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 two odourless dye spots. The test spot is on a part of the animal that would be visible in front of a mirror, while the control spot is in an accessible but hidden part of the animal's body. Scientists observe whether the animal reacts in a manner consistent with it being aware that the test dye is located on its own body while ignoring the control dye. 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 limb while viewing the mirror.
Animals that have passed the mirror test are common chimpanzees, bonobos, orangutans, dolphins, elephants, and humans. Initially, it was thought that gorillas do not pass the test, but there are now several well-documented reports (such as one gorilla, Koko) of gorillas passing the test. Human children tend to fail this test until they are at least 1.5 to 2 years old - mirror stage - . Dogs, cats 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.
Capuchin monkeys react in a way that indicates an intermediate state between seeing the mirror as another individual and recognizing the image as self.
There is some debate in the scientific community as to the value and interpretation of results of the mirror test. While this test has been extensively conducted on primates, there is also debate as to the value of the test as applied to animals who rely primarily on senses other than vision, such as dogs. As dogs have very poor visual resolution and acuity with red/green blindness, they have little chance of recognizing themselves or a dot (commonly red) in a mirror. However, dogs do recognize their own scent invariably with 40x more neurons than humans dedicated to processing smell. The key point being that the mirror test is only a measure of ability closely matching humans, not a statement of consciousness, as is popularly believed. Additionally, as mentioned with gorillas, many animals may regard eye contact as a threatening gesture, so the application of the mirror test is unclear. Some mammalian species do not have stereoscopic vision, including rabbits and deer, which may be a factor in determining the value of the test.
In 1981, Epstein, Lanza and Skinner published a Science paper in which they argued that the pigeon also passes the mirror test, making it the only non-mammal to do so. However, the methodology of the experiment has been criticised for explicitly training the pigeons to perform the criterion response (i.e. pecking at the mark.) See pigeon intelligence. This is, of course, quite different from the tests with apes and other large-brained animals, which pass the mark test without any explicit training.
- American Psychologist May 1977
- The World First Self-Aware Robot and the Success of Mirror Image Cognition (Lecture at the Karlsruhe University and the Munich University, Germany), 8-Nov.-2006.
- Elephants pass mirror test (The Guardian)
- Elephants' jumbo mirror ability (BBC News)
- Elephant study published in Proceedings of National Academy of Sciences, USA
- Elephants see themselves in the mirror (Newscientist.com with video)