FIRST, you realise it’s a gorilla. The opening strains of Phil Collins’s “In the Air Tonight” are playing; the beast is enraptured. As the camera pulls back, you see that he’s seated at a drum kit. He flexes, raises his drumsticks, then brings them resoundingly down. Only in the final frames do you discover that the gorilla is pitching Cadbury’s Dairy Milk chocolate. The advert, released in 2007, should not have worked. Conventional wisdom doubted that a jolt of joy from a drumming primate, however rhythmically gifted, would spur sales of chocolate bars. A member of the team that developed the ad says that when it was passed to Millward Brown, the world’s biggest tester of adverts, the firm found that it scored poorly among women on its measures of “awareness” and “brand appeal” and about average among men (Millward Brown says it did better on other measures). Yet Cadbury went ahead, and was rewarded with millions of online views, better perceptions of its brand and higher sales. Return on investment was three times the average for packaged-goods marketing campaigns.