In this test, the subject is marked on parts of its body normally invisible to it and placed in front of a mirror. If it recognizes the mark it sees in the mirror as actually being located on its body, it must possess some representation of the concept of self. Usually, the test counts as passed if the subject attempts to scratch the mark away from its body but other variants exist (e.g. using videos). Evidence for self-awareness is even stronger if the subject also uses the mirror to investigate other parts of the body normally hidden from the visual field, such as the inside of the mouth.
Self-recognition is considered an important aspect of theory of mind, the capacity to take the perspective of another organism in order to anticipate its behaviour. Traditionally, both theory of mind and the passing of the mirror test had been considered uniquely human but since the last third of the 20th century, data have accumulated that contradict this view.
The comparative study of mirror self-recognition is thus an area of active research, especially in primates, since the evidence available so far hints at a cognitive divide between great apes who pass the test, and monkeys who fail it. Mirror self-recognition has also been reported in dolphins, elephants and magpies.
In children, the capacity develops around the middle of the second year of life, about the same time as the use of personal pronouns.