How do you know when someone else is paying attention to you? If they’re staring at you intensely, that’s a pretty obvious giveaway. But there are also far subtler signals — such as the size of their pupils.

As Clara Colombatto and Brian Scholl at Yale University note in a new paper in the Journal of Experimental Psychology: General, our pupils automatically and uncontrollably dilate when we’re emotionally aroused, working something out, or just attending to something. Pupil size has been used as an objective indicator of all these things in a wealth of recent studies.

But if another person is directing their attention towards you, you need to know about it. It might be attention that you should reciprocate, to build a relationship, or it might signal a potential threat. So, Colombatto and Scholl wondered, “If the apprehension of pupil size is so helpful to scientists, might it be similarly helpful to us in everyday life?”

It seems, that our visual system is indeed attuned to large pupils, which carry a social meaning.

In revealing that we unconsciously pay more attention to someone with large pupils, who seems to be paying attention to us, the work helps to illuminate the processes by which we come to share mutual attention. “In short, the current results suggest that the perceived attentional state of other can in turn cause us to attend to them — a novel form of ‘attentional contagion’,” the researchers conclude.