The mirror test, also known as the mirror self-recognition (MSR) test, was developed by psychologist Gordon Gallup Jr. in 1970 as a method to assess self-recognition and self-awareness in animals. The test involves marking an animal with a visual mark, typically using odorless dyes or stickers, on an area of the body that is normally unseen, such as the forehead or ear. The animal is then given access to a mirror, and its behavior is observed. If the animal touches or investigates the mark on its own body while viewing the mirror, it is interpreted as evidence that the animal recognizes the reflected image as itself, rather than another individual. This mark-directed behavior is considered an indication of self-awareness and the ability for visual self-recognition.

The inspiration for the mirror test comes from Charles Darwin observing an orangutan named Jenny at the London Zoo in 1838. Darwin watched Jenny gaze into a mirror, leading him to ponder the possibility of self-recognition in orangutans.

In 1970, Gordon Gallup Jr. conducted the first experimental mirror self-recognition study with chimpanzees. He exposed four wild preadolescent chimps, who had never seen mirrors before, to their reflections over 80 hours. Initially threatened by their images, the chimps eventually used the mirrors for self-directed behaviors like grooming unseen areas.

Gallup then anesthetized the chimps and applied an odorless dye mark. After regaining consciousness, he recorded how frequently they touched the marked areas with and without a mirror present. The chimps inspected the marks significantly more often when the mirror was available, suggesting they recognized the reflected image as themselves.

This marked the first empirical evidence of mirror self-recognition in a non-human species. Subsequent variations used tactile markers or skipped anesthesia, but the core method of observing mark-directed behaviors remained. The test has since been conducted across many species to probe self-awareness capabilities.

In an effort to assess self-recognition in species that rely heavily on olfaction, researchers have proposed an "olfactory mirror" test as an alternative to the traditional visual mirror test. This approach involves modifying an animal's own scent in some way and observing if it investigates the altered odor more intently when presented with its own scent sample versus an unmodified control. While intriguing, the olfactory mirror test has its own limitations, as it can be difficult to ensure the scent modifications are truly imperceptible through other senses.

Beyond sensory adaptations, some researchers argue that different species may exhibit self-awareness through behaviors not captured by mirror tests, such as distinctive vocalizations or scent-marking patterns. These modality-specific expressions of self-recognition highlight the need for innovative, multi-modal assessments tailored to each species' unique capabilities and ecological contexts. As our understanding of animal cognition deepens, a diverse toolkit of tests may be required to fully probe the boundaries of self-awareness across the tree of life.