Behavior or behaviour (see spelling ifferences) refers to the actions or reactions of an organism, usually in relation to the environment. Behavior can be conscious or unconscious, overt or covert, and voluntary or involuntary. In animals, behavior is controlled by the endocrine system and the nervous system. The complexity of the behavior of an organism is related to the complexity of its nervous system. Generally, organisms with complex nervous systems have a greater capacity to learn new responses and thus adjust their behavior. Human behavior (and that of other organisms and mechanisms) can be common, unusual, acceptable, or unacceptable. Humans evaluate the acceptability of behavior using social norms and regulate behavior by means of social control. Animal behavior is studied in comparative psychology, ethology, behavioral ecology and sociobiology.