Have you ever seen a demon, spirit, or other malevolent entity in the mirror instead of your own reflection?
Despite popular misconception, psychology as a science often knows less than what’s known by the first hand experience of occultists, mages, and in this case -- even little kids who like to scare the hell out of their friends by exposing them to the possibility of accidentally selling their soul to the Devil and burning in hell forever. (Or something similar.)
If you’ve been a kid at any point in your life, you’re familiar with the traditional party / sleepover activity of revolving around intentionally contacting some malevolent spirit (or something similar) through the portal connecting to every house’s bathroom mirror. (Everyone knows the routine, so I won’t outline it here.)
The particular evil entity to be contacted tends to vary according to local folklore. In many places it was the Devil himself, looking for souls. In mine, it was the Bell Witch, still pissed off and willing to wreak revenge on anyone stupid enough to contact her. Regardless, the game usually ends up with one or two people being getting totally freaked out by actually seeing the malevolent entity, and everyone else thinking the whole damn thing was nothing more than a practical joke.
If you’re one of those who thought it was a prank to scare idiots, I can tell you without a doubt, some of the people who claimed to see something in the mirror weren’t lying. In 2010, the perceptual effect underlying the experience was experimentally verified and given the name the “strange face in the mirror illusion”.
Here’s the results of that mindfuck of an experiment, in a nutshell:
The participants reported that apparition of new faces in the mirror caused sensations of otherness when the new face appeared to be that of another, unknown person or strange 'other' looking at him/her from within or beyond the mirror. All fifty participants experienced some form of this dissociative identity effect, at least for some apparition of strange faces and often reported strong emotional responses in these instances. For example, some observers felt that the 'other' watched them with an enigmatic expression – situation that they found astonishing. Some participants saw a malign expression on the ‘other’ face and became anxious. Other participants felt that the `other’ was smiling or cheerful, and experienced positive emotions in response. The apparition of deceased parents or of archetypal portraits produced feelings of silent query. Apparition of monstrous beings produced fear or disturbance. Dynamic deformations of new faces (like pulsations or shrinking, smiling or grinding) produced an overall sense of inquietude for things out of control.
You can read more at good ol mindhacks: https://mindhacks.com/2010/09/18/the-strange-face-in-the-mirror-illusion/