It’s a common and intuitive assumption that the abilities and actions of individuals determine where they fit in the hierarchy of a large group. But in a new study published today in PLOS One, a pair of scientists posit that this perfectly reasonable premise is incorrect.

Ivan D. Chase, a sociologist at Stony Brook University, and W. Brent Lindquist, an applied mathematician from Texas Tech University, say that pecking orders—whether within chicken pens or human social organizations—are surprisingly unpredictable and to a degree maybe even random. Using experimental and mathematical evidence, they showed that differences in individual ability are excellent predictors of who dominates whom in pairs of individuals, but they are poor predictors of hierarchy rank in larger groups. They found the same limitation holds for winning and losing streaks.

“This goes against the standard way both biologists and social scientists think about dominance hierarchies in social organizations,” Chase said. “Whether it’s humans or animals, when you think about rankings within organizations—how much money people make, how popular they are in their network, or why a hermit crab might have a larger shell than others—the idea is that the rank is all based on the abilities, actions and strategies of individuals. But it’s not.”

To test assumptions about pecking orders, Chase and Lindquist conducted observational experiments with African cichlids, a tropical fish that Chase has used in his research for years. They first observed pairs of fish, recording acts of aggression—nipping and chasing—to determine which one was the alpha fish. Then they separated the fish for two weeks, long enough to assure they wouldn’t remember each other. When they brought them back together, the same fish was dominant in 94 percent of the 31 pairs in the experiment.

But when they did the experiment with four fish, the hierarchies in the first and second rounds were the same only 27 percent of the time---nearly three-quarters had different outcomes, in a variety of new combinations. In 18 percent of the quartets, all four fish ranked in a different position the second time around.

“We tend to think that individual differences in ability, genetics, personality, nature, nurture are the predominant factors in social outcomes,” said Chase. “But while those things have an influence, there are other things that override differences in individuals. And because there are so many variables, hierarchies are so complex as to be unpredictable and somewhat random. So our theory is that it’s impossible to predict which individual will be the alpha, which the second in rank, and so on. But there are rules of interaction that are crucial.”

The first rule governing hierarchies, Chase says, is that all individuals interact – no one is left out. The second is that hierarchies are re-established when lower-ranking individuals displace higher-ranking ones—but not in a predictable pattern. “It’s similar to crystal formation,” Chase said. “Physicists can’t predict which atom of table salt will be where in a crystal, but they know the rules that make the crystal a cube.”

The researchers suggest that many species of animals, including human preschoolers, use the same two rules in forming their hierarchies, and they are now collecting data to test this hypothesis in fish.

Biologists and social scientists use individual attributes and behaviors to explain other social hierarchies, from the distribution of wealth in humans to nesting territories in birds. Given the shortcomings of individual-based explanations for pecking orders, the researchers suggest such assumptions limit our understanding of other social inequality systems and should be revisited.

“We want things to be predictable in science and when we find something is not predictable that’s upsetting to folks,” Chase observed. “People think that hierarchies in social organizations are entirely based on the individual and that hierarchies have a structure that is stable and unchanging. But that will never really help us understand how social organizations have the shape they do. They are actually dynamic systems that change over time.”

Journal reference: Ivan D. Chase & W. Brent Lindquist. The Fragility of Individual-based Explanations of Social Hierarchies: A Test Using Animal Pecking Orders. PLOS ONE, 2016, 11(7):e0158900, doi:10.1371/journal. pone.0158900

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