A selection of the images shown to the monkeys in the trial.
A selection of the images shown to the monkeys in the trial. Public Domain

We see faces that don’t exist. A toilet’s two-buttoned flush looks like a wonky scowl. Two cardboard boxes, stacked on top of each other, resemble an astonished beast-with-two-backs, caught open-mouthed in an unnatural act. A suitcase, peering out from its overhead locker, is somewhere between disgusted and irate. (All these are from the excellent Twitter feed, Faces in Things.)

Pareidolia, as this phenomenon is technically known, gets used in artificial intelligence, when training computer networks to recognize faces. It also has a role to play in psychological assessment, in the Rorschach “inkblot” test, where patients look for objects in seemingly random shapes.

But it turns out we aren’t the only animal that sees sloths in pains au chocolat or Jesus Christ in our morning coffee. A new study from a team of researchers at the U.S. National Institute of Mental Health suggests that rhesus monkeys do, too.

Rhesus monkeys are a particularly social species, with impressive cognitive abilities. They can sort the wheat from the chaff, understand simple rules, and live in complicated matriarchal troops of 20 to 200 monkeys. (In 2014, a Kanpur rhesus monkey stole hearts, and clicks, when it helped to revive a simian friend who had electrocuted itself on a train line.) Because of these factors, researchers say, a predisposition toward pareidolia isn’t totally surprising.

Monkeys were shown trios of pictures on a computer screen, and timed to see which one held their attention for the longest. Prior research suggests that rhesus monkeys, like humans, are far more interested in looking at pictures of faces than they are of objects. Face-tracking software showed that they particularly focused on objects that appeared to be eyes or mouths. Even more strangely, the monkeys preferred to look at objects that looked like faces more than pictures of other monkeys.

Perhaps they, like @FacesPics’ 583,000 followers, just thought it was funny that a cute little black coffee appeared to be smiling out at them.