When we care we share, according to Jonah Berger, author of "Contagious: Why Things Catch On". Absorbed with the role social influence plays in spreading the word about products, ideas or behaviours, Berger believes it is psychology that drives people to share rather than the technology itself. Anger and excitement can be motivators, as can things that make us look smarter. And stories with a message people like can all help build a brand's message and ultimately make it go viral.

A marketing professor at Wharton's School of Business, Berger's research interests include consumer decisionmaking, product adoption and abandonment, social contagion, viral marketing, and word of mouth.