Anorexia nervosa develops from a combination of genetic vulnerability, brain chemistry differences, personality traits, and environmental pressures. No single factor causes it. Twin studies estimate that genetics account for 28% to 74% of the risk, meaning biology plays a substantial role, but life experiences and cultural forces shape whether that vulnerability ever becomes a disorder. Understanding what drives anorexia requires looking at all of these layers together.
Genetics Set the Foundation
Anorexia runs in families, and twin studies have been the primary tool for measuring how much of that is genetic versus environmental. Estimates of heritability vary depending on how strictly researchers define the disorder, but they consistently point to a strong genetic component. One reanalysis of twin data found that genetic effects accounted for 88% of the liability to anorexia, while other studies have placed the figure between 28% and 58%. A study of 17-year-old female twins estimated heritability at 74%.
What’s notable across these studies is that shared environment, meaning the family home, meals together, parenting style, had little measurable effect. The environmental factors that mattered were unique to each individual: personal experiences, relationships, and stressors that one twin encountered but the other didn’t. This doesn’t mean parenting is irrelevant, but it does mean the disorder isn’t caused by how someone was raised in any simple, direct way.
Brain Chemistry That Resists Eating
People with anorexia appear to have a fundamentally different relationship between the brain’s reward system and its threat-detection system. In most people, eating produces a pleasurable signal. In anorexia, that signal is muted or even reversed. Brain imaging research shows that the chemical messenger involved in reward actually triggers anxiety rather than pleasure in people with the disorder. Where a healthy brain associates food with satisfaction, an anorexic brain may associate it with discomfort.
At the same time, the brain’s inhibitory control system appears to be unusually strong. People with anorexia show a heightened ability to delay gratification and resist impulses, including the drive to eat. They also tend to be highly sensitive to punishment and low in reward reactivity, traits that persist even after recovery. This combination means that restricting food may actually feel like relief: it reduces the anxiety that eating produces. One study found that when researchers lowered serotonin activity in people with anorexia, their anxiety decreased, suggesting that the brain’s serotonin system contributes to the distress they experience around food.
The amygdala, a brain region involved in processing fear and disgust, also behaves differently. Brain scans show increased activity in the right amygdala when people with restrictive anorexia view images of food, paired with decreased activity in regions that regulate emotional responses. This pattern suggests that food triggers a fear-like reaction that the brain struggles to calm down.
Personality Traits That Precede the Disorder
Certain personality traits show up before anorexia develops, not just alongside it. Perfectionism is one of the strongest and most consistent. People who score high in perfectionism and later develop anorexia tend to have more severe symptoms, higher levels of depression, greater anxiety, and lower self-esteem. The relationship between perfectionism and anorexia appears to be especially strong in the restrictive type, where the person limits food intake rather than binging and purging.
Anxious temperament is another reliable predictor. Many people with anorexia describe being anxious children long before any eating problems appeared. This trait connects back to the brain chemistry findings: an anxious brain that finds restriction calming has a ready-made path toward disordered eating once the right trigger appears. Low self-directedness and difficulty cooperating with others also show up in research, painting a picture of someone who turns rigid self-control inward when the outside world feels unmanageable.
Cultural Pressure and the Thin Ideal
Thin-ideal internalization, the degree to which someone genuinely buys into cultural standards of thinness as personally important, is one of the few risk factors with enough evidence to be considered causal. This goes beyond noticing that thinness is valued. It’s about absorbing that value so deeply that it shapes how you judge your own worth.
Researchers have measured this on standardized scales. People with disordered eating score significantly higher on internalization measures (averaging 4.27 on a 5-point scale) compared to those with healthy eating patterns (averaging 3.29). A score of about 3.78, representing moderate internalization, appears to mark the threshold where risk for clinically significant eating problems rises sharply. Reducing thin-ideal internalization through intervention programs reliably reduces eating disorder symptoms, which strengthens the case that the relationship is causal rather than coincidental.
Social media has amplified this pressure by making idealized body images constant and unavoidable, but the thin ideal existed long before Instagram. Fashion, film, advertising, and peer groups all contribute. The key isn’t exposure alone but how deeply someone absorbs those messages as personal goals.
Life Events That Trigger Onset
Clinicians consistently report that people with eating disorders can identify specific events that triggered their illness. Research supports this: adverse life events in the year before onset are significantly more common among people who develop eating disorders. Major moves, serious illness, pregnancy, and experiences of physical or sexual abuse all appear more frequently in the histories of people who go on to develop disordered eating. The more stressful events someone experienced in a single year, the higher their risk.
Not all stressful events carry equal weight. Bereavement, a partner’s illness, or the start or end of a romantic relationship did not significantly differentiate people who developed eating disorders from those who didn’t. The events that mattered most seemed to involve disruption to someone’s sense of control or physical safety. For a person already carrying genetic vulnerability, anxious temperament, and perfectionist tendencies, a major life upheaval can be the catalyst that turns latent risk into active illness.
How the Body Gets Trapped in a Cycle
Once anorexia takes hold, the body’s hunger-regulating hormones shift in ways that make recovery harder. Ghrelin, the hormone that signals hunger, rises dramatically during starvation. People with anorexia have significantly higher fasting ghrelin levels than people at a healthy weight, and those levels stay elevated throughout the day rather than rising and falling with meals. In healthy people, ghrelin drops after eating. In anorexia, it often doesn’t, suggesting the body loses its ability to respond normally to the hormone’s signal.
Leptin, which signals fullness and helps regulate energy balance, drops as body fat decreases. So the body is simultaneously screaming for food through high ghrelin and losing its ability to register satisfaction through low leptin. Despite these powerful biological signals to eat, people with anorexia don’t respond to them. Researchers believe the brain becomes insensitive to ghrelin’s effects over time, and the growth hormone system also adapts, responding less to ghrelin than it would in a healthy person. These hormonal disruptions don’t cause the disorder, but they help maintain it by breaking the normal feedback loop between hunger and eating.
Depression, Anxiety, and Overlapping Conditions
Anorexia rarely exists alone. Between 36% and 80% of people with anorexia also meet criteria for depression, depending on how it’s measured. Anxiety disorders are similarly common, and obsessive-compulsive symptoms frequently accompany the illness. People who have anxiety, depression, and obsessive symptoms alongside anorexia tend to have more severe eating disorder symptoms than those with anorexia alone.
The relationship between these conditions is tangled. Anxiety often predates the eating disorder, suggesting it’s part of the underlying vulnerability rather than a consequence. Depression can develop before, during, or after anorexia onset, and starvation itself worsens mood and cognitive rigidity, creating a feedback loop. Malnutrition alters brain chemistry in ways that deepen both the eating disorder and the co-occurring conditions, which is one reason why weight restoration is typically a prerequisite for other treatments to work effectively.
Who Is Most Affected
Anorexia is roughly three times more common in females than males, with global prevalence rates of about 22.5 per 100,000 in females and 7.5 per 100,000 in males. Adolescent girls between 15 and 19 are the most affected group. But these numbers almost certainly undercount males, who are less likely to be screened, less likely to recognize their symptoms as an eating disorder, and less likely to seek treatment.
The disorder exists across all ethnicities, socioeconomic backgrounds, and body types. The stereotype of anorexia as a condition affecting only thin, wealthy, white teenage girls has historically delayed diagnosis in everyone who doesn’t fit that profile. People in larger bodies can meet every diagnostic criterion for anorexia, including dangerous medical complications, while still being told by clinicians that they don’t “look” like they have an eating disorder.