This is but one of the ways we adjust to being small fish in a big pond. Fish happen to be a good model for what we do: research on fish “traffic” management has led to the formulation of three simple rules they follow to avoid congestion while moving together with hundreds or thousands of other fish. The same rules explain the remarkable synchrony of flocks of birds, swarms of army ants and even mass migrations of wildebeests and whales.

First, avoid bumping into others (while staying comfortably close). What counts as comfortably close — the “personal” space an animal attempts to maintain between itself and others — will vary by species. Little brown bats roost tightly packed together, with full-body contact, and Emperor penguins are happy setting up shop a mere couple of flippers’ lengths from other birds, while humpback whales appear to prefer to stay more than a mile away from one another — still close enough to react to the behavior of those around them.

A second rule: Follow whoever is in front of you. No matter that she may herself be following another, or may be only a temporary leader (the head of a flock of birds, for example, is only momentarily, and circumstantially, at its nose, and will cede leadership at the next turn right or left). For humans, following the person in front of you helps, on the most basic level, to form natural walking routes that become well populated. While we do not settle exactly in someone else’s slipstream, as fish do, we hover, preferring to look over the shoulder of the person ahead of us. On a sidewalk, this tendency sets up ever widening and narrowing channels of walkers going in the same direction.

The final rule: Keep up with those next to you. Everyone must speed up or slow down with attention to those around them. This seems like a difficult calculation, until you realize how little effort you have to exert to walk next to a friend down the street.

These rules of “attraction” (staying with others ...), “avoidance” ( ...while not too close), and “alignment” (going the same direction and speed as those around you) are sufficient to explain all herd, school, flock and swarm behavior — not to mention that of big-brained and busy human pedestrians.