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Behavior or behaviour (see American and British spelling differences) refers to the actions of a system or organism, usually in relation to its environment, which includes the other systems or organisms around as well as the physical environment. It is the response of the system or organism to various stimuli or inputs, whether internal or external, conscious or subconscious, overt or covert, and voluntary or involuntary.



In humans, behavior is believed to be controlled primarily by the endocrine system and the nervous system. It is most commonly believed that complexity in the behavior of an organism is correlated to the complexity of its nervous system. Generally, organisms with more complex nervous systems have a greater capacity to learn new responses and thus adjust their behavior.

Behaviors can be either innate or learned. However, current research in the Human Microbiome Project points towards a possibility that human behavior may be controlled by the composition of the microbe population within a human body.[1]

More generally, behavior can be regarded as any action of an organism that changes its relationship to its environment. Behavior provides outputs from the organism to the environment.[2]


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. In sociology, behavior is considered as having no meaning, being not directed at other people and thus is the most basic human action, although it can play a part in diagnosis of disorders such as the autism spectrum disorders. Animal behavior is studied in comparative psychology, ethology, behavioral ecology and sociobiology. According to moral values, human behavior may also depend upon the common, usual, unusual, acceptable or unacceptable behavior of others.

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