Why (and How) People of a Feather Flock Together

Seeking the hidden causes of behavior, some scientists work on the scale of brain regions and neurons, searching inside people's heads. Others work on the scale of crowds, neighborhoods and nations, seeking hidden patterns in the way multitudes behave. What's unusual about this paper in PLoS One is that it combines both those perspectives: Mehdi Moussaïd and his co-authors have worked out the physical effects of a psychological motivation. That gave them a new way to predict how people walk on city streets.

There are other physics-style models of crowd behavior, but, as the authors point out, those have assumed that each human "particle" on the street is an isolated individual. Give each "agent" a few simple rules to follow, and a computer can generate a simulation of actual people's movements pretty well. But we're a social species, and such models fail to capture the fact that people don't walk alone.


The paper's authors set up a camera on streets in Toulouse and simply recorded pedestrian traffic on a workday and then again on a weekend—4559 people in all. If people talked, smiled, gestured or laughed together, they counted them as part of a single group. More than half the pedestrians in the workday streetscape were in such groups; on the weekend, 70 percent were.

Physical facts have an important effect on how people walk, of course. As molecules of a gas move more quickly when they're less densely packed, so too people on the street walk faster when they're less crowded. However, Moussaïd and Co. found that walkers in groups put physical imperatives below psychological ones: The bigger the collection of people, the slower all its members walked. Degree of crowding had no effect on this pattern.

Were there any predictable laws in the way these walking groups organized themselves? To find out (this is the part that scientists properly call "heroic" for its superhuman care and patience) the researchers looked at all the pedestrians they'd recorded in groups and measured the angle and distance of an imaginary line between each person and his/her right-hand neighbor. That gave the team a way to define and track patterns in movement over time.

When streets weren't too crowded (or, in physics-speak, when density was low), the social groups walked in a horizontal line, shoulder to shoulder. When streets get more crowded, though, groups reform into a predictable formation: The middle person or persons hang back, and others draw closer. The result is a "V" or "U" of people.

It's much like the "V" you can see in the sky when geese are migrating, except that the human version points away from the direction of travel. As the authors point out, a flexible structure moving against a flow would be most efficient if it did what geese do. (That is, of course, why geese do it.) But people don't. Physical laws push them one way, but a psychological law (people in a group want to communicate with each other) pushes the other way.

Armed with this information, the team devised a new model of pedestrian behavior, adding group motivations to the usual variables that represent the effects of density, physical barriers and so on. This model, which assumes that people will do what makes it easiest to keep each other in sight and in earshot, fits real-life behavior quite well. Groups of walkers are slower than they "should" be, because they aren't organized to walk in the quickest, most "aerodynamic" formation. So, as the groups get bigger, they get slower, regardless of how many strangers are also walking on the street.

This doesn't mean that human V's lack leaders. Conversational groups tend to be made up of a few big talkers and many listeners, so, Moussaïd and Co. expect that the "leaders" of pedestrian groups are usually around the point of the inverted V. Again, like birds, but in reverse, because it's sociodynamic instead of aerodynamic.

Speaking of bird flight, this paper on pigeon flocking in last week's Nature coincidentally touches on the same theme as the Moussaïd paper in PLoS. By tracking pairs of pigeons as they flew, Tamás Vicsek and his co-authors found that pigeons, too, have leaders: Birds in the front (especially if they were on the followers' left) had more influence over the movements of the whole group than did others. In fact, they found a stable hierarchy of influence, with some pigeons definitely more equal than others. Both papers combine the individual and group levels of analysis to find rules in motion that looks random to the naked eye.

Further Reading:

Moussaïd, M., Perozo, N., Garnier, S., Helbing, D., & Theraulaz, G. (2010). The Walking Behaviour of Pedestrian Social Groups and Its Impact on Crowd Dynamics PLoS ONE, 5 (4) DOI: 10.1371/journal.pone.0010047  

Nagy, M., Ákos, Z., Biro, D., & Vicsek, T. (2010). Hierarchical group dynamics in pigeon flocks Nature, 464 (7290), 890-893 DOI: 10.1038/nature08891

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New fossils suggest human ancestors evolved in Europe, not Africa

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Homo sapiens have been on earth for 200,000 years — give or take a few ten-thousand-year stretches. Much of that time is shrouded in the fog of prehistory. What we do know has been pieced together by deciphering the fossil record through the principles of evolutionary theory. Yet new discoveries contain the potential to refashion that knowledge and lead scientists to new, previously unconsidered conclusions.

A set of 8-million-year-old teeth may have done just that. Researchers recently inspected the upper and lower jaw of an ancient European ape. Their conclusions suggest that humanity's forebearers may have arisen in Europe before migrating to Africa, potentially upending a scientific consensus that has stood since Darwin's day.

Rethinking humanity's origin story

The frontispiece of Thomas Huxley's Evidence as to Man's Place in Nature (1863) sketched by natural history artist Benjamin Waterhouse Hawkins. (Photo: Wikimedia Commons)

As reported in New Scientist, the 8- to 9-million-year-old hominin jaw bones were found at Nikiti, northern Greece, in the '90s. Scientists originally pegged the chompers as belonging to a member of Ouranopithecus, an genus of extinct Eurasian ape.

David Begun, an anthropologist at the University of Toronto, and his team recently reexamined the jaw bones. They argue that the original identification was incorrect. Based on the fossil's hominin-like canines and premolar roots, they identify that the ape belongs to a previously unknown proto-hominin.

The researchers hypothesize that these proto-hominins were the evolutionary ancestors of another European great ape Graecopithecus, which the same team tentatively identified as an early hominin in 2017. Graecopithecus lived in south-east Europe 7.2 million years ago. If the premise is correct, these hominins would have migrated to Africa 7 million years ago, after undergoing much of their evolutionary development in Europe.

Begun points out that south-east Europe was once occupied by the ancestors of animals like the giraffe and rhino, too. "It's widely agreed that this was the found fauna of most of what we see in Africa today," he told New Scientists. "If the antelopes and giraffes could get into Africa 7 million years ago, why not the apes?"

He recently outlined this idea at a conference of the American Association of Physical Anthropologists.

It's worth noting that Begun has made similar hypotheses before. Writing for the Journal of Human Evolution in 2002, Begun and Elmar Heizmann of the Natural history Museum of Stuttgart discussed a great ape fossil found in Germany that they argued could be the ancestor (broadly speaking) of all living great apes and humans.

"Found in Germany 20 years ago, this specimen is about 16.5 million years old, some 1.5 million years older than similar species from East Africa," Begun said in a statement then. "It suggests that the great ape and human lineage first appeared in Eurasia and not Africa."

Migrating out of Africa

In the Descent of Man, Charles Darwin proposed that hominins descended out of Africa. Considering the relatively few fossils available at the time, it is a testament to Darwin's astuteness that his hypothesis remains the leading theory.

Since Darwin's time, we have unearthed many more fossils and discovered new evidence in genetics. As such, our African-origin story has undergone many updates and revisions since 1871. Today, it has splintered into two theories: the "out of Africa" theory and the "multi-regional" theory.

The out of Africa theory suggests that the cradle of all humanity was Africa. Homo sapiens evolved exclusively and recently on that continent. At some point in prehistory, our ancestors migrated from Africa to Eurasia and replaced other subspecies of the genus Homo, such as Neanderthals. This is the dominant theory among scientists, and current evidence seems to support it best — though, say that in some circles and be prepared for a late-night debate that goes well past last call.

The multi-regional theory suggests that humans evolved in parallel across various regions. According to this model, the hominins Homo erectus left Africa to settle across Eurasia and (maybe) Australia. These disparate populations eventually evolved into modern humans thanks to a helping dollop of gene flow.

Of course, there are the broad strokes of very nuanced models, and we're leaving a lot of discussion out. There is, for example, a debate as to whether African Homo erectus fossils should be considered alongside Asian ones or should be labeled as a different subspecies, Homo ergaster.

Proponents of the out-of-Africa model aren't sure whether non-African humans descended from a single migration out of Africa or at least two major waves of migration followed by a lot of interbreeding.

Did we head east or south of Eden?

Not all anthropologists agree with Begun and his team's conclusions. As noted by New Scientist, it is possible that the Nikiti ape is not related to hominins at all. It may have evolved similar features independently, developing teeth to eat similar foods or chew in a similar manner as early hominins.

Ultimately, Nikiti ape alone doesn't offer enough evidence to upend the out of Africa model, which is supported by a more robust fossil record and DNA evidence. But additional evidence may be uncovered to lend further credence to Begun's hypothesis or lead us to yet unconsidered ideas about humanity's evolution.