It’s a staple in wildlife documentaries: the gleaming mass of slender silver fish swimming in unison, an unconscious coalescence flashing and spilling like liquid mercury around the predators that dart among them.
Schools of fish are but a single example of the mesmerizing group formations that occur within the animal kingdom. Tens of thousands of starlings can move as if with one mind in sweeping, synchronous murmurations, and entire flocks of sheep will roam like a single woolen entity across miles and miles of pasture.
From where humans sit at the crown of the planet’s sentience hierarchy, this sort of swarming behavior may seem like the product of a lesser-evolved animal subconscious. Research indicates, however, that vestiges of the same instinct persist in us as well.
Stand on the corner of Fifth Ave and 49th Street for an hour and you’ll see thousands of people passing through Rockefeller Center, walking alone or moving in hordes, at times flowing like rivulets and at others sputtering by in congested knots.
Unlike the ripples and undulations formed by flocks of birds, the movement of human crowds appears entirely chaotic, hardly the result of coordination at an individual level. But on a large-scale, crowds are actually influenced by the same basic, mathematically predictable principles that govern flocking behavior in all species.
“We think of ourselves as being incredibly complicated creatures,” said Iain Couzin, professor of Ecology and Evolutionary Biology at Princeton University. “But when you’re in a crowd, your interactions tend to be much simpler than you think.”
Couzin specializes in the study of how living things move in unison. Though many different animals engage in flocking–or swarming, or schooling, depending on the creature–the instinctual motivation underpinning the behavior varies species to species.
Consider, for example, a minnow swimming all alone in a vast expanse of open water. If a barracuda appears, there’s very little that our minnow could do to avoid being eaten. For fish, schooling is as simple as seeking safety in numbers. The tight, synchronized movements of the group confuse predators and increase each individual’s odds of survival.
In humans, the evolutionary root of collective behavior likely has more to do with the transferral of information than with evading predation. If an individual knew where to find a source of food or water, it would have been advantageous for our small group-living primate ancestors to stick together as a whole. If multiple individuals disagreed on the correct direction, or had conflicting information, the group would have had to come to a consensus decision in order to remain united.
In 2006, Couzin and his colleagues conducted a study to assess how humans arrive at these consensus decisions without active signaling or individual recognition. They found that in group situations, “a small informed minority could guide a group of naive individuals to a target without verbal communication or obvious signaling.”
Similar findings emerged from a series of experiments conducted by Jens Krause, a collective behavior specialist and professor at the University of Leeds. Krause asked groups of people to walk around at random in a large hall, with the parameters that they must stay within arm’s length of another person and that they may not communicate with anyone. His team then secretly instructed several individuals of where to walk.
In every trial, the naive majority ended up following the few individuals with specific information, unintentionally forming snakelike patterns.
“We’ve all been in situations where we get swept along by the crowd,” said Krause. “But what’s interesting about this research is that our participants ended up making a consensus decision despite the fact that they weren’t allowed to talk or gesture to one another. In most cases the participants didn’t realize they were being led by others.”
This unconscious, self-organized collective behavior was observed across the board in groups of varying sizes. Surprisingly, as the number of people in a group increased, the necessary number of informed individuals decreased. For example, in a crowd of 200 people, only five percent constituted sufficient leadership for the entire mass to follow.
Understanding the mechanisms that govern how people move in groups can help researchers develop more effective methods of directing the flow of large crowds. More importantly, better-informed spatial planning could have major implications for evacuating crowds in disaster scenarios, in which verbal communication is likely to be inhibited.
“There are many situations where this information could be used to good effect,” said Krause.