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Have you ever noticed how your footfalls tend to line up when you walk alongside someone? Seriously, try walking so your footsteps aren't in sync with the person next to you.

It's harder than it sounds. There's actually scientific research showing that pairs of people tend to walk in sync, too. And other studies have found that people's heart rates and breathing also tend to line up when they're doing things together, like singing in a choir, for example.

But it's not just your breathing or heart rate or footsteps that can sync up. It's the electrical activity in your brain, too. People's brain activity syncs during all kinds of social interactions, like playing music together or solving a problem together.

And by learning about when and how people's brains connect, researchers are beginning to uncover a whole lot more about what it means to be truly connected to another person. Any activity in your brain creates brain waves: electrical signals that can have different wavelengths and patterns. And whenever you do something that requires coordination, brain waves from different regions of your brain can sync up.

But it wasn't until the last decade or so that scientists learned how these waves can also sync between brains. That research really took off with a 2010 paper on people playing card games; a type of social interaction known as interactive decision-making. In the study, published in the journal Brain Topography, a group of Italian neuroscientists had 14 pairs of subjects play an Italian card game while hooked up to EEGs.

Basically, they had a bunch of electrodes stuck to their heads to monitor the electrical activity in their brains. And they played a game called tressette, which is a four-player game with two people on each team. That means the researchers were able to compare the brain waves of people who were working together as well as those who were working against each other.

The main goal of this study was to show that you could use EEGs to measure multiple people's brain waves at the same time, which they called EEG hyperscanning. But in the process, the team also found something weird. They happened to find that the brain waves of players on the same team were very much in sync, while those of players on opposing teams weren't.

It's worth mentioning that because EEG hyperscanning is a very new field, practically all of the studies that use it are small. They're meant to introduce new ideas for further research, not conclusively prove anything. And with a new field, there's still room to explore these phenomena in all kinds of brains, including those that might process social interactions in atypical ways.

But the results of this card game study led to a whole slew of research using this new technique to measure the connections between people's brain waves during interactive decision-making, as well as a bunch of other types of social interactions. For example, shared attention is a type of interaction where you communicate with someone else using non-verbal signals, like by looking at each other. A 2017 study published in the journal PNAS investigated possibly the most adorable example of this: an adult singing to a baby.

Researchers from the UK and Singapore had 19 babies listen to an adult singing nursery rhymes while she looked at the baby either directly or indirectly. The team found that when the adult was looking directly at the baby, their brain waves were much more in sync with each other. And the more the baby vocalized during the singing, the stronger the similarity was.

So even in interactions where you're not necessarily working together towards some goal, there's evidence that a stronger connection between people also involves a stronger connection between their brain waves. But that's just a simple example of people looking at each other. Affective communication takes non-verbal interaction a step further, where people communicate using more complex emotional signals.

Like when they kiss. In a 2014 study published in the journal PLOS One,. German neuroscientists studied the brain waves of 15 heterosexual couples that they brought into the lab for a smooch sesh.

Hey, great date night idea: how about, laboratory!? They asked the couples to kiss each other normally, then kiss each other while they solved math problems in their heads. As you do.

They also asked each partner to kiss their own hand. All for science, of course. The researchers studied the subjects' brain waves in each scenario and also asked them to rate the quality of the kisses.

They found that, yup, the couples' brain waves were the most in sync while they were kissing each other. And the more satisfied they said they were, the stronger the connection was between their brains. And, surprise!

People tended to report more satisfaction from the kisses that didn't involve solving math problems. So, pro tip: skip the mental math. In any case, all of these studies found connections between people's brain waves that reflected the connections they had with each other.

But we're starting to learn that each person plays an individual role in building that connection, too. In a 2018 study published in the Annals of the New York Academy of Sciences, researchers monitored the brain waves of a guitar quartet while they played. That's a type of social interaction called joint action, where people are coordinating with each other.

The researchers on this 2018 paper had already done a bunch of studies on people playing guitar duets, where they found that the musicians' brain waves synced up with each other. For this study, though, they decided to study four musicians at once, and they used computer models to map the connections between the subjects' brain waves. And the researchers found that when the connections within an individual guitarist's brain were stronger, like when one guitarist was playing a solo, the inter-brain connection tended to be stronger, too.

In other words, the connection didn't just emerge from the experience they were sharing, it was also built on the contribution each individual made. If there's anything we've learned from the research that's been done so far, it's that the connections between people run much deeper than we thought. We humans are connected not just by our shared planet or biology or technology, but by the actual patterns in our brain waves.

Thanks for watching this episode of SciShow Psych! If you liked this video, you might be interested in our episode on brain waves, where we talk about different kinds of brain waves and the activities you do that create different ones. You can check that one out next! [♪ INTRO].