Behavior (U.S.) or behaviour (U.K.) refers to the actions or reactions of an object or organism, usually in relation to the environment. Behavior can be conscious or unconscious, overt or covert, and voluntary or involuntary. 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.
The behavior of people (and other organisms or even mechanisms) falls within a range with some behaviors being common, some unusual, some acceptable, and some outside acceptable limits. The acceptablity of behavior is evaluated relative to social norms and regulated by various means of social control. For behavior of people see human behavior. Animal behaviour is studied in comparative psychology, ethology, behavioural ecology and sociobiology.