Why do we weigh an argument carefully one day and just follow a famous face the next?

Because how much we think about a message depends on whether we care and are able to. The elaboration likelihood model, developed by the psychologists Richard Petty and John Cacioppo in 1986, says people can process a message with anything from low thought to high thought. It describes two routes to persuasion. Under the central route, we carefully consider the true merits of the arguments. Under the peripheral route, we are swayed by cues unrelated to the logical quality of the message, such as how credible or attractive the speaker is, or how slick the production looks.

The razor experiment shows both routes in action. Students were told either that a new disposable razor would soon be test marketed locally and that they could get one, which made them highly involved, or that it would be marketed in a distant city and they would get a toothpaste instead, which made them low-involvement. Some saw ads with famous athletes, some with ordinary citizens, and some with strong arguments and some with weak ones. When people did not care much, famous athletes won them over regardless of how good the product's arguments were. When people cared a lot, only the strength of the arguments mattered.

Two things decide the route: motivation, the desire to think about the message, and ability, such as having no distractions and enough knowledge. Because people are cognitive misers, looking to save mental effort, they often take the peripheral route and rely on mental shortcuts, the kind that Cialdini's principles exploit.

The central route has a payoff: attitudes formed through careful thought tend to last longer and resist persuasion more. So when something matters to you, give it your attention.