In Contagious: Why Things Catch On, Wharton marketing professor Jonah Berger examines the irresistible spread of ideas and products.

Why are the mechanics of word of mouth so important?

We get information from ads, and from our friends. But we really don’t believe what we hear from ads; we’re much more likely to believe our friends. Because of that, word of mouth is much more effective at driving sales and popularizing ideas than is traditional advertising.

You spotlight outrageous marketing coups, like the Philadelphia restaurant that charges $100 for a cheesesteak. How does that draw business?

They’ve leveraged important psychological principles into word-of-mouth interest. We’re used to cheesesteaks that cost five or six dollars, maybe $10. But charging $100 for a cheesesteak is so unexpected that it becomes remarkable, so we’re much more willing to talk about it.

You analyze a much-maligned Budweiser ad that shows a bunch of dudes standing around saying, “What’s up?” How is that ad really a stroke of genius?

We might hate that Budweiser ad, but we think about it very often. People frequently say that phrase, and the phrase brings to mind the ad. Products and ideas not only need to be creative, they need to be cued by the social environment.

You argue that antidrug public service announcements that tell kids to resist peer pressure backfire. Why do they fail?

It’s because of “social proof”: we tend to follow the lead of other people—but only if we know what they’re doing. Those antidrug ads make the fact that people are doing drugs much more public. Imagine you’re a child sitting at home and you’ve never thought about doing drugs before, and you see this ad that says, “Hey! Kids in school might ask you if you want to do drugs.” You’re sitting there thinking, “Wow! I had no idea that other kids in school were doing drugs! And it might be the cool kids! Maybe if they’re doing drugs, I should too.”

With all the possibilities for manipulating social cues and word of mouth, can we be convinced to buy or believe anything with the right marketing ploys?

Marketing can’t make us buy a terrible product; at the end of the day, if people don’t like your restaurant or don’t find your book interesting, word of mouth won’t help. What word of mouth can do is help you cut through the clutter and encourage people to try your product. Choosing to buy something is often just about awareness.