At some point in the spring, when Bernie Sanders had lost any chance of winning the Democratic nomination, Hillary Clinton’s campaign suddenly started to perk up. According to Scott Adams, creator of Dilbert and contrarian pundit, much of the improvement was due to one simple word: dark. Clinton’s team had made a point of using this word to describe Donald Trump’s speeches — thus inciting voters to engage in a little confirmation bias. Next time they saw him kiss a baby or do anything at all, they would think: how dark.

The man behind this strategy — at least on Mr Adams’ telling of the story — is Robert Cialdini, professor at Arizona State University and author of Influence, one of the best-selling books on behavioural psychology ever. This week he publishes Pre-Suasion, in which he argues that people spend too much time fretting about the message itself, and not enough about what goes on immediately before they deliver it.

So when I rang Mr Cialdini to ask if he was helping Mrs Clinton, what he did in the moment before he replied struck me as unusually important. This amounted to a longish pause, and an intake of breath. Very slowly he said: “It’s my policy not to speak about any campaign that’s ongoing. The emotions are too deep.”