Crucially – and this is the really important point – the Montagu principle held even among Trump's own supporters, while there was no evidence for the red meat hypothesis. The researchers found that it also held true when Trump was responding with a “counterpunch” to personal criticism of himself – a situation where he might appear to be more justified to respond in kind.

Overall, it seems the public prefer politicians to make no response to an insult, rather than to react spitefully themselves.

To make sure that this was a general effect and not simply shaped by their existing expectations, Frimer and Skitka broadened their analysis to examine participants’ reactions to statements of fictitious politicians from across the political divide. Once again, the Montagu Principle predicted their reactions, irrespective of the participants’ own political affiliations.

Frimer and Skitka also performed textual analyses to measure civility in the transcripts of congressional debates between 1990 and 2015, and examined public approval of Congress over the same period. It’s worth noting that here, they were examining the Congress as one body, rather than the contributions of individual politicians – but they still saw a similar effect. The less civil the debates, the less the public approved of Congress’s handling of various issues. In other words, rudeness reduced trust in the whole political process.

The researchers conclude that good manners can still “buy everything and cost nothing" – as Lady Montagu had asserted – and politicians campaigning in the US mid-term elections would do well to take note.

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David Robson is a senior journalist at BBC Future. He is d_a_robson on Twitter.

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