Explanations for Obesity

Obesity is an excess of body fat that raises the risk of ill health, including type 2 diabetes, cardiovascular disease and some cancers. It is most commonly defined using body mass index (BMI) — a person's weight in kilograms divided by the square of their height in metres — with a BMI of 30 or above classed as obese.

The specification asks you to explain obesity from two distinct angles:

Type of explanation	Focus	Key ideas
Biological	The body and brain	Genetic inheritance; neural control of appetite
Psychological	Thinking and behaviour	Restraint theory; disinhibition; the boundary model

A strong answer treats these as complementary rather than competing. Obesity is best understood as a condition in which a biological predisposition interacts with a person's environment and psychology, so most exam questions reward candidates who can weigh both sides.

Obesity is defined by an excess of body fat (commonly BMI ≥ 30) that raises health risk. Keep the definition brief and respectful — the marks are for the explanations, not for describing the condition.

The genetic explanation argues that obesity has a substantial heritable component: a tendency to gain and store fat can be inherited, influencing appetite, metabolism and fat storage.

The strongest evidence comes from twin and adoption studies:

• Adoption studies — Stunkard et al. found that the body weights of adoptees resembled those of their biological parents far more than their adoptive parents, even though the adoptees were raised in the adoptive home. This points to genes rather than the shared family environment.
• Twin studies — monozygotic (identical) twins show much higher concordance for body weight than dizygotic (non-identical) twins, again implicating inherited factors.

Researchers have identified candidate genes that plausibly carry this effect:

Gene	Proposed role
FTO gene	Associated with increased appetite and higher BMI; carriers tend to feel less full after eating
ob gene (leptin)	Codes for leptin, the hormone that signals satiety; disruption weakens the "stop eating" signal

Genes do not act alone: they influence how strongly a person feels hunger and fullness, how efficiently they store energy, and how they respond to food cues. This is a predisposition, not a guarantee of obesity.

The neural explanation locates obesity in the dysregulation of the brain systems that control appetite. Three mechanisms are worth knowing.

1. Leptin resistance. Leptin is released by fat cells and normally tells the brain that energy stores are full, suppressing appetite. In obesity, leptin levels are often high, yet the brain stops responding to the signal — a state called leptin resistance. The satiety message is effectively ignored, so the person keeps eating.

2. Hypothalamic dysfunction. The hypothalamus integrates hunger and satiety signals. If the pathways that register fullness are impaired, appetite regulation breaks down and food intake is not properly switched off.

3. Altered reward circuitry. Eating energy-dense food triggers dopamine release in the brain's reward pathways. In some people this reward response is heightened or dysregulated, making high-calorie food especially rewarding and driving overeating beyond physical need.

Leptin resistance is the key term. The problem is not a lack of leptin but the brain failing to respond to it — high leptin, weak effect. Contrast this with leptin deficiency, which is rare.

Together, these neural accounts explain why appetite control can fail even when the body has ample energy reserves.

Restraint theory, proposed by Herman and Polivy, offers a psychological explanation. Its central and counter-intuitive claim is that deliberately trying to restrict food intake — dieting — can paradoxically increase the likelihood of overeating.

People who chronically try to limit what they eat are called restrained eaters. Rather than losing weight steadily, they are prone to episodes of binge eating. The act of restraint sets up a psychological tension: food is constantly monitored, resisted and thought about, which makes a loss of control more likely, not less.

This produces the dieting paradox: the very behaviour intended to reduce intake can end up raising it, so restraint can be counter-productive and contribute to weight gain over time.

The exam-critical point: restraint theory does not say dieting reduces eating. It argues that restraint can increase overeating in restrained eaters — the opposite of what dieters intend.

This explanation is valuable because it accounts for why so many diets fail, but note that it applies specifically to people who restrain, a point we return to in the evaluation.

Disinhibition describes how restraint breaks down. Once a restrained eater believes they have broken their diet — for example by eating a single "forbidden" food — they abandon control and overeat. This is the "what-the-hell" effect: having crossed the line, the person reasons that the diet is ruined anyway, so they may as well continue.

The boundary model (Herman and Polivy) explains this using two physiological limits with a zone of psychological control between them:

Boundary	What it marks
Hunger boundary (lower)	The point below which physiological hunger drives a person to eat
Satiety boundary (upper)	The point above which physiological fullness makes a person stop

Between the two lies a range governed by cognition rather than physiology. Restrained eaters have a larger gap between their hunger and satiety boundaries and impose a self-set "diet boundary" within this zone. While they stay below it, control holds. But once they cross the diet boundary, disinhibition kicks in and they tend to keep eating all the way up to the satiety boundary — which explains the binge episode.

Restraint = trying to limit intake. Disinhibition = the breaking of that limit. The boundary model ties them together: crossing the self-set diet boundary triggers eating up to the satiety limit.

A top-band answer weighs the explanations against evidence and against each other (AO3).

Genetic evidence is strong, but genes are not the whole story. Twin and adoption studies (Stunkard et al.) provide robust support for a heritable component. However, genetics alone cannot explain the rapid rise in obesity over recent decades: the human gene pool has not changed in that time, but the environment has. This points to an obesogenic environment — cheap, energy-dense food and reduced activity — interacting with genetic risk. Leptin resistance is also complex and not yet fully understood, so the neural account remains incomplete.

The dieting paradox is well evidenced. Laboratory studies show that restrained eaters often eat more after a high-calorie "preload" than unrestrained eaters do, supporting restraint theory and usefully explaining why diets so frequently fail. Its main limitation is scope: restraint theory cannot explain obesity in people who do not diet or restrain their eating at all, so it is at best a partial account.

The boundary model has explanatory power for restrained eaters' overeating and for individual differences in the size of the "control zone". Yet it is somewhat reductionist, compressing complex eating behaviour into two boundaries, and the boundaries themselves are hard to measure or test directly.

The most complete account is interactionist: a biological predisposition (genes, neural appetite systems) combines with a modern obesogenic environment and psychological factors such as restraint. No single explanation is sufficient on its own.

1. Saying restraint theory means dieting reduces eating

Restraint theory claims the opposite: deliberately restricting intake can paradoxically increase overeating in restrained eaters. Writing that "dieting lowers food intake" misrepresents the whole theory and loses AO1 marks.

2. Confusing restraint with disinhibition

Restraint is the attempt to limit eating; disinhibition is the breaking of that limit (the "what-the-hell" effect). They are two stages of the same process — do not use the terms interchangeably.

3. Treating genes as the sole cause

Genes create a predisposition, not a certainty. An answer that ignores the obesogenic environment cannot explain why obesity rates have risen so fast while the gene pool has not changed. Strong evaluation is interactionist.

4. Muddling the hunger and satiety boundaries

In the boundary model the hunger boundary is the lower limit (below it you are driven to eat) and the satiety boundary is the upper limit (above it you stop). Restrained eaters cross a self-set diet boundary between them, then eat up to satiety.

5. Describing leptin resistance as a lack of leptin

Leptin resistance means the brain fails to respond to leptin despite high levels — not that leptin is missing. Framing it as a deficiency confuses the neural explanation.