An app that sends the word “Yo” to a phone contact with a single tap. That’s it.

Last week, when investors poured $1.2 million into this simplest of ideas — built by an Israeli engineer in just eight hours — reactions varied from laughter to outright disbelief. Comedian Stephen Colbert lampooned the app, saying, “When I first learned about an app that boils down all your communication into two letters, I expressed myself in one: Y.”

Or Arbel, creator of the new app, and Moshe Hogeg, CEO of the social photo- and video-sharing company Mobli, may have created the app for practical reasons, but it’s caught fire in part because it appeals to basic human brain processes. Arbel and Hogeg are tapping into the brain’s circuitry to activate a “reward center” that stimulates us and, more importantly, keeps us coming back for more.

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Your Brain on "Yo's"

When the brain receives a reward, the chemical messenger dopamine floods a pathway to the hypothalamus, the part of the brain that produces hormones that controls basic functions like body temperature, hunger, sleep, and mood.

Kent Berridge, a professor of psychology and neuroscience at the University of Michigan, explains that the addictive nature of rewards that give us hits of dopamine is the result of the interplay between two different systems in the brain: the dopamine, or “wanting,” system and the opioid, or “liking,” system. 

And according to Berridge, the wanting system is “more robust, bigger,” than the liking system. 

“A small taste will turn on the system and absolutely primes and amplifies the system to want more,” Berridge said. “It’s like the cocktail peanut situation. You can’t just have one.”

Dopamine is released not just when we receive a reward — such as food, drugs, or a "Yo" — but also when we anticipate one. The wanting switch can be flipped on by cues and triggers that the brain learns over time, activating the system in anticipation of a future reward.

“The whole brain is wired together, and it only takes, say, three or four synapses to go from the sensation to the system,” Berridge said. “The signals converge on the reward circuitry that loops around the brain.”

Information is enough to power those circuits, fueling our natural curiosity and drive. 

“The other thing that can activate the system are information nuggets,” Berridge said. “So cues in that sense are information nuggets.” 

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Anyone who checks their phone over and over again has experienced this drive for more, seeking the reward of an information package in the form of a text or tweet (or a "Yo"). Dr. Gary Small, a professor psychiatry and behavioral sciences at UCLA, said that texts lights up the same areas of the brain as other addictive substances, like drugs. If teenagers are interrupting their sleep to text or  are "hyper-texting," it could lay the groundwork for other addictive behaviors.

Short and Sweet

Arbel and Hogeg had to appeal for the app to be made available to the public at all — Apple originally rejected it from the iTunes Store for its lack of substance. Ironically, it's this lack of substance that gets us hooked in the first place.

Research suggests that the smaller the packet of information, the more we’re left wanting and the more we seek it out. As we move from emails to texts, from texts to tweets, and from tweets to "Yo's", those dopamine circuits are fired more and more rapidly. 

“A small taste will turn on the system and absolutely primes and amplifies the system to want more,” Berridge said. “It’s like the cocktail peanut situation. You can’t just have one.”

Engagement and unpredictability are other factors that can trigger this desire, which is why video games and gambling can be so exciting, Berridge explained. “What happens is the dopamine system comes on even stronger than it would if [the cue] were a perfect predictor,” Berridge said. Sending "Yo's" triggers the dopamine system, and the unpredictability of if and when you'll receive one back makes the reaction even more intense.

“This is the stupidest, and most addictive app I’ve ever seen in my life,” reviewers told Hogeg in the app’s early days. Yo reported on Twitter this week that the app has been downloaded over one million times.