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The mirror test is an experiment developed in 1970 by psychologist Gordon Gallup Jr. to determine whether an animal possesses the ability to recognize itself in a mirror. It is the primary indicator of self-awareness in non-human animals and marks entrance to the mirror stage by human children in developmental psychology.
The classic mirror test is performed by surreptitiously marking the animal with two scentless dye spots. The "test" spot is put on a part of the animal that would only be visible to it through use of a mirror; the "control" spot is on an accessible but completely visually hidden part of the animal's body. If the animal reacts in a manner consistent with awareness that the test dye is located on its own body, yet ignores the control dye, it can be arguably concluded that the animal recognizes the mirror as an image of itself. Such behavior includes turning and adjusting of the body in order to better view the marking in the mirror, or tactile examination of the marking with a limb while viewing the mirror.
Several species of animals have been documented to pass the mirror test. Children gain the ability to pass the mirror test around 18 months of age. A specific version of the mirror test called the rouge test involves the use of rouge makeup as a test spot and assess the degree of self-awareness based on the child's response to his or her mirror image. It has also been noted that some animals, young children, and people who have their sight restored after being blind from birth, often (at least initially) react to their reflection in the mirror as though it were another individual.
The inspiration for the mirror test comes from an anecdote about Charles Darwin and a captive orangutan. While visiting the London Zoo in 1838, Darwin had occasion to observe an orangutan named Jenny throwing a tantrum after being teased with an apple by her keeper. This started him thinking about what the subjective experience of an orangutan is like. Among other observations about how intelligent and human-like Jenny was, he also watched the ape gaze into a mirror and noted the possibility that the animal recognized itself in the reflection.
In 1970, Gordon Gallup Jr. reenacted Darwin's initial experiment with two male and two female wild preadolescent chimpanzees, none of whom had presumably ever come into contact with a mirror prior to the experiment. Each chimpanzee was put into a room by itself for two days. Next, a full length mirror was placed in the room for a total of 80 hours at periodically decreasing distances. A multitude of behaviors were recorded upon introducing the mirrors to these wild chimpanzees.
Initially, the chimpanzees made threatening gestures at their own images, ostensibly seeing their own reflections as threatening. Eventually, the chimps used their own projected images for self-directed responding behaviors, such as grooming parts of the body before unseen without a mirror, picking their noses, making faces, and blowing bubbles at their own reflections. Gallup expanded the experiment by manipulating the chimpanzee's appearance and observing its reaction to the mirror.
Gallup built on these observations to devise what is now commonly known as the mirror test, a method to gauge the self-awareness of an animal by determining whether it recognizes its own reflection in a mirror.
The Rouge test is a specific version of the mirror test used with children. Using rouge makeup, an experimenter surreptitiously places a dot on the nose and/or face of the child. The child is then placed in front of a mirror and their reactions are monitored; depending on the child's development, distinct categories of responses are demonstrated. This test is widely cited as the primary measure for mirror self-recognition in human children.
From the age of 6 to 12 months, the child typically sees a "sociable playmate" in the mirror's reflection. Self-admiring and embarrassment usually begin at 12 months, and at 14 to 20 months most children demonstrate avoidance behaviors. Finally, at 18 months half of children recognize the reflection in the mirror as their own and by 20 to 24 months self-recognition climbs to 65%. Children do so by evincing mark-directed behavior; they touch their own nose and/or try to wipe the mark off.
It appears that self-recognition in mirrors is independent of familiarity with reflecting surfaces. The rouge test in some cases has been shown to have differing results depending on sociocultural orientation. For example, a Cameroonian Nso sample of infants 18 to 20 months of age had an extremely low amount of self-recognition outcomes at 3.2%. The study also found two strong predictors of self-recognition: object stimulation (maternal effort of attracting the attention of the infant to an object either person touched) and mutual eye contact. A strong correlation between self-concept and object permanence have also been demonstrated using the rouge test. 
The rouge test is a measure of self-concept; the child who touches the rouge on their own nose upon looking into a mirror demonstrates basic ability of understanding global awareness. Current views of the self in psychology position the self as playing an integral part in human motivation, cognition, affect, and social identity. Other theorists have expounded on this significance of this time period in a child's life, such as psychoanalyst Jacques Lacan's use of a similar test in marking the mirror stage when growing up.
Animals that have been observed to pass the mirror test include:
While the mirror test remains the "gold-standard" indication of self-awareness, only a very small number of species have been recorded to pass it. However, varying levels of self-awareness in other animals has been postulated from experiments which are similar to the mirror test.
Primates, other than the great apes, have so far universally failed the mirror test. However, mirror tests with three species of Gibbon (Hylobates syndactylus, H.gabriellae, H. leucogenys) have shown convincing evidence of self-recognition despite the fact that the animals failed the standard version of the mirror test. 
Capuchin monkeys react to their reflection either with hostility or affection, and mark test experiments have shown that they are incapable of spontaneous mirror self-recognition. Similar tests performed using video technology support these findings, but suggest that the monkeys do at least possess the raw input systems required for explicit self-recognition.
Pigs are able to utilize visual information seen in a mirror to find food, but do not show evidence of self-recognition when presented with their reflection. In an experiment, 7 of the 8 pigs tested were able to find a bowl of food hidden behind a wall and revealed using a mirror. The eighth pig looked behind the mirror for the food.
It is speculated that, in addition to the European magpie, some other Corvids have at least some degree of self-awareness, evidenced by behaviors like re-caching food when initially watched by another bird.
In 1981, Epstein, Lanza and Skinner published a paper in the journal Science in which they argued that the pigeon is also capable of passing the mirror test, but only after extensive behavioral conditioning. 
In the experiment, a pigeon was trained to look in a mirror to find a response key behind it which the pigeon then turned to peck—food was the consequence of a correct choice (i.e., the pigeon learned to use a mirror to find critical elements of its environment). Next, the pigeon was trained to peck at dots placed on its feathers; food was, again, the consequence of touching the dot. The latter training was accomplished in the absence of the mirror. The final test was placing a small bib on the pigeon—enough to cover a dot placed on its lower belly. A control period without the mirror present yielded no pecking at the dot. When the mirror was revealed, the pigeon became active, looked in the mirror and then tried to peck on the dot under the bib.
While the mirror test has been extensively conducted on primates, it is of limited value when applied to animals who rely primarily on senses other than vision.[verification needed] In light of this, the mirror test has been adapted to function through other sensory modalities, such as scent. Biologist Marc Bekoff developed a scent-based paradigm using dog urine in testing self-awareness in canines.
There is some debate as to the interpretation of the results of the mirror test, and researchers in one study have identified some potential problems with the test as means of gauging self-awareness in young children.
Proposing that a self-recognizing child may not demonstrate mark-directed behavior because they are not motivated to clean up their faces, thus providing incorrect results, the study compared results of the standard rouge test methodology against a modified version of the test.
In the classic test, the experimenter first played with the children, making sure that they looked in the mirror at least three times. Then, the rouge test was performed using a dot of rouge below the child's right eye. For their modified testing, the experimenter introduced a doll with a rouge spot under its eye and asked the child to help clean the doll. The experimenter would ask up to three times before cleaning the doll themselves. The doll was then put away and the mirror test performed using a rouge dot on the child's face. These modifications were shown to increase the number of self-recognizers.
The results uncovered by this study at least suggest some issues with the classic mirror test; primarily, that it assumes that children will recognize the dot of rouge as abnormal and attempt to examine or remove it. The classic test may have produced false negatives, because the child's recognition of the dot did not lead to them cleaning it. In their modified test, in which the doll was cleaned first, they found a stronger relationship between cleaning the doll's face and the child cleaning its own face. The demonstration with the doll, postulated to demonstrate to the children what to do, may lead to more reliable confirmation of self-recognition.
Similar criticisms may apply to non-human animals; they may not recognize the dot as abnormal, are not vision-oriented enough to notice it, or are not motivated to react to it, although this does not mean that they are not self-aware. For example, in a mirror test conducted on three elephants by Diana Reiss and Joshua Plotnik in 2006, only one elephant passed, but the two elephants that failed still demonstrated self-aware behavior, including movements that showed they connected the mirror image to themselves. Reiss and Plotnik say that the elephants might not have gone after the mark because it was not important enough to them.
On a more general level, it remains debatable whether recognition of one's mirror image is necessary for self-awareness.