The mirror test is a measure of self-awareness developed by Gordon Gallup Jr in 1970.
The test gauges self-awareness by determining whether an animal can recognize its own reflection in a mirror as an image of itself.
This is accomplished by surreptitiously marking the animal with an odourless dye, and observing whether the animal reacts in a manner consistent with it being aware that the dye is located on its own body.
Such behaviour might include turning and adjusting of the body in order to better view the marking in the mirror, or poking at the marking on its own body with a finger while viewing the mirror.
Animals which have passed the mirror test are common chimpanzees, bonobos, orangutans, dolphins, elephants, humans and possibly pigeons.
Surprisingly, gorillas have not passed the test, although at least one specific gorilla, Koko, has passed the test; this is probably because gorillas consider eye contact an aggressive gesture and normally try to avoid looking each other in the face.
Human children tend to fail this test until they are at least 1.5 to 2 years old.
Dogs and 1 year old children, for example, usually react to a mirror in fear or curiosity, or simply ignore it, while birds often attack their own reflections.