One of the most powerful ideas in advertising is the idea that everyone is doing it. Whether you’re looking for a holiday destination, buying a mobile phone, or just choosing a new CD, most people are more likely to get out their wallets if they think that other people are doing the same.

This is one reason why Amazon has those ‘X% of people who viewed this bought this’ boxes. This is more than just cross-promotion, more than just suggesting products related to the one that you’ve shown an interest in; it’s a subtle endorsement of the decision to buy, appealing to the fact that lots of other people have done the same. “How could it be a mistake when so many people have done it?” Or, to make the old trope unmistakable, “Millions can’t be wrong”.

Persuasive though this train of thought can be, it lacks logical rigour. Popular decisions can be mistakes; millions can be wrong. And if X% of people have messed up and bought rubbish, then you wouldn’t want to join them.