as rating systems have become more popular — and, as Resnick shows, valuable — there has been what some would say is a predictable response: the emergence of scammers, spammers, and thieves bent on manipulating the mob. Call it crowdhacking.

In some cases, crowdhackers are looking to boost sales or increase traffic to their Web sites. In other instances, they’re simply ripping off unsuspecting consumers. Either way, the more we base decisions on the wisdom of crowds, the greater the incentive to cheat.