a tendency or desire to avoid one stimulus that results from its pairing with another stimulus that is experienced as unpleasant. Such an aversion may arise after a single pairing of the stimuli (see conditioned taste aversion) or as part of a process of deliberate repeated pairings (as in aversion conditioning). For example, a person who is trying to stop nail biting may paint her fingernails with a bitter substance, such that the foul taste experienced each time she chews a nail eventually leads to cessation of the behavior.