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Intelligence is a characteristic defined by several characteristics in humans, including the ability to think abstractly, the ability to understand the function and operation of the world, being aware of one's own self (including one's mortality), the ability to make rational connections, the ability to learn new facts and skills, the ability to retain information, the ability to make plans and the ability to solve problems. Although many animals, including many mammals and most apes, show some innate intelligence, humans are generally believed to be the species that has the most highly developed intelligence as well as the greatest dependence on this characteristic in order to survive.
The root and nature of intelligence has come under much scrutiny, but it has been known since ancient times that the characteristic resides entirely within the brain. Current theories also believe that most of an individuals is due to genetics rather than environmental factors during childhood.
Although the nature of intelligence develops through childhood and adolescence, changes after a human reaches adulthood are usually more subtle. However, age alone usually does not affect intelligence, although certain skills like the ability to learn and the ability to immediately recall facts wane with age.
A gradual or sudden change in intelligence is usually a sign of an underlying disease. Dementia is always the sign of a serious underlying illness, and often an untreatable one. However, several treatable illnesses, such as pellagra and Korsakoff's syndrome, present as if the person were of limited intelligence. In addition, many dehabilitating diseases appear as if they have an effect on intelligence, where in fact the person's intelligence may remain completely normal (or in the case of ALS sufferer Stephen Hawking, well above average) but problem with speech or hearing may in fact masquerade as intelligence deficiencies.