Human social interactions are rich with non-verbal cues, one of which is the ‘side eye’ – a sidelong glance that conveys disapproval, skepticism, contempt, or suspicion. It can also express other things such as veiled curiosity. It’s made by looking at someone indirectly, out of the corners of the eyes. By comparing human behavior with that of our closest relatives, the great apes, we can explore the possibility that the ‘side eye’ is an evolutionary adaptation for non-confrontational communication of distrust.

In human social dynamics, the ‘side eye’ is often employed in situations where direct confrontation is undesirable or risky. This subtlety in expression allows individuals to convey their feelings while maintaining a level of social decorum or safety.

The behavior of great apes provides a compelling parallel to this human non-verbal cue. It is known that chimpanzees and gorillas will face and look straight at friends and companions but they will look to the side at group members that they don’t trust or are afraid of. This is because when an ape looks right at another ape, it expresses intent, and that could be an intent to attack. To avoid this, they look to the side to conceal the fact that they are observing the other ape. This behavior suggests a method of observation that does not escalate to direct confrontation.

I think that when a human enters side it is because they have a feeling that they shouldn’t trust the other person enough to look right at them. So we use side eye when someone disappoints or insults us because we don’t see them as a trusted ally but rather as competition or opposition. Due to cultural factors and propriety, its original function has been changed, or co-opted. Now it seems that people often do it consciously as a way to demote others (sometimes playfully), and it is used as a nonverbal signal of dominance or even humor. But originally, and at its root, it may be a neurological mode of covert vision that, as close relatives of apes, we still have an instinct to engage in.

From a neurological perspective, the ‘side eye’ may be linked to brain regions associated with social judgment and threat assessment. This response could be deeply rooted in the evolutionary need to quickly covertly assess potential threats and allies. The ubiquitous nature of this response suggests a hardwired, instinctual behavior pattern developed over primate evolution.

The hypothesis presented here posits that the ‘side eye’ in human communication is an evolutionary adaptation inherited from our primate ancestors. It serves as a non-aggressive way to express distrust or disapproval, but may be rooted in a neurological instinct, to hide the object of attention. Understanding this behavior through the lens of evolutionary psychology offers valuable insights into the subtleties of human social interactions.