An eating disorder is a disturbance in accepted norms for eating behaviours. These are usually due to an overvalued desire to be seen as being thin, although there are other possible causes. These include a biological vulnerability, and a range of both social and environmental factors. Then, once such a disorder develops, psychological changes strengthen the behaviours and help sustain irrational thought patterns. As an example of this: if an individual is starving themselves, this then increases their preoccupation with food and increases the risk of their binge eating.