The living world turns chaos into order by making large structures out of small units. The sublime coordination of a flock of birds or a school of fish—built iteratively from the twitches and bumps of single individuals—turns instinctive behaviors into something vast and graceful. It’s not just for show. A lone gazelle can’t evade a fast-moving lion anywhere near as well as a slippery, undulating herd.
With that in mind, take a look at this video of a truck barreling through protesters in Minneapolis in May. The densely packed protesters swirl out of the truck’s path, then close ranks back around it when the truck stops. For a few moments, the movements of the protesters look very much like a murmuration of starlings responding to an attacking falcon, or maybe little fish to a shark—parting like water in front of a boat, swirling in eddies around the invader, regrouping behind it. The protesters respond as a collective.
This is speculative, but roll with me a little: Individual persons move in unpredictable ways, but people—crowds—behave very much like any other collective group of living creatures. The motion of those groups switches from fluidic swirls around and through barriers to jammed-up particulate movement, in spontaneously self-organized formations that accomplish tasks like migration—or respond to predator-like behavior from things like vehicles and, yes, law enforcement moves. Think of “kettling”—walls of armored men encircling protesters—or troops mounted on bikes or horses cutting across lines of demonstrators. And, like all living things, these groups adapt. Protesters adapt to police tactics, and the police respond, often with greater force. But maybe understanding this kind of distributed, collective behavior can provide some options beyond a violent response to peaceful protest.
Collectives of living things generally take one of three fundamental structures: a localized swarm (think bees or bats on the hunt), a sort of milling or orbiting around a central core (a school of anchovies), or a fluid-like flow in a direction (like this swarm of fire ants so big it showed up on weather radar).
Now imagine a public space like a park full of protesters or a march down a boulevard. A crowd of people can take on any of those forms as well, from milling mass to parade. But like the phases of physical matter—gas, liquid, and solid—those structures can transition from one to the other. The changes aren’t exactly like classical phase transitions, like ice melting to water and then evaporating to steam, because those involve billions or trillions of individual atoms or molecules. But in groups of just a few dozen living things, similar changes do happen. Small and local interactions propagate with startling speed through the group, gumming up the works or making the whole collective move more smoothly.
“When you have energy in a system—like people pushing and pulling—there’s all sorts of phase transition-like behavior,” says Iain Couzin, director of the Max Planck Institute of Behavior at the University of Konstanz in Germany. “At the local interactions, as the change in density percolates out, the speed of individual movement changes.”
In animal groups, that propagating wave of change in behavior—like a flock changing direction—moves without the birds, bugs, or whatever thinking much about it. In fact, Couzin’s team has argued that a collective is smarter than any of its members, like a brain being better at thinking than a single neuron. Nonhuman animals make these phase changes work to their advantage. “If birds hear the screech of an eagle or gazelle smell a lion, they'll dramatically change configuration to more effectively propagate information across the collective,” Couzin says. “The network of interactions does the computation for you.” The change in the network’s spatial configuration literally allows it to become more defensive, to keep all the members safer.
Humans didn’t evolve to do any of that. “We don’t have that capability,” Couzin says. “We’re limited in our ability to interact with large numbers of others.” High-stress, high-intensity situations don’t convert big groups of people into emergent, calculating networks. They don’t always know what to do with the fast-moving information around them. That can include nonverbal cues like the direction of someone else’s gaze, or the sight of other people running or crowding together. Or it can be verbal communication signals like shouts or, on a wider spatial and temporal scale, text messages or phone calls. But humans don’t always use those signals in positive ways. Misunderstood, they can lead to chaos and disaster—“crowd panics” like the crush during the Hajj in 2015 in Mecca that killed perhaps as many as 2,400 pilgrims, caused when two massive groups of people tried to pass the same bottlenecked intersection.
Spatiotemporal scale is the key difference. Cues move fast and can have a profound effect on changing behavior. Which brings us back to the context of a protest. “In any kind of crowded situation that’s at the edge of a phase transition, in this critical-like state, a few individuals can have a huge impact,” Couzin says. “There’s tension in the crowd, and then someone screaming ‘Bomb!’ creates a huge wave of panic, because the crowd has sufficient density.” (Fast-propagating cues don’t have to sow chaos among people, of course—in Hong Kong, protesters last year used simple hand signs to send complex communication signals about organization and the deployment of specialized teams of demonstrators like medics).
Importantly, it doesn’t matter which side freaks out first. A sudden or unexpected move from law enforcement can panic demonstrators, and vice versa. But the apparent widespread use of violence by law enforcement, or of chemical weapons like smoke and tear gas, can be a panic starter. “That’s why it’s so damn dangerous when you have these types of events,” Couzin says. “Both of them are ready to blow up. You don’t need much to trigger it.”
Law enforcement tactics like kettling seem like they might make bad outcomes more likely. One bit of violence or apparent violence—from an extremist in a crowd of peaceful protesters, or from a wall of unidentified paramilitary troops in riot gear—and not only does the center not hold, the edges don’t either. In a disaster, crowds sometimes panic and bash themselves against walls looking for exits. The modern police tactic of forming a skirmish line with shields and moving inward toward protesters might—emphasizing the might; this is hypothetical—be the inverse, a wall moving toward them. Sometimes it’s not even intentional. In 2010, 21 people died in a crush incident at the Love Parade electronic dance festival in Germany, in part because police formed barricade lines hoping to guide people toward the exits.
But if protesters know that they’re going to almost involuntarily act as a collective while engaging in their constitutional right to demonstrate—and if law enforcement personnel know they will too—and if both sides know the other side knows—that could open up a channel to cool off violent tactics. “I think strategic police responses to these types of events are going to take into account how to interrupt those back-and-forth, tit-for-tat types of exchanges, some of which are sort of retaliatory in nature,” says Edward Maguire, a criminologist and the associate director of Arizona State University’s Center for Violence Prevention and Community Safety. “Police departments that are not thinking about those things can be mindlessly caught up in it, and what we need is for the police to be the adult in the room.”
Maguire, who’s working on training police to engage earlier and more closely with protesters and organizers, worries that things will only get worse if they don’t—people will die, law enforcement will keep upping the ante, and protesters will respond in kind. The kettling, chemical weapons, and rubber bullets will keep coming. “As long as the police behave in a manner perceived as illegitimate, or in this case blatantly unconstitutional, it’s a really difficult sell for moderate protesters to talk down those among them who prefer more extreme tactics,” he says. “The more extreme protesters feel morally justified in what they're doing. You have a much greater share of moderate protesters who may not disagree with them.” That’s its own kind of phase transition-like change—from demonstration to uprising.