Why is Greek yogurt suddenly trendy?
Why is Greek yogurt suddenly trendy? - 
Listen To The Story

Walk by the dairy section of any grocery store, and you'll notice a half a wall devoted to Greek yogurt, that extra thick and creamy, strained yogurt that has erupted in popularity in recent years. There is Chobani and Fage and Oikos, not to mention house brands like Trader Joe's. And to think that most of these did not exist five years ago.

We have been eating yogurt in the U.S. for a long time and the Greeks have been making yogurt in Greece for a long time. So why did the yogurt suddenly catch on? Why do any trends -- be it food, clothes or music -- catch on?

Jonah Berger is a marketing professor of the University of Pennsylvania's Wharton School of Business and he studies trends. He has wrapped up his research in a new book, "Contagious: Why Things Catch On."

"Greek yogurt," said Berger, "is really a great example of the power of social influence and word of mouth. They did spend some money on advertising, but it was really people trying the product and telling people how fabulous it was."

The reasons people pass on information and, thereby, induce trends, are numerous. For some, it is about keeping up with the Joneses -- people want to appear to be "in the know rather than behind the times," said Berger.

Berger noted that people tend to talk about what is top of mind, like the weather, and that is triggered by their envionment. We are also motivated by emotions. Berger looked at six months of articles on the New York Times most-emailed list and found that it was those pieces that aroused us -- either positively and negatively -- that we forwarded on.

"Positive thing were shared  more than negative ones," said Berger. "But even some negative ones were shared if they were arousing. Things like anxiety or anger -- they make us want to get out of our chair and do something."

This, he said, explains why YouTube videos go viral, and political rants do, too.

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Follow Kai Ryssdal at @kairyssdal