The body's start-and-stop signals for eating converge on the hypothalamus. The classic dual-centre model assigns two roles: the lateral hypothalamus (LH) is the 'hunger centre' — stimulate it and an animal eats, lesion it and the animal starves (aphagia) — while the ventromedial hypothalamus (VMH) is the 'satiety centre' — lesion it and the animal overeats to obesity (hyperphagia). Modern work has complicated this tidy picture; the arcuate nucleus and neurotransmitters such as neuropeptide Y do much of the real signalling, so treat the LH/VMH split as a useful model rather than the literal wiring.
Two hormones feed the hypothalamus its information. Ghrelin — the 'hunger hormone' — is secreted by the stomach lining when the stomach is empty; it rises before meals, acts on the arcuate nucleus, and drives the feeling that it is time to eat. Leptin works on the opposite side of the ledger: it is released by adipose (fat) tissue in proportion to how much fat is stored, and it signals long-term energy sufficiency, damping appetite. Ghrelin is the short-term 'eat now' pulse; leptin is the slow 'we have enough in the tank' background signal. The system is elegant, but its failures are exactly where the topic turns — leptin that the brain stops listening to (leptin resistance) is a leading account of obesity, which the next blocks pick up.
Consider a 6-mark 'outline and briefly evaluate the role of ghrelin and leptin' question. A model answer opens with precise AO1: Ghrelin is secreted by the stomach when it is empty and stimulates the arcuate nucleus of the hypothalamus, increasing appetite; leptin is secreted by fat cells in proportion to fat stores and signals satiety, decreasing appetite. That is two clear, contrasting mechanisms — full description marks. The AO3 sentence then earns the analysis: A strength is supporting evidence from leptin-deficient individuals whose extreme overeating is corrected by leptin injections, showing a causal role; however, most obese people have high leptin yet keep eating, suggesting leptin resistance and that the hormonal account is incomplete without cognitive and environmental factors. Notice the shape — state the mechanism, give one piece of evidence that supports it, then one qualification that limits it. That three-move rhythm is what separates a Level 2 from a Level 3 answer.
ModelAnorexia nervosa — the biological explanation
Anorexia nervosa is a serious psychiatric illness — it has the highest mortality rate of any mental disorder — characterised by restriction of intake, an intense fear of gaining weight and a distorted body image. The biological explanation comes in two parts. Genetic evidence rests on family and twin studies: anorexia runs in families, and concordance rates are markedly higher in identical (MZ) than non-identical (DZ) twins — Holland's twin study reported roughly 56% MZ against 5% DZ concordance, a gap that points to heritable vulnerability rather than a shared home environment alone. No single 'anorexia gene' exists; the inheritance is polygenic, raising susceptibility rather than guaranteeing the disorder.
The neural account focuses on neurotransmitters. Disturbed serotonin activity is linked to the anxiety, obsessionality and appetite dysregulation seen in patients, and altered dopamine in the caudate nucleus (Kaye's work) may distort the reward value of food and even make restriction feel rewarding. The honest evaluation is that these biological findings are strong on association but weak on causation: starvation itself changes brain chemistry, so abnormal serotonin and dopamine may be a consequence of the illness as much as a cause. The genetic evidence is more robust, but even 56% concordance leaves nearly half the variance to non-genetic factors — which is the cue to bring in the psychological explanations rather than treat biology as the whole answer.
ModelAnorexia nervosa — the psychological explanations
Three psychological accounts sit on the spec. Family systems theory (Minuchin) locates the disorder not in the individual but in a dysfunctional family pattern — the 'psychosomatic family' marked by enmeshment (blurred boundaries, over-closeness), overprotection, rigidity and poor conflict resolution. Refusing food becomes the child's only route to control and autonomy within a family that allows none. Social learning theory emphasises modelling and reinforcement: the thin ideal is modelled relentlessly by media figures and peers, imitation is more likely when the model is admired, and thinness is vicariously reinforced by the praise and status it attracts. Cognitive theory points to distorted thinking — a genuinely disturbed body image (patients overestimate their own size), perfectionism, and irrational beliefs about weight and self-worth that keep the restriction going.
These explanations are not rivals so much as layers. Family dynamics may create vulnerability, social learning supplies the thin-ideal target, and cognitive distortions maintain the behaviour once it starts. The powerful synthesis — and the one examiners reward at the top band — is diathesis-stress: a genetic and neural predisposition (the diathesis) is triggered and sustained by psychological and social stressors (family conflict, media pressure, dieting). This lets you use the biological block and this one together rather than writing two disconnected halves of an essay.
ModelObesity — the biological explanation
Obesity is defined clinically by a body mass index over 30, and the biological explanation again splits into genetic and neural strands. The genetic case is strong: adoption studies (Stunkard) find adoptees' weight tracks their biological parents far more closely than their adoptive ones, and specific genes have been identified — most famously the FTO gene (Frayling et al., 2007), where carrying two high-risk copies is associated with substantially higher obesity risk. As with anorexia, inheritance is polygenic and probabilistic: genes load the gun, environment pulls the trigger.
The neural account centres on the hypothalamic signalling from the earlier block. Leptin resistance is key — obese individuals typically have high leptin levels, yet the hypothalamus no longer responds to the satiety signal, so appetite is not switched off despite ample fat stores. Damage or dysfunction in the ventromedial hypothalamus can likewise produce hyperphagia. The evaluation to reach for is that the biological explanation struggles with the recent rate of the obesity epidemic: gene frequencies cannot change in a few decades, so a purely genetic or neural story cannot explain why obesity has surged since the 1980s. That change is environmental — cheap, calorie-dense, heavily-marketed food — which is exactly why the psychological explanations of obesity carry so much weight.
CaseObesity — restraint theory, the boundary model and dieting
The leading psychological explanation is a paradox: trying not to eat can cause overeating. Restraint theory (Herman and Mack, 1975) proposes that chronic dieters — 'restrained eaters' — impose a cognitive limit on intake that is easily breached. Once it is breached, disinhibition takes over: the famous 'what-the-hell effect', where a restrained eater who believes they have already blown the diet abandons control and overeats. Herman and Mack's preload study captured it neatly — restrained eaters who were first given a high-calorie milkshake went on to eat more ice cream afterwards than those given nothing, the reverse of what appetite alone predicts.
The boundary model (Herman and Polivy) explains the mechanism. Eating is bounded by a physiological hunger boundary at the low end and a satiety boundary at the high end; between them lies a zone of indifference governed by cognition. Restrained eaters have a wider gap between the two boundaries and impose a self-set diet boundary well below satiety. Cross that cognitive boundary and they eat all the way up to satiety — hence the disinhibited binge. This is why dieting so often fails: restriction itself creates the conditions for overeating, and preoccupation with forbidden food (plus the effects of variety and stress) undermines control. The success stories tend to replace restriction with sustainable habit change rather than white-knuckle willpower.
Take a 16-mark essay: 'Discuss psychological explanations for obesity.' A top-band paragraph reads: Restraint theory offers a counter-intuitive but well-supported account of obesity. Herman and Mack argued that restrained eaters impose a cognitive limit that, once broken, triggers disinhibited overeating — the 'what-the-hell effect'. This is supported by preload studies in which restrained eaters ate more ice cream after a milkshake than after nothing, a finding appetite regulation cannot explain. A strength of the theory is its practical value: it predicts that rigid dieting is counter-productive and that flexible, habit-based approaches work better, which is borne out by weight-loss outcome research. However, restraint theory cannot account for obese individuals who are not restrained eaters and never diet, so it explains the maintenance of overeating in dieters better than the origins of obesity itself. This suggests it is best combined with the biological explanation — leptin resistance and FTO-linked appetite differences may create the vulnerability, while restraint and disinhibition maintain the behaviour. Read that back and notice the marks: one clear point, named evidence, a strength with real-world value, a genuine limitation, and a synthesis that links back to biology. That is AO1 and AO3 woven together, not stacked.
VocabularyKey terms the mark scheme pays for
TrapsMisconceptions that cost marks
ExamWhat examiners want
Eating behaviour is an optional Paper 3 topic, examined with short AO1/AO3 questions, application (AO2) items and 16-mark essays marked over four levels. In a 16-marker the marks split roughly 6 AO1 (accurate, detailed description) and 10 AO3 (thorough, effective evaluation), so plan for evaluation to be the larger half — three or four developed critical points beat six one-line assertions. Always name your evidence: 'Holland's twin study found ~56% MZ vs 5% DZ concordance' scores where 'twin studies support genetics' does not.
For evaluation, build points using the PEEL/three-move rhythm modelled above: state the point, give named evidence, then a limitation or 'this suggests…'. Reach for the issues-and-debates hooks the topic invites — biological reductionism (reducing anorexia to serotonin), nature-nurture (food preferences), and social sensitivity (research on obesity and eating disorders can stigmatise, so discuss its real-world implications). Your strongest conclusion is almost always a diathesis-stress synthesis that links the biological and psychological explanations rather than declaring one 'correct'.
On application questions, quote the stem: if the scenario describes a dieter who abandons control after one biscuit, name restraint theory and the disinhibition/what-the-hell effect explicitly and tie your theory to the character's behaviour. The AO2 mark is for the link, not for reciting the theory in the abstract.