Human beings are champions at spotting patterns, especially faces, in inanimate objects—think of the famous "face on Mars," which is essentially a trick of light and shadow, in images taken by the Viking 1 orbiter in 1976. And people are always spotting what they believe to be the face of Jesus in burnt toast and many other (so many) ordinary foodstuffs. There was even a now-defunct Twitter account devoted to curating images of the "faces in things" phenomenon.

The phenomenon's fancy name is facial pareidolia. Scientists at the University of Sydney have found that, not only do we see faces in everyday objects, our brains even process objects for emotional expression much like we do for real faces rather than discarding the objects as "false" detections. This shared mechanism perhaps evolved as a result of the need to quickly judge whether a person is a friend or foe. The Sydney team described its work in a recent paper published in the journal Proceedings of the Royal Society B.

Lead author David Alais, of the University of Sydney, told The Guardian:

We are such a sophisticated social species and face recognition is very important... You need to recognize who it is, is it family, is it a friend or foe, what are their intentions and emotions? Faces are detected incredibly fast. The brain seems to do this using a kind of template-matching procedure. So if it sees an object that appears to have two eyes above a nose above a mouth, then it goes, "Oh I'm seeing a face." It’s a bit fast and loose and sometimes it makes mistakes, so something that resembles a face will often trigger this template match.

Alais has been interested in this and related topics for years. For instance, in a 2016 paper published in Scientific Reports, Alais and his colleagues built on prior research involving rapid sequences of faces that demonstrated that perception of face identity, as well as attractiveness, is biased toward recently seen faces. So they designed a binary task that mimicked the selection interface in online dating websites and apps (like Tinder), in which users swipe left or right in response to whether they deem the profile pictures of potential partners attractive or unattractive. Alais et al. found that many stimulus attributes—including orientation, facial expression and attractiveness, and perceived slimness of the online dating profiles—are systematically biased toward recent past experience.

This was followed by a 2019 paper in the Journal of Vision, which extended that experimental approach to our appreciation of art. Alais and his co-authors found that we don't assess each painting we view in a museum or gallery on its own merits. They also found that we're prone to a "contrast effect": that is, perceiving a painting to be more attractive if the work we've seen before it was less aesthetically appealing. Instead, the study revealed that our appreciation of art shows the same "serial dependence" systemic bias. We judge paintings as being more appealing if we view them after seeing another attractive painting, and we rate them less attractive if the prior painting was also less aesthetically appealing.

The next step was to examine the specific brain mechanisms behind how we "read" social information from the faces of other people. The phenomenon of facial pareidolia struck Alais as being related. "A striking feature of these objects is that they not only look like faces but can even convey a sense of personality of social meaning," he said, such as a sliced bell pepper that seems to be scowling or a towel dispenser that seems to be smiling.