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If you've ever been part of a huge crowd, like at a sporting event, you've probably seen people clap, sing, and chant together in sync. How do big groups of individuals all manage to do the same thing at the same time, even when there's no one leading them? Well, it turns out, they kind of can't help it!

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If you've ever been part of a large crowd, like cheering for your team at a big game, you know that people seem to transform when they're in a group. They chant, they clap, they sing, they do things they'd never do in public on their own.

The weirdest thing? They manage to do it in sync, sometimes even when there's nobody up there leading them. It might sound weird, but a throng of people chanting ‘Dee-Fense!' actually tells us a lot about how we interact with one another, in, and out of, a group.

So how do people know how to coordinate a chant or a cheer? It turns out that they kind of can't help it. When people are together, they tend to unconsciously sync their movements.

When researchers ask a group of people to, say, move one arm around, they generally start to coordinate. In one study in 2006, this happened even when the participants were specifically told not to sync up. This tendency is similar to something scientists call automatic mimicry.

That's what happens when you yawn after someone else does, or smile when someone smiles at you. Researchers hypothesize that our tendency to copy each other plays a big role in the development of empathy -- or relating to someone else's mental and emotional state. Being able to recognize and mimic other people's actions may actually lead to taking on their emotions.

Sharing emotions helps us communicate and cooperate, which makes it easier to live in groups and may help ensure the survival of our species. That might explain why studies have found that when people are feeling excluded from a group, they're more likely to mimic the people in that group. Maybe in hopes of being invited back into the fold.

And no wonder you'd want back in. Syncing your actions with a group helps create a feeling of unity. When people move or chant together, their breathing and sometimes even their heart rates literally sync up.

It affects their interactions afterward, too. For example, a 2009 study by researchers at Stanford University tested whether doing things in synchrony as a group would make people more likely to cooperate. The researchers had around 100 American college students listen to music on headphones and sing along while they moved a cup in time with the music.

In some conditions, the music played out of sync, so that people were basically doing their task independently. In others, the songs played in unison, so that the participants would be singing out loud together. Afterward, the researchers had the groups play a couple of games to see how well they worked together.

The groups who sang in sync were better at cooperating and were more likely to share their resources. Seems like those weird icebreaker games you do at work retreats might actually bring people together, huh? But the effects of group synchrony aren't always positive.

The leader of the Stanford team published another study a few years later in 2012 showing that people basking in the joy of coordinated activity can also be destructive. He had groups march around the parking lot behind an experimenter. Some groups were in step, some weren't.

Then, the experimenter took them into a lab set up with little cups of sow bugs and a funnel. Leading to a coffee grinder. Yeah.

Full disclosure, there was actually a catch in the grinder that saved the little guys before they hit the blades. But the participants didn't know that! The experimenter told them to toss as many bugs into the grinder as they could in 30 seconds.

Those who had walked in step together sent 38% more bugs to their deaths than those who walked out of step. And remember how synchrony boosts cooperation with your group? Well, another study by that same researcher, this one from 2011, showed that it may also boost aggression toward outsiders.

In this experiment, 156 people in groups of three performed the same moving-cups-to-music task as before. But beforehand, they had to memorize a list of cities, then recall it afterward for a chance at a cash prize. That created some incentive for them to want the other groups to do worse.

Next, they got to choose the music that the next group would listen to during their task. One of those choices was a 90-second blast of static, which would make it harder for them to perform the cup task. One of the group members, who was secretly an experimenter, would suggest they choose it.

The groups who had moved their cups in sync were significantly more likely to choose the noise blast than those who moved out of sync. Which undermined the other groups. They also reported feeling more emotional connection to their group, which the researcher found could explain most of the difference.

When you feel emotionally connected to someone, you might be more inclined to obey them. This could explain why militaries perform marching drills, even though marching has long since been replaced by other tactics on the actual battlefield. It might also explain why leaders of rallies can drum up so much agreement.

But the good side of group synchrony is powerful too. Together, crowds can pool their money for charity, take a stand for a political movement, or sing “Sweet Caroline” at the top of their lungs. Whether it's positive or negative, the synchrony of crowds shows that our need to bond with our fellow humans runs deep.

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