Back in the 1970s, NASA sent two spacecraft called Viking 1 and Viking 2 to Mars. While in orbit, they took lots of pictures of the surface, including a region called Cydonia. Cydonia is pretty cool! It’s a distinctive colour that makes it obvious if you look at it through a telescope on Earth and it might once have been a coastal region. Oh, and it contains a 1.5km-wide hill in the shape of a human face.

Sort of.

Some people got very excited and started yelling about how the Face on Mars meant there were aliens on Mars, hooray, neighbours! The Drake equation tells us that there are almost certainly aliens somewhere, but they are unlikely to be local aliens, so NASA and other space agencies took a more measured approach and went back to examine Cydonia with a camera that didn’t have the same megapixel rating as a potato – which revealed that it was, alas, just a hill with some interesting shadows on it.

Alas.

The thing is, even though I know it’s just a hill, I can’t quite get over it looking like a face. What gives?

Well, this is a classic example of a weird thing our brain does called pareidolia, which is Ancient Greek for “beside-image”, so thank you to the person who gave us this difficult-to-spell and etymologically-unclear word. Pareidolia is your mind’s tendency to decide that things that look vaguely familiar are actually something familiar: the Face on Mars, the Man in the Moon, the Horsehead Nebula, and here on Earth, Jesus in a Marmite cap.

Gaining pareidolia

When we think about human interaction carefully, it’s no surprise that our brains do this. At least for sighted people, a lot of interacting with other people relies on recognising familiar faces (is this person my mother or a stranger?) and on detecting the emotions they are showing (did they find the joke funny or should I run away in shame?). It’s so vitally important that we do these two tasks well that our brains are oversensitive to things that could be faces: pareidolia is basically a false positive. Much better to think something is a face when it’s not than to blank someone we rely on for physical or social needs, because the consequences of that second mistake are potentially much more severe. This importance is reflected in the activity of the brain, because the way we react to seeing faces and to seeing things that look like faces is very similar indeed [i].

Archimboldo’s ‘Vertumnus’ (public domain), the very apex of playing with your food

Of course, looking at faces is only one type of social interaction – we also very commonly interact using speech. So it should be no surprise that as well as seeing faces where there aren’t any, we also hear voices when there aren’t any [ii].

We’re not the only species where social interaction is really important – the same is true for a lot of primates, for example. I went looking for examples of pareidolia in other species and I am pleased to tell you that rhesus monkeys also have it, though of course for things that look like rhesus monkey faces rather than things that look like human faces [iii]. I didn’t find any examples in other animals, but I think that might be because we haven’t been looking rather than because pareidolia isn’t there.

Back to humans! Pareidolia starts happening when we’re around 8 months old [iv] [v], but we don’t all show it to the same extent. Young women tend to have it to a greater extent than young men, for example [vi]. There are also some neurological differences that can make some people less likely to experience pareidolia than others: if you have autism, you will probably be less susceptible to pareidolia than most [vii], and if you have Williams syndrome, you’re likely to be even less susceptible [viii]. However, we’re not clear on why these individual differences exist, yet.

We have a clearer idea on some other individual differences, like religious people and paranormal believers being more likely to experience pareidolia than non-religious people and paranormal sceptics [ix]. This fits with a general pattern among both religious people and paranormal believers of assigning meaning and intention where others would probably not. To understand this, let’s imagine someone’s playing Bananagrams (or Scrabble, but Bananagrams is superior). Over the course of several turns, they pick out the exact letters of their own name, in order. If this person is a nonreligious sceptic, they would probably decide it was a strange coincidence and think no more of it. If they were a religious person, they might decide it was meaningful, a message from God. If they were a paranormal believer, they might also decide it was meaningful, but with a different interpretation, perhaps a psychic message. Similarly, these three people looking at something that could be a face or could just be the way a rock has eroded might decide it was a coincidence, evidence of reincarnation, or evidence of elves trapping giants in the earth.

Trick of the light, or ELVES?

Using pareidolia

Pareidolia is more than just a side effect of our brains making sure we don’t miss faces – we can also use it! To finish, let’s look at a few examples of those uses.

You might know about Parkinson’s disease, a long-term disorder that gradually destroys the nervous system. While Parkinson’s isn’t curable, its progress can be slowed down with treatment, so for maximum effectiveness, you’d want to catch and treat Parkinson’s as early as possible. Amazingly, we might be able to use pareidolia to do this, because one of the things that we see in the early stages of Parkinson’s is an increase in pareidolia [x]. This is probably related to the visual hallucinations that a lot of people start developing as the disease progresses, because the specific thing that happens is not saying, “Hey, that looks like a face!” but really and truly thinking that not-faces are faces.

Medical professionals also purposely make use of pareidolia as a memory technique [xi]! I was amazed at the variety of pareidolias that are out there, so I won’t go through them all, but a couple that I thought were particularly cool were the ‘starfish sign’ for a subarachnoid haemorrhage – bleeding in the brain that shows up as a five-pointed ‘star’ on a brain scan – and the ‘winking owl’ sign which shows up on X-rays of the spine when a part of the vertebra called the pedicle is damaged on one side but not the other.

Lastly and perhaps most weirdly, we might be able to use pareidolia as a means of interesting people in conservation. In one study [xii], researchers showed people images of butterflies with and without eyespots. Hold up…

Spot the eye, lol

Then the researchers asked them how they felt about conservation efforts focused on each of those types of butterflies. People tended to be more positive about conservation efforts focused on the butterflies with eyespots… but the eyespots didn’t affect people’s willingness to get more informed about or donate to butterfly conservation. So, if you’ve just excitedly picked up a bag of googly eyes and some superglue, I’m afraid it’s probably not going to work.