Yet not all members of a certifiably self-reflective species will pass the mirror test. Tellingly, said Diana Reiss, a professor of psychology at Hunter College who has studied mirror self-recognition in elephants and dolphins, “animals raised in isolation do not seem to show mirror self-recognition.”

For that matter, humans do not necessarily see the face in the mirror either. In a report titled “Mirror, Mirror on the Wall: Enhancement in Self-Recognition,” which appears online in The Personality and Social Psychology Bulletin, Nicholas Epley and Erin Whitchurch described experiments in which people were asked to identify pictures of themselves amid a lineup of distracter faces. Participants identified their personal portraits significantly quicker when their faces were computer enhanced to be 20 percent more attractive. They were also likelier, when presented with images of themselves made prettier, homelier or left untouched, to call the enhanced image their genuine, unairbrushed face. Such internalized photoshoppery is not simply the result of an all-purpose preference for prettiness: when asked to identify images of strangers in subsequent rounds of testing, participants were best at spotting the unenhanced faces.

How can we be so self-delusional when the truth stares back at us? “Although we do indeed see ourselves in the mirror every day, we don’t look exactly the same every time,” explained Dr. Epley, a professor of behavioral science at the University of Chicago Graduate School of Business. There is the scruffy-morning you, the assembled-for-work you, the dressed-for-an-elegant-dinner you. “Which image is you?” he said. “Our research shows that people, on average, resolve that ambiguity in their favor, forming a representation of their image that is more attractive than they actually are.”

Image Asian elephants are among the few nonhuman animals found to recognize themselves in mirrors. Credit... Wildlife Conservation Society

When we look in the mirror, our relative beauty is not the only thing we misjudge. In a series of studies, Dr. Bertamini and his colleagues have interviewed scores of people about what they think the mirror shows them. They have asked questions like, Imagine you are standing in front of a bathroom mirror; how big do you think the image of your face is on the surface? And what would happen to the size of that image if you were to step steadily backward, away from the glass?

People overwhelmingly give the same answers. To the first question they say, well, the outline of my face on the mirror would be pretty much the size of my face. As for the second question, that’s obvious: if I move away from the mirror, the size of my image will shrink with each step.

Both answers, it turns out, are wrong. Outline your face on a mirror, and you will find it to be exactly half the size of your real face. Step back as much as you please, and the size of that outlined oval will not change: it will remain half the size of your face (or half the size of whatever part of your body you are looking at), even as the background scene reflected in the mirror steadily changes. Importantly, this half-size rule does not apply to the image of someone else moving about the room. If you sit still by the mirror, and a friend approaches or moves away, the size of the person’s image in the mirror will grow or shrink as our innate sense says it should.