The groups walked toward each other at a normal pace. When the groups met, the students intuitively performed a maneuver familiar to those who study crowds: They formed lanes. When a person at the front of one group found a way through the oncoming group, others fell in behind that person, creating several ribbons of walkers going past one another. This was effortless, and almost instantaneous.

The researchers then asked three of the students to perform a task on their phones while they walked — simple single-digit addition, not too taxing but enough to keep their gazes directed down instead of forward.

When those students were placed at the back of their group, the distraction did not affect how the groups moved past each other. But when the distracted walkers were at the front of the pack, there was a dramatic slowdown in the whole group’s walking pace. It also took longer for clear lanes to form.

Distracted people also did not move smoothly.