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Identifying unique features of human beings is a cottage industry in psychology. In his book "Stumbling on Happiness," the Harvard psychologist Daniel Gilbert jokes that every member of his profession lives under the obligation at some time in his career to complete a sentence which begins: "The human being is the only animal that..." Those who have completed the sentence with phrases like "makes tools," "is conscious" or "can imitate" have generally now conceded that some other animals also have these traits.