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Cialdini, a social psychologist, studied research on persuasion and also went undercover in training programmes for salespeople, fundraisers and advertisers. He found that much of our decision making runs on automatic shortcuts, like a recording that starts playing when a button is pressed.

These shortcuts usually serve us well, because we cannot think through every choice. Simply giving a reason, even a weak one, makes people more likely to agree to a request. The contrast principle works in a similar way: after a big purchase, a smaller add-on seems cheap. Professionals who know these triggers can use them to get a yes.