Anorexia nervosa is an eating disorder characterised by refusal to maintain a healthy body weight, and an obsessive fear of gaining weight due to a distorted  self image which may be maintained by various cognitive biases that alter how the affected individual evaluates and thinks about their body, food and eating. Anorexia nervosa is a serious mental illness with a high incidence of  comorbidity and also the highest mortality rate of any psychiatric disorder.